Physics
Please send comments to: J.Elfick@uq.edu.au
Updated: 2008-08-02
Table of contents
4.155.0 Inertia
4.158.0 Centipetal force
4.162.0 Force and motion
4.170.0 Machines
4.188.0 Liquid pressure
4.200.0 Buoyancy
4.212.0 Surface tension
4.155.0 Inertia
4.155 Inertia with a stone
4.156 Inertia with two drink-can pendulums
4.157 Inertia tricks
4.158.0 Centipetal force
4.158 Centripetal force with a liquid
4.159 Centripetal forces with a fresh and hard-boiled egg
4.160 Centripetal force with a water bucket
4.161 Measure centripetal force
4.162.0 Force and motion
4.162 Equal forces on light and heavy bodies
4.163 Equal forces from spring clothes-pegs
4.164 Action and reaction pushing forces
4.165 Action and reaction when stepping forward
4.166 Action and reaction with balloons
4.167 Thrust from a hose, rifle
4.168 Action and reaction pulling forces
4.169 Electric fan on a sailing boat
6.11 Forces on coins on a slope (Primary)
4.170.0 Machines
4.170 Three orders of lever, machines
4.171 First order lever, type 1 lever
4.172 Second order lever, type 2 lever
4.173 Third order lever, type 3 lever
4.174 Wheel and axle
4.175 Simple pulley
4.176 Single fixed pulley
4.177 Single movable pulley
6.10 Pull with pulleys (Primary)
4.178 Inclined plane, screw, thread, wedge
4.179 Belt drive
4.180 Gear wheel
4.181 Sliding friction
4.182 Rolling friction
4.183 Mount a box on wheels
4.184 Reduce friction with oil
4.185 Reduce friction with ball bearings
4.186 Reduce friction with an air stream
6.13 Forces of friction (Primary)
4.187 Propeller
4.188.0 Liquid pressure
4.188 Weight and pressure
4.189 Measure pressure with a U-tube
4.190 Water pressure changes with depth
4.191 Liquid pressure depends on the density of the liquid
4.192 Water pressure does not depend on the size of the container
4.193 Water pressure is the same in all directions
4.194 Balancing water columns
4.195 Raise heavy weights with water pressure
4.196 Water does not compress
4.197 Make a hydraulic lift
4.198 Make a hydraulic ram, water ram
4.199 Make water wheels
4.200.0 Buoyancy
4.200 Buoyancy of water
4.201 Cartesian diver
4.202 Density of irregular solid, overflow can
4.203 Weight of a floating body
4.204 Float a lighted candle
4.205 Float different kinds of wood
4.206 Float an egg in tap water and salt water
4.207 Float grapes at different levels in water
4.208 Drinking straw hydrometer
4.209 Floating in different density liquids
4.210 Model diving bell
4.211 Float a metal boat, Plimsoll line
6.12 Floating and sinking (Primary)
4.212.0 Surface tension
4.212 Soap and surface tension
4.213 Float a needle on water
4.214 Float a razor blade
3.38 Float a pin (Primary)
4.215 Lift the water surface
4.216 Hold water in a sieve
4.217 Heap up water in a glass
4.218 Pinch together water streams
4.219 Drive a boat with surface tension
4.220 Blow soap bubbles
1.40 Blow soap bubbles (Primary)
4.221 Soap bubble support
4.222 Soap film and sliding wire
4.223.0 Atmospheric pressure
4.223 Plastic syringes and air pressure
4.224 Find air
4.225 Air takes up space
4.226 Air has mass
4.227 Air pressure in all directions
4.228 Push a drinking straw through a potato
4.229 Mercury barometer, barometric pressure
4.230 Aneroid barometer
4.231 Measure atmospheric pressure with a bicycle pump
4.232 Measure atmospheric pressure with a plumber's force cup, suction cup
4.233 Syringe lift pump
4.234 Test-tube force pump
4.235 Siphon
4.236 Siphon fountain
4.237 Lift water with air pressure
4.238 Volume and pressure of air
4.239 Pressure affects boiling point of water
4.240 Make a model of the lungs
4.241 Oxidation and air pressure, steel wool over water
4.242 Air streams
1.43 Air streams (Primary)
4.243 Cold air is heavier than warm air
6.35 Burn candle over water (Primary)
4.155 Inertia with a stone
See diagram 4.155: Inertia with a stone
Use a stone weighing about 1 kg. Suspend the stone with a light string that is just strong enough to support the stone. Attach two pieces of the same string to the stone and let them hang down.
1. Grasp firmly the lower end of one hanging string, B, and give it a quick jerk with a sudden impulsive pull. The lower string breaks and leaves the stone suspended by string A because of the inertia of the stone. If you leave some slack in string B then pull it you have a greater force. Also you can attach a short iron bar to the end of string B.
2. Pull steadily on the other hanging string, B. The upper string A breaks and the stone falls. The steady application of force has set the stone in motion. The stone was "reluctant" to accelerate because of its inertia.
4.156 Inertia with two drink-can pendulums
See diagram 4.156: Inertia with two drink-can pendulums
Use long strings to suspend from the ceiling two large identical tin cans or buckets. Fill one can with sand. Use the hook of a spring balance to push each can in turn. Note what force is necessary to start the cans moving. Use your hand to stop the cans when they are moving. You can feel the difference in inertia of the two cans.
4.157 Inertia tricks
1. Put a coin on a stiff playing card placed over the mouth of an empty glass. The coin must be placed over the edge of the glass. Try to remove the card but not the coin. Flick the card away quickly with your finger. The coin falls into the glass. The coin does not move sideways because of its inertia.
2. Make a pile of books. Grasp the book at the bottom of the pile and pull very quickly. You can remove the bottom book without upsetting the pile because of the inertia of the books above it.
3. Make a pile of coins or checkers. Strike the lower most coin sharply with a ruler to dislodge it without the pile falling over.
4. Repeat the experiment by dislodging the lowermost coin with a tight string held in both hands and pulled quickly against the bottom coin.
5. Scoop up a spade full of dry earth. Pitch the earth away from you. When the spade stops, the earth keeps moving because of its inertia.
4.158 Centripetal force with a liquid
See diagram 4.158: Centripetal force with a liquid
Tie a wire securely around the neck of a plastic goldfish bowl. Attach a string to the wire. Clamp a hook in a hand drill chuck and attach it in the centre of the string. Put water coloured with ink in the bowl. Put a plastic "fish" in the bowl. Turn the drill handle to spin the bowl and water or whirl the bowl around you by hand. The water begins to climb up the sides of the bowl because of the "centrifugal force" on the water, not the centripetal force on the water. Note the effects of inertia of the water when starting and stopping spinning the bowl. When starting spinning, the water tends to remain stationary. When stopping spinning, the water tends to continue spinning inside the bowl.
4.159 Centripetal forces with a fresh and hard-boiled egg
Use your fingers to spin a fresh egg and a hard-boiled egg end-on. The hard-boiled egg spins longer because the inertia of the fluid contents of the fresh egg brings it to rest sooner.
4.160 Centripetal force with a water bucket
1. Almost fill a small bucket with water. Swing it around rapidly at arm's length in a vertical circle. The water does not spill because centrifugal, not centripetal, force acts on the water to keep it in the bucket.
2. Some people can do the water in glass trick by turning a glass of water quickly through 360o, without spilling a drop.
4.161 Measure centripetal force
See diagram 4.161: Measure centripetal force
Use a metal or wooden tube about 15 cm long. Tie a two-holes rubber stopper to the end of 1.5 m of string. Pass the other end of the string through the tube and attach iron washers for weights. Adjust the string so that the distance from the top of the tube to the cork is 1 m. Grip the tube as a handle and swing it in a small circle above your head so that the rubber stopper moves in a horizontal circle. The force of gravity on the washers provides the horizontal force needed to keep the stopper moving in a circle. Attach a clip to the string below the tube. Swing the tube such that the level of the clip remains constant and so the circular motion is constant. To find the frequency, record the number of revolutions per minute. Record the number of washers when the stopper moves in a path of radius 1 m at constant revolutions per unit time. If you increase the number of washers, you must increase the speed of the stopper to keep the washers at the same height, F = mg = m (v2 / r). If you halve the radius of the stopper, you must decrease the speed of the stopper to keep the washers at the same height, F = mg = m (v2/ r).
4.162 Equal forces on light and heavy bodies
See diagram 4.162: Equal forces on light and heavy bodies
When you apply equal forces to light and heavy bodies, the light body moves farther than the heavy body. Draw a spot on a rubber band and attach two spring clothes-pegs. Put a metre stick on the table. Pull the clothes-pegs apart an equal distance between each clothes-peg and the spot. Note the position of the spot compared with the metre stick and call this the mid point. Release the clothes-pegs simultaneously. They impact at the mid point. The rubber band exerted an equal and opposite force on each clothes-peg. Attach two clothes-pegs on the left side of the rubber band and one on the right side. Stretch the rubber band and release the clothes-pegs. The clothes-pegs impact to the left of the mid point. Clamp more clothes-pegs on the left side and release. The clothes-pegs on opposite sides of the rubber band impact further to the left of the mid point.
4.163 Equal forces from spring clothes-pegs
See diagram 4.163: Equal forces from spring clothes-pegs
Tie a spring clothes-peg open by tying a thread around the mid point of the long end. Put the clothes-peg on the table and put two marbles of the same size and weight at the end of the long ends where you would normally put your forefinger and thumb. Burn or cut the thread. The clothes-peg springs open and exert an equal and opposite force on the marbles, giving them equal speeds in opposite directions. Repeat the experiment using a large and a small marble. The small marble is given a faster speed than the large marble.
4.164 Action and reaction pushing forces
Forces work in pairs. If you push against a wall, the wall pushes back with equal force. Use two kitchen spring balances with square platform tops. Put the tops together, with the dial faces up. Ask a student to push on one spring balance while you push on the other balance. Note that when you push together each balance reads the same, although you may push harder than the student. According to Newton's third law of motion, to every action there is an equal and opposite reaction.
4.165 Action and reaction when stepping forward
1. Throw a heavy ball to a student on roller skates. The student moves in the opposite direction to the motion of the ball. If the student then takes a step forward, the other roller skate tends to move backwards.
2. Stand in a boat and take a step forward on to a wharf step. The boat tends to move backwards in the opposite direction to the step.
4.166 Action and reaction with balloons
1. Inflate a rubber balloon and then release it. The balloon moves forward with a spluttering sound in the opposite direction to the compressed air leaving the balloon. The propulsive force produced by a jet is called the thrust. Put the balloon in a box so that the opening of the balloon is fixed with a tube through a hole in the back of the box. Put a cardboard tube in the nozzle of the balloon. Inflate the balloon and release it. The boat moves forward like a jet boat.
2. Attach a drinking straw to the side of a long balloon. Pass a thin wire through the drinking straw and attach it to the opposite sides of the room. Put a cardboard cylinder in the nozzle of the balloon. Inflating the balloon and then release it. The balloon travels along the wire.
4.167 Thrust from a hose, rifle
1. Use a bow and arrow or catapult. The force exerted by one arm to pull back equals the force used to hold steady the bow or the fork of the catapult.
2. Fire a rifle. As the bullet leaves the rifle, you feel the recoil force on your shoulder. When large guns are fired, they tend to move backwards.
4.168 Action and reaction pulling forces
Use two spring balances. Make a loop in each end of a short piece of strong string. Attach a spring balance to each end and to pull in opposite directions. Note the readings on both balances and compare them.
4.169 Electric fan on a sailing boat
Put a battery operated fan on a model sailing boat. When it blows against the sail the boat does not move forward because there is an equal and opposite force on the fan. Put the fan on the shore where the wind from the fan can reach the sailing boat. The wind from the fan blows the sailing boat forward.
4.170 Three orders of lever, machines
1. Machines allow a force to be applied, the effort, E, to overcome other forces, load, L. The mechanical advantage (force ratio) MA, of a machine is the ratio of the load to the effort, load / effort, L / E. The velocity ratio (distance ratio, gear ratio) VR, of a machine is the ratio of the distance travelled by the effort to the distance travelled by the load. The efficiency of a machine is the ratio of the work done by the machine to the work supplied to the machine. Work = force X distance. For a machine with efficiency = 1, i.e. 100%, MA = VR. However, the efficiency of a machine is less than 100% cent because some energy loss always occurs. Machines usually allow a smaller applied force, the effort, to overcome a larger resistance force, the load. However, machines may be used to change the direction of a force or to handle small objects, e.g. tweezers (forceps).
2. Levers have a rigid beam supported a one point, the fulcrum, F, with a load force, L, applied at one point and an effort force, E, applied at another point. The lever principle is load X length of load arm = effort X length of effort arm. Each side of this equation is a moment, i.e. force X perpendicular distance to the pivot. So moments clockwise = moments anticlockwise.
The three orders of levers depend upon the relative positions of F, L, and E:
A first-order lever has the fulcrum between the load and the effort (E F L).
A second-order lever has the load between the effort and the fulcrum (F L E).
A third-order lever has the effort between the fulcrum and load (L E F).
4.171 First order lever, type 1 lever
See diagram 4.171: First order lever, type 1 lever, class 1 lever
1. Use a metre stick with a hole drilled in the centre. Hammer a nail horizontally into the side of a table. Suspend the metre stick at the centre by the nail through the hole. Use a loop of string and a small mass to balance the metre stick. Tie a loop of string around the metre stick, each side of the nail. Attach a spring balance to one loop, hanging down. Attach a weight to the other loop. Tie a loop of string to a weight. Move the loops to any position along the bar. Pull down on the ring end of the spring balance to raise the weight. Note the weight, the reading on the spring balance, the distance from a weight loop to nail, the distance from a spring balance loop to nail. Also, note how far the spring balance loop moves down and how far the weight loop moves up.
2. Use a board the same height as a desk. Place a stick across the board and use it as a lever to raise the table. Note that the longer end of the stick moves farther than the shorter end. The force exerted by the shorter end, the load, is greater than the force used to move the longer end, the effort.
3. Close a wooden match box and try to crush it between your thumb and fingers. You cannot do it. Hold the match box in the jaws of a pair of pliers. You can easily crush it by squeezing the handles together. Pliers, tin snips, and bolt cutters have two Type 1 levers with each fulcrum as a pivot. When cutting paper or cloth with scissors the effort < load because you want a long length of scissors blade and cloth does nor require much force to cut it. Try using a pair of scissors as tin snips to feel the difference.
4. Hammer a nail into a big piece of wood. Try to pull the nail out with your fingers. You cannot do it. Use a claw hammer to pull out the nail. The load is the force of the nail on the claw. The fulcrum is the round part of the hammer head. The effort is your pull on the handle. You are using the hammer as a bent lever to pull out the nail.
4.172 Second order lever, type 2 lever
See diagram 4.172: Second order lever, type 2 lever,  class 2 lever
Use a metre stick with a hole drilled in the centre near one end. Hammer a nail horizontally into the side of a table. Suspend the metre stick at one end by the nail through the hole and attach a spring balance to the other end. Tie a loop of string to a weight. Pass the bar through the loop so that the bar can support the weight. Move the loop to any position along the bar. Examples include the wheelbarrow and the nutcracker.
4.173 Third order lever, type 3 lever
See diagram 4.173: Third order lever, type 3 lever, class 3 lever
1. Use the same apparatus as for Type 2 levers, but put the weight, load, at the end of the bar and suspend the bar by a loop of string attached to a spring balance, effort. Since a Type 3 lever has the effort between the fulcrum and the load, the effort is always greater than the load, M. A. < 1. Pick up something heavy with tweezers, forceps, or chopsticks. They consist of two Type 3 levers joined at the fulcrum. For chopsticks the fulcrum is the angle between your thumb and forefinger. The force you apply with your fingers, effort, is greater than the force exerted by the ends of the tweezers or chopsticks, load. Type 3 levers are convenient for picking up small things.
2. Catch a fish with a rod and line. The load is the pull of the fish. The effort is your pull on the rod. The fulcrum is where you hold it lower down or where the rod touched the ground.
3. Keep your upper arm vertical and your forearm horizontal in front of the body. Put a heavy stone in the palm of your hand and move it up towards your mouth without moving the upper arm. The load is the weight of the stone. The effort comes from the shortening of the biceps muscle in your upper arm. The fulcrum is the elbow joint.
4.174 Wheel and axle
See diagram 4.174: Wheel and axle
Tie one end of a string to books. Grab the other end of the string, pull up the books and feel their weight. Remove the cover from a pencil sharpener and tie the end of a string around the end of the shaft. Turn the handle of the pencil sharpener to raise the books. The force needed to turn the handle is much less than the force needed to pull up the books by grabbing the string.
4.175 Simple pulley
See diagram 4.175: Simple pulley
Use a wire clothes-hanger and a cotton reel. Cut the hanger wire 20 cm each side of the hook. Bend the cut ends until horizontal then slip the ends into a cotton reel. Push the cut ends through the cotton reel then turn them down where the come out of the other side.
4.176 Single fixed pulley
See diagram 4.176: Single fixed pulley
The single fixed pulley allows the use of a downward force, the effort, E, to lift a load, L. It is just a convenient way to lift something by pulling down instead of pulling up. The tension in the rope is equal to the weight of the body supported. Mechanical advantage = 1, because E = L (ignoring friction). Velocity ration = 1 because distance of pulling down = distance body moves up.
Hang masses at A to find how much force you need to lift 50, 100 and 200 g placed at B. You need the same force. Pull 20 cm down on A and measure the distance moved by a mass at B. The distances are the same.
4.177 Single movable pulley
See diagram 4.177: Single movable pulley
The single movable pulley has a fixed pulley to change direction and a moving pulley. If the effort at the end of the rope = E, the total upward force on the moving pulley = 2E because it is supported by two parts of the rope. So the load = 2E (ignoring friction) M.A. = 2. To raise the load by 1 m requires the rope on each side of the moving pulley to shorten by 1 m so 2 m of rope must be taken from the pulling end, V.R. = 2. Use a spring balance to measure the weight of three books. Suspend two pulleys on a string and use the books for a load. Attach a spring balance to the end of the string and pull down on the ring at the end of the spring balance. The force needed to lift the books is equal to half the weight of the books, ignoring friction and the mass of the pulley. However, you must pull down at twice the distance needed to raise the books using a single fixed pulley. Friction in the pulleys and the weight of the movable pulley lowers the efficiency of this system of pulleys.
4.178 Inclined plane, screw, thread, wedge
See diagram 4.178: Simple screw thread
1. Put a roller skate or a heavy toy car on the table, attach a spring balance with a string. Steadily raise the spring balance and note the weight of the roller skate. Pull the roller skate up an inclined plane with constant speed and note the force required. You need less force to pull up the roller skate up the inclined plane than to lift it vertically. However, you must apply the force for a greater distance up the inclined plane than when the roller skate is lifted vertically through the same vertical height.
2. Make a simple screw thread. Cut a piece of white paper to make a right angle triangle with sides 50, 40 and 30 cm. Roll the triangle on a round rod, beginning at the shortest side and rolling towards the point of the triangle. Keep the base line of the triangle even as it rolls. Note the hypotenuse is like an inclined plane. Note how it spirals up the rod like the thread of a screw. A screw thread is a type of inclined plane.
3. Make a simple lifting jack. Bore a hole through a block of wood to fit a bolt threaded for nearly its entire length. Sink the head of the bolt in the wood so that it is flush with the surface of the wood. Nail a piece of board over it. Attack a hexagonal nut, a washer and short piece of metal pipe with the diameter slightly larger than the diameter of the bolt. Turn the nut with a wrench do that the device acts as a lifting jack.
4.179 Belt drive
See diagram 4.179: Belt drive
Drive two long nails into a block of wood. Place spools, one larger than the other, over the nails so you can use them as axles. Slip a rubber band over both spools. Rotate the larger spool through one turn and note whether the smaller spool makes one full turn. Note the direction in which the small spool turns. Cross the rubber band and note the result.
4.180 Gear wheel
See diagram 4.180: Gear wheel
1. Punch holes exactly in the centres of bottle tops. Put two of the bottle tops on a block of wood so that the tooth-like projections mesh. Fasten the bottle tops to the wood with nails through the hole, but make sure that the bottle tops can still turn easily. Turn one of the bottle tops and note the direction that the other turns. Add a third bottle top and note the direction that each bottle top turns.
2. Turn a bicycle upside down. Turn the pedal wheel exactly one turn and note the number of turns made by the rear wheel. Examine the gear mechanism.
4.181 Sliding friction
See diagram 4.181: Push and pull
1. Attach a string from a heavy box to a spring balance. Move the box across the table by pulling it with the spring balance. Record the force needed to pull the box at constant slow speed. The force of friction opposes the motion. The friction between the bottom of the box and the table is called sliding friction.
2. Put a sheet of glass on the table. Record the force needed to pull the box at a slow constant speed. If the surface of the glass is smoother than the surface of the table, the sliding friction is less.
3. Use chalk or water or oil to lubricate the surface of the table. Record the force needed to pull the box at a slow constant speed.
4.182 Rolling friction
See diagram 4.182: Rolling friction
Put round pencils under the box to act as rollers. Record the force needed to pull the box at a slow constant speed. This friction is called rolling friction.
4.183 Mount a box on wheels
Record the force needed to pull the box at a slow constant speed. Has the rolling friction decreased? Put ball bearings or marbles under the box. Record the force needed to pull the box at a slow constant speed. Pour oil on the ball bearings or marbles. Record the force needed to pull the box at a slow constant speed. This may be the least force need to pull the box.
4.184 Reduce friction with oil
Lay two panes of glass side by side and place a few drops of oil on one. Feel the difference when you rub a finger back and forth on the unoiled pane and on the oiled pane.
4.185 Reduce friction with ball-bearings
See diagram 4.185: Reduce friction with ball-bearings
Use two paint pots with a deep groove around the lid. Put marbles in one groove and invert the other paint pot over the marbles to form a ball-bearing race. Put a book on top of the paint pots and note how you can easily move the top paint pot around. Add oil to the marbles and the top paint pot turns more easily.
4.186 Reduce friction with an air stream
See diagram 4.186: Reduce friction with an air stream
A Balloon, B Cotton reel, C Cardboard
Cut out a disc of cardboard about 10 cm in diameter. With a red-hot pin, burn a hole through the centre. Saw a small cotton reel in half and glue the original end of one half over the middle of the disc. Find a piece of bamboo or another tube which just fits the hole in the reel. Push this into the neck of a small balloon, using cotton or a rubber band to secure the joint. Blow up the balloon, pinch the neck, and insert the tube into the hole in the cotton reel. Place the disc on the table and release the air. The expanding air, escaping through the hole in the disc, will lift the card so that, given a flick, it will shoot across the table with practically no friction. This experiment illustrates the principle of the hovercraft.
4.187 Propeller
See diagram 4.187: Propeller
Make a rotor from the lid of a drink-can. Roll the outer edge to avoid cuts. Draw the three blades on the lid. Make cuts first along the thick lines and then along the dotted lines. Remove the smaller sections leaving three blades. Put the drink-can lid on a block of wood and cut out the shape with a chisel. Drill at the centre two 5 mm diameter holes 5 mm apart, then remove the little bridge of metal between them to make a central slot. Use a twisted strip of thick metal 1 cm X 25 cm to fit the above slot or use two strong wires. To twist the wire, fold a 60 cm length in half with a large loop at the bend. Slip rod B through the loop and clamp the free ends close together in a vice. Then twist up the doubled piece to give a long uniform twist of angle about 20o to the axis. The holes in the rotor may need a little trimming so that it will spin freely up and down the twist. Use a short tube made from tin plate that slides easily along the wire. Twist the blades so that the angle of the rotor blades gives lift when the rotor is spun by being pushed off the wire. The assembly has 3 parts: 1. The wire, held vertically 2. the tin tube, which should rest on the loop at the foot of the twisted wire and 3. the rotor, which should rest on top of the tin tube. To launch this flying saucer, hold the arrangement steady above your head by the tube and strongly pull the wire twist down with the other hand. Use different blade angles, or different numbers of blades to get the best flight effect. Use a rubber band drive as a source of power for your propellor in a model aircraft or model boat.
4.188 Weight and pressure
See diagram 4.188: Weight and pressure
1. Use a block of wood with two different dimensions, e.g. 10 cm X 15 cm. Put the block on Plasticine (modelling clay) or mud, with the larger face down. Repeat the experiment with the smaller face down. Record the different depths the block sinks. Add a weight to the block to make it sink deeper. When the smaller face is down, the block sinks deeper than when the larger face is down. Pressure = force/area. The force down, i.e. the weight, is the same but the area of the smaller face is less than the area of the larger face. When the block has the smaller face down, it exerts more pressure.
2. Stand on mud wearing flat shoes and high heel shoes. You sink deeper wearing high heel shoes because the surface area is less. Formerly, ladies could not wear high heel shoes in aircraft because the pointed heel might make holes in the aluminium floor.
3. Cut with a sharp knife and a blunt knife, or dig with a sharp spade and a blunt spade. You can cut deeper with the sharp knife because the surface area of the knife edge is less and applies more pressure. Pressure = force / area, so the greater the area, the less the pressure.
4.189 Measure pressure in liquids with a U-tube
See diagram 4.189: Measure pressure in liquids with a U-tube
Half fill a U-tube with coloured water. Stretch a piece of thin rubber loosely over the mouth of a filter funnel and tie it securely. Attach the stem of the filter funnel to one arm of the U-tube with rubber tubing. Hold the mouth of the filter funnel at different depths in a container of water. Record the depths of the mouth of the funnel in the water corresponding to the differences in heights of the coloured water in the U-tube. This U-tube is being used as a pressure gauge.
4.190 Water pressure changes with depth
Cut one end off a tall plastic drink bottle to make a tall container. Use the pressure manometer from the previous experiment. Place the funnel at different depths to measure the pressure.
4.191 Liquid pressure depends on the density of the liquid
Repeat the experiment with a container of methylated spirit, pure water and salt water. Hold the mouth of the filter funnel at the same depths in the three liquids and note the corresponding differences in the heights of the coloured water in the U-tube. At the same depth, the less dense methylated spirit exerts less pressure than pure water and the more dense salt water exerts more pressure than pure water.
4.192 Water pressure does not depend on the size of the container
Jet aircraft are usually refuelled by pumping fuel up through a hole in the bottom of the fuel tank. The pump has to overcome only the weight of the column of liquid fuel above the hole, and with the same diameter as the hole, not the weight  of the whole fuel contents of the fuel tank.
Repeat the experiment using a large container of water and a small container of water. Hold the mouth of the filter funnel at the same depths as before. The corresponding differences in height of the coloured water in the U-tube are the same.
4.193 Water pressure is the same in all directions
See diagram 4.193: Spurting tennis ball
1. Punch holes around the base of a tall metal can with a nail. Cover the holes with a strip of tape. Fill the can with water and hold it over a sink. Strip off the tape. The distance the streams shoot out from the holes is the same in all directions.
2. Cut six pieces of glass tubing 3 cm long. Cut small holes in a tennis ball just big enough to insert the glass tubes. Place the holes at top and bottom, right and left, nearest you and farthest from you. Put the tennis ball and glass tubes in a bucket of water. Squeeze out all the air so the tennis ball is full of water. Take out the tennis ball and hold it in your hand with your fingers around it but not over the glass tubes. Squeeze the tennis ball. The same amount of water squirts out through the glass tubes in all directions.
4.194 Balancing water columns
See diagram 4.194: Balancing water columns
Cut the bottoms from different shape plastic bottles. Fit one bottle with a one-hole and the other with two-holes stoppers. Connect the first bottle with a two-holes stopper to a tap or reservoir and the second bottle. Connect all the bottles with glass tubing and rubber connectors. The last bottle has the one-hole stopper. Turn the attached bottles upside down. Turn on the tap so that water flows into the bottles. The level of water is the same in the differently shaped bottles. Pressure in a liquid is independent of the size or shape of the vessel and depends only on the depth. Some people say "Water finds its own level."
4.195 Raise heavy weights with water pressure
See diagram 4.195: Raise heavy weights with water pressure
Use a rubber hot water bottle. Put a one-hole stopper carrying a short glass tube tightly in the neck. Punch a hole in the bottom of a plastic container and make it large enough to take a one-hole stopper. Put a short piece of glass tubing through the stopper. Connect the water bottle and the container with 1.25 m of rubber tubing. Wind wire or adhesive tape firmly around the connection at the bottle. Fill the bottle, tube and can with water. Place the bottle on the floor and put a wooden board, books or other heavy objects on it. Raise the plastic container above the level of the floor. The books rise. Note how heavy a weight you can lift by raising the plastic container.
4.196 Water does not compress
Some teachers have tried this several times and cannot get the desired effect. Water just squirts out. Fit a bottle with a one-hole stopper with a long glass tube passing through it. Fill the bottle with water. Insert the stopper tightly until the water rises in the medicine dropper. Grasp the bottle in your hands and squeeze as hard as you can. Water rises in the tube because you cannot compress water.
4.197 Make a hydraulic lift
Water pressure can raise freight and passenger lifts. Connect a rubber hose to a motor car hand pump and bind the connections with wire and adhesive tape. Connect the other end of the hose to a water tap. Fix a weight to the handle of the pump. Turn on the water tap and see the water pressure lift the weight.
4.198 Make a hydraulic ram, water ram
See diagram 4.198: Hydraulic ram
Hydraulic rams are sometimes used to raise water from a low level to a higher level. A flowing stream of water operates them. You can make a model hydraulic ram in the following way. Remove the bottom of a plastic drink bottle. Fit the bottle with a one-hole rubber stopper carrying a short length of glass tubing. Connect the glass tubing to a glass or metal T-tube that has a piece of rubber tubing on one end and a jet tube connected to it with a rubber tube. Fill the bottle with water and pinch the tube at the end. Let the water run from the end of the tube. Stop the flow suddenly by quickly pinching the tube, and note the height to which the water squirts from the jet tube. Let the water flow and stop alternately, and you have a working model of the hydraulic ram.
4.199 Make water wheels
See diagram 4.199: Water wheels
Use a cylindrical cork and push in one the cutting edges of safety razor blades to make paddles. Insert two needles in the centre of each circular face to act as an axle. Grasp the needles in each hand to hold the water wheel in a stream of water from a tap. See undershot and overshot water wheels.
4.200 Buoyancy of water
See diagram 4.200.1: Archimedes' principle
1. According to Archimedes' principle, (Archimedes of Syracuse 287-212 B.C.), an object wholly or partly immersed in a fluid will be subjected to an upward force, upthrust, equal to the weight of the fluid it has displaced. If the density of the object is greater than the density of the fluid, its weight will be greater than the upthrust and it will sink. If the density of the object is less than the density of the fluid, the upthrust will be the greater than its weight and the object will be pushed upwards towards the surface. As the object raises above the surface, how much fluid it displaces decreases. Also, the upthrust decreases until the upthrust acting on the submerged part of the object equals the weight of the object and the object floats.
Let v = submerged volume of floating regular solid, and V = volume of whole solid. Let d = density of floating solid, and D = density of liquid
Weight of floating solid = upthrust (weight of liquid displaced)
d = m / v, so m = v X d
V X d = v X D, so v / V = d / D
d / D is the relative density of the solid compared with the liquid and v / V is the fraction submerged
So fraction submerged = density ratio
If the floating solid has uniform cross-section area, v/V = submerged length / total length
So relative density of liquid = submerged length of floating solid / total length of floating solid.
Buoyancy does not increasae with depth.
See diagram 4.200.2: Buoyancy of water
2. Use a metal container with a tightly fitting cover, e.g. a treacle tin. With the cover on, push the container into a bucket of water, with the cover end down, and quickly let go of it. Note the upthrust on the container. Put some water in the container and repeat the experiment. Keep adding water until the container can no longer float. Fill the container can with water and put the cover on. Put a double loop of string around the side of the container and then attach a large rubber band to the other end of the string. Lift the container by holding the rubber band and note how much the band stretches. Lower the container into a bucket of water and note the stretch in the rubber band. The buoyant force a fluid exerts on a submerged object is equal to the weight of the volume of fluid displaced.
3. Float a small wooden boat carrying a heavy piece of lead in a bucket of water. Note the level of the water at the side of the bucket. Remove the piece of lead and drop it into the water. Again note the level of the water at the side of the bucket. The water level has fallen because when in the boat the piece of lead displaces its weight of water. However, when at the bottom of the bucket of water the piece of lead displaces its volume.
4.201 Cartesian diver
See diagram 4.201: Cartesian diver
1. Wrap copper wire around the narrow part of the rubber bulb from a medicine dropper to make a rubber diver. Fill a tall plastic container with water. Put enough water in the rubber diver so that it only just floats in the container of water. Most of the rubber will be under water. Adjust how much the rubber diver floats by pinching the bulb to remove air. Cover the container with a sheet of plastic and fix it tightly around the rim of the container with string or rubber bands. Press on the tight plastic cover to make the diver sink. Stop pressing on the tight plastic cover to make the diver rise.
2. Repeat the experiment using a very small glass tube to make a diver. Add ink to the water to help you see the water level in the glass tube. Note that when the cover is pressed down, the pressure is transmitted through the water to decrease the volume of the air bubble in the glass diver. So the water level rises in the tube. When the volume of the air bubble is too small to hold up the glass diver by displacement of water the glass diver will sink. When you stop pressing on the cover, the decrease in pressure is transmitted to the air bubble that expands so the water level in the glass diver decreases. The increased volume of the air bubble in the glass diver displaces enough water to provide an upward force by displacement of water to allow the glass diver to float again.
3. Push a wooden match stick into a hollow plastic ball. The plastic ball by itself just sinks in water but the match gives it enough buoyancy to just float. Shorten the match so that its end floats level to the surface of water in a plastic drink bottle. Close the bottle with a plastic cap. The pressure of the fingers on the walls of the plastic bottle is transmitted to compress the air in the plastic ball and it sinks.
4. Cut a fresh piece of orange peel in the shape of a submarine. Make portholes in the side with the end of a ball point pen. Put the orange peel submarine in a container of water sealed with a plastic cap. Bubbles in the orange peel allow the submarine to float. Pressure on the plastic cap is transmitted to decrease the size of the bubbles and the submarine sinks.
5. Use a plastic ball point pen top with a pocket clip. If air can pass through the upper end seal it with Plasticine (modelling clay). Attach a chain of paper-clips to the pocket clip so that the pen top can float near the surface of water with the paper-clips hanging down. Almost fill a large plastic drink bottle with water. Hold the pen top vertically over the bottle with the paper-clips hanging down then gently lower it into the water. Screw the drink bottle cap on tightly. Squeeze the sides of the plastic drink bottle with your thumbs to make the pen top sink. The pen top contains an air bubble. When you squeeze the sides of the drink bottle, you also squeeze the air bubble, so more water enters the pen top and it sinks because the air bubble displaces less water.
6.  For a diver use a plastic sachet of sauce, mayonnaise or butter. The type you see used in airlines. The sachet contains some air. When you put the sachet in a plastic bottle full of water and squeeze the bottle,  some the air is compressed, the volume of the sachet decreases and it sinks.
7. Use a tall wide mouth container or a plastic drink bottle. Wrap copper wire around the narrow part of the rubber bulb from a medicine dropper to make a rubber diver. Fill the container with water. Put enough water in the rubber diver so that it only just floats in the container of water. Most of the rubber will be under water. You can adjust how much the rubber diver floats by pinching the bulb to remove air. Cover the container with a sheet of rubber or plastic sheet and tie it to the sides of the container. If you press on the tight cover, the rubber diver will sink. If you stop pressing on the tight cover, the rubber diver will rise. Repeat the experiment using a very small glass tube of a medicine vial instead of the rubber bulb to make a glass diver. Add a small amount of ink to the water so you can see the level of water in the glass diver. Note that when the cover is pressed down, the pressure is transmitted through the water to decrease the volume of the air bubble in the glass diver so the water level rises in the tube. When the volume of the air bubble is too small to hold up the glass diver by displacement of water the glass diver will sink. When you stop pressing on the cover, the decrease in pressure is transmitted to the air bubble that expands so the water level in the glass dive decreases. The increased volume of the air bubble in the glass diver displaces enough water to provide an upward force by displacement of water to allow the glass diver to float again.
4.202 Density of irregular solid, overflow can
See diagram 4.202: Overflow can
Use an overflow can, a stone, and a catch bucket. Fill the overflow can with water to the level of the spout. Attach a string to the stone and weigh it with a spring balance. Weigh the catch bucket and put it underneath the spout of the overflow can so that it catches the water displaced when you put the stone in the water. Immerse the stone in the water and record its weight. It weighs less than in the air. Find the weight of the displaced water by subtracting the weight of the bucket from the weight of the bucket and water. The loss of weight of the object in water is equal to the weight of the water displaced by the object.
4.203 Weight of a floating body
Fill an overflow can with water and let it run out until the surface is level with the spout. Select a piece of wood that floats half or more submerged in the overflow can. Weigh the piece of wood with a spring balance. Weigh the catch bucket. Put the catch bucket under the spout. Put the wood block in the overflow can and note the balance reading. Find the weight of the displaced water by subtracting the weight of the catch bucket from the total weight of catch bucket and water. The weight of the water displaced is equal to the weight of the object.
4.204 Float a lighted candle
Push nails or pins in the lower end of a candle so that the candle floats vertically with its top a little above the surface of the water. Light the candle and watch it burn. The candle constantly loses mass as it burns. The candle continues to float if it displaces a mass of water greater than its own mass.
4.205 Float different kinds of wood
See diagram 4.205: Floating wood
1. Put pieces of wood and cork with the same dimensions in a pan of water and note how each piece of wood floats. Measure the ratios of lengths above and below water.
2. Place lengths of wood with equal dimensions in a graduated cylinder containing water. Insert a drawing pin (thumb tack) up into the bottom of the lengths of wood to make them float upright. Measure the ratios of whole length to length below water.
4.206 Float an egg in tap water and salt water
See diagram 4.206: Floating egg
If an egg floats in tap water it is a bad egg. It floats because hydrogen sulfide gas (rotten egg gas) has formed from the decomposition of proteins in the egg. So before using an egg for cooking, make sure that it sinks in water and does not float. Place an egg in a glass of tap water. Add salt to the water to make the egg float. Ships float higher in salt-water than in freshwater because salt-water is more dense than freshwater.
4.207 Float grapes at different levels in water
Be careful! Do NOT taste chemicals in the laboratory.
Prepare 4 beakers of water. Put a grape in beaker 1 then fill the beaker with tap water. Put a grape in beaker 2, add some tap water then add sugar until the grape floats on the surface of the water. Prepare beaker 3 in the same way as for beaker 2 then pour out half the water. Wait until the solution in beaker 3 is still, then very slowly add tap water until the beaker is full. The grape now floats between the lower sugar in water and the upper pure tap water. Carefully increase the concentration of sugar in the water, while stirring, until the grape floats at the same level as in beaker 3. To investigate why the grapes float at different levels, taste the water in each beaker by touching the surface. You can taste the difference between beaker 1 and beaker 2, but beaker 3 tastes the same as beaker 1, and beaker 2 tastes the same as beaker 4.
4.208 Drinking straw hydrometer
Seal one end of a drinking straw. Put some sand in it until it floats in water in a vertical position. Put a rubber band round the stem so that you can slide it up and down as a marker. Mark the drinking straw at water level. Measure the length from the bottom end of the drinking straw to the water level mark, X cm. Assume the relative density of water = 1, and assume that the drinking straw has a uniform cross-section area. Mark the drinking straw for different relative densities, e.g. from 0.6 to 1.2. Check the accuracy of your drinking straw hydrometers with a glass hydrometer.
4.209 Floating in different density liquids
See diagram 4.209: Floating in different density liquids
Use a measuring cylinder or tall glass container, water and kerosene. You will also need a piece of wood that at sinks in water paraffin wax or candle wax and a piece of cork. Pour water into the container then carefully pour the kerosene into the container on top of the water. Drop in the solid substances. The wood sinks in two liquids. The paraffin sinks in the kerosene but floats on the water. The cork floats on the kerosene.
4.210 Model diving bell
See diagram 4.210: Model diving bell
Use a small wide mouth bottle with a two-holes stopper. Put some stones or metal washers in the bottle so it floats in an upright position. Insert one arm of a U-tube through the stopper so that it extends to the bottom of the bottle. Insert a short length of glass tubing through the other hole and attach a long rubber tube. Put the bottle in water. Suck on the rubber tube. Water enters the bottle through the U-tube until the bottle sinks. You can make the bottle rise by blowing through the rubber tube. This model illustrates the principle of the tanks or pontoons used to lift sunken ships. Fasten a weight to the bottle, sink both in water and lift the weight by blowing air into the bottle.
4.211 Float a metal boat, Plimsoll line
See diagram 4.211: Plimsoll line
1. Shape a piece of aluminium foil into the form of a little boat. Float the boat on water. A floating boat displaces the volume of the boat under water. This volume is greater than the volume of the ball of metal foil. The weight of this volume of water displaced is equal to the weight of the boat, so the boat floats.
2. Squeeze the boat into a ball. Try to float the ball on water. The ball sinks. Buoyancy force = weight of water displaced. The ball of aluminium foil displaces its own volume of water. The ball is heavier than its own volume of water, so it sinks.
3. Plimsoll line, Plimsoll mark, load lines
Plimsoll lines (Samuel Plimsoll 1824-1898) are lines painted on both sides of a ship to show the minimum freeboard, load line,  allowed in different parts of the world and at different seasons to prevent dangerous overloading of the ship. If you live near a sea port, look for the Plimsoll lines on the sides of the big boats.
4.212 Soap and surface tension
Fill the large clean plate with cold water and let it stand for a time on the table until the water is still. Sprinkle some talcum powder lightly over the surface of the water. Wet a piece of soap in water and touch it to the water near the edge of the plate. The talcum powder will be drawn to the opposite side of the plate at once. The soap reduced the surface tension at one point. The increased surface tension on the other side contracts the surface and pulls the talcum with it. Try a similar experiment but substitute flowers of sulfur for the powder and synthetic liquid detergent instead of the soap. If you use a transparent dish, you can place it on an overhead projector and display the results on a screen.
4.213 Float a needle on water
See diagram 4.213: Float a needle
1. Use a steel needle and dry it thoroughly. Place it on the tines of a dinner fork and gently break the surface of some water in a dish with the fork. If you are careful, the needle will float as you take the fork away. Look at the water surface closely. See how the surface film seems to bend under the weight of the needle.
2. Float a paper clip on water. Hold a paper clip in a sling made of a paper towel. Dip the paper-clip on to the water surface. Take away the paper towel. See the water surface bending under the paper-clip.
3. Float needles, paper-clips, rings of wire, and a razor blade on water.
4. Float an aluminium sheet on the surface of deionized water and add weights until the metal sinks.
4.214 Float a razor blade
See diagram 4.214: Float a razor blade | See also 3.2: Mosquito life cycle (Primary) - See mosquito larva using surface tension of water
Use a razor blade of the double edge type. Try floating it on the surface of water. Again note the surface and see the surface film.
4.215 Lift the water surface
Bend the pointed end of a pin or use a piece of fine wire to make a hook. File the point of the hook until it is very sharp. Put your eye on a level with the surface of the water in a drinking glass. Put the hook under the surface of the water and gently raise the point to the surface. If you are careful, the point will not penetrate the surface film but will lift it slightly upwards.
4.216 Hold water in a sieve
Pour oil over the wire mesh of a kitchen sieve and shake out the excess so that the holes are open. Use a pitcher of water and carefully pour it into the sieve by letting it run down the side of the sieve. When the sieve is about half full, hold it over a sink or bucket and note the bottom. You will see water pushing through the openings but the surface tension keeps it from running through. Touch the bottom of the sieve with your finger and the water should run through.
4.217 Heap up water up in a glass
Place a drinking glass in a shallow pan or on a saucer. Rub the top edge of the glass with a dry cloth. Pour water into the glass until it is full to the brim. You will note that you can fill the glass several millimetres above the top. Now drop coins or thin metal washers into the water edgewise. By dropping these in see how far you can heap the water up before it runs over.
4.218 Pinch together water streams
See diagram 4.218: Pinch together water streams
Punch five holes in the side of a metal can, 5 mm apart. Fill the can with water. Note how the water leaves the metal can in five streams. Pinch the jets of water together with your thumb and forefinger to make one stream. Brush your hand across the holes in the can and the water again flows in five separate streams.
4.219 Drive a boat with surface tension
See diagram 4.219: Surface tension boat
Cut out the shape of a 4.5 cm boat from stiff paper. Cut a notch in the middle of the stern large enough to hold a small lump of gum camphor or naphthalene mothball in contact with the water without letting it fall out. Float the boat in a large round dish of water. Make other boats with the notch in the stern on the right or on the left of the middle. You can achieve the same effect with a drop of dishwashing detergent.
4.220 Blow soap bubbles
Make a soap bubble solution by putting three level tablespoonfuls of soap powder or soap flakes into four cups of hot water. Let the solution stand for three days before using. Try blowing bubbles with a bubble blower or a drinking straw by slitting the end of the drinking straw with a razor blade into four parts, extending about 1 cm from the end. Bend these pieces outwards.
4.221 Soap bubble support
See diagram 4.221: Soap bubble support
Make a soap bubble support with a wire loop about 10 cm in diameter. Dip the loop in soap solution. Blow a large soap bubble and put it in the loop. Now wet a drinking straw in the soap solution and put it through the large bubble. Blow a smaller bubble inside the large bubble.
4.222 Soap film and sliding wire
See diagram 4.222: Soap film and sliding wire
Twist wire to make a rectangle with one side missing. Twist each end of another piece of wire to make a slider. The slider forms the fourth side of the rectangle. Dip the wire rectangle in the soap solution. Pull the slider out slightly and watch the film stretch. Release the slider. The contraction of the film pulls back the slider. In sliding wire experiments, the soap film provides the force to pull a light wire on a U-shape frame. A sliding wire frame film with a spring on one end and a string pull on the other shows that tension does not increase with length.
4.223 Plastic syringes and air pressure
See diagram 4.223: Syringes and air pressure
[Some school systems do not allow the use of syringes in the classroom.]
With the tip sealed, use a syringe to compress air or to produce a partial vacuum. Attach a small piece of plastic tubing to let you seal the tip with a pinch clamp or seal the syringe by pushing the tip into a wooden block drilled to the appropriate size. With this base as a platform, use the syringe in a vertical position as a balance for measuring weight by air compression. You can quantify all the following experiments because syringes are already graduated.
1. Fill the syringe with a small amount of air and hang it inverted to serve as a "spring type" balance.
2. Compress moist air within a syringe to cause water condensation and make "artificial rain".
3. Attach a length piece of plastic tubing to make a simple syringe pump.
4. Put water in the tube to make an air thermometer or use 12 m of tubing to make a water barometer.
5. Couple two syringes with a piece of tubing to show pressure changes within closed systems.
4.224 Find air
1. Push a narrow neck bottle mouth down into a container of water. Slowly tip the mouth of the bottle towards the surface of the water. Note the rising air bubbles.
2. Place a lump of soil in a container of water and note air bubbles rising.
3. Put a house brick in a container of water and note bubbles of air rising.
4. Fill a glass with water and let it stand in a warm place for several hours. Note bubbles of air rising.
4.225 Air takes up space
See diagram 4.225.1: Funnel, drinking glass | See diagram 4.225.2: Fish tank
1. Place the funnel in the neck of the bottle. Seal the space between the funnel and the neck of the bottle with heavy grease or Plasticine (modelling clay). Pour water slowly into the funnel. The water stops running because the air takes up space. Repeat the experiment and pour in water until it comes nearly to the top of the funnel. Use a nail to punch a hole through the seal. All the water drops into the bottle. The water replaces the air that comes out through the punched hole.
2. Pour water into a large glass container until it is half full. Float a cork on the water and lower a drinking glass, mouth downward, over the cork. Repeat the experiment with a piece of paper wedged tightly into the bottom of the glass. The paper does not get wet.
3. Almost fill a fish tank with water. Lower a drinking glass, mouth downward, into the fish tank. With your other hand lower another glass into the fish tank. Let this second glass fill with water by tilting its mouth upwards. Now hold this glass above the first one mouth downwards. Carefully tilt the first glass to let the air escape slowly. Fill the second glass with air from the first glass to transfer the air under water. Air replaces some water in the second glass.
4.226 Air has mass
1. Inflate a balloon and put it on a sensitive balance. Record the weight. Deflate the balloon and weigh again. The weight is less than before because air exerts the weight force. The first weight is less than the true weight because a balloon has a large volume relative to its mass, so there has been a significant upward force because of buoyancy. You live in a "sea of air" but you can usually disregard the effect of buoyancy when you weigh things because the volume of what you are weighing is small in comparison to its mass.
2. Place a 1 m flat stick on a table so nearly half the length hangs over the edge of a table. Lay a sheet of newspaper over the end of the flat stick on the table and smooth it down. Give the other end of the flat stick a sharp blow. The flat stick breaks over the edge of the table. The stick breaks because the air pressure on the large sheet of paper exerts a force down on the paper.
4.227 Air pressure in all directions
See diagram 4.227.1: drinking straw | See diagram 4.227.2: Glass jar | See diagram 4.227.3: Jar at different angles | See diagram 4.227.4: Force cups
1. Finger on drinking straw
Hold a finger over the end of a piece of straight glass tube or drinking straw and lower the tube into a container of coloured water. Water does not replace the air in the tube. Remove the finger and water enters the tube. Replace the finger on the top of the tube and then lift the tube from the container. The water remains in the tube because the effect of the air pressure up the tube is greater than the weight of the water. Remove your finger and the water falls out of the tube.
2. Inverted glass
Fill a drinking glass with water. Cover the glass with a flat piece of glass or cardboard so that no air exists between the cover and the water. Turn the glass and cover upside down. The cover remains in place because the pressure of the air pushing up is greater than the pressure of the water pushing down.
3. Holes in a drink-can
Make a small hole in the top of a drink-can. It is very difficult to suck the drink through the hole or to pour the drink into a glass. Make a second hole in the drink-can. Now it is easy to suck the drink through the hole or to pour the drink into a glass. Sucking reduces pressure at one hole in the can so the air pressure acting through the second hole forces drink into your mouth or lets you pour the drink into a glass.
4. Control water from a hole in a drink-can
Make a hole with a nail near the bottom of an open metal can. Fill the can with water. Hold the palm of the hand tightly over the top and water stops running from the hole. Remove the hand and water runs from the hole.
5. Sucking water up
Fit a flask with a two-holes stopper with a straight and a bent piece of glass tubing fitted through the holes. Pour water into the flask and put the stopper in tightly. You can suck water up the straight tube. Close the end of the bent tube with your finger. You cannot suck up water through the straight tube.
6. Suction disc
A rubber suction disc stays on a smooth window because no air exists between the disc and the window. The pressure of the atmosphere on the rubber disc keeps it pressed to the window.
7. Plumber's force cups
7.1 Press a plumber's force cup against a flat surface, e.g. the top of a stool, and lift the stool. The force cup works better if the rubber is wet. The force cup works because almost no air remains between the object and the force cup. However, the air in the atmosphere is pressing down on the rubber with atmospheric pressure.
7.2 Wet the rims of two plumber's force cups. Press the rubber cups tightly together. Try to pull them apart. This experiment is similar to the historical demonstration of air pressure called the Magdeburg hemispheres.
8. Vacuum cleaner
A vacuum cleaner pumps some air away from over the dirty carpet creating a "partial vacuum". Air rushes in to replace the air pumped out and when it is pumped away it takes with it the dirt from the carpet. The more air that you remove the greater the force from the atmospheric pressure to replace it. In a laboratory, it is impossible to pump all the air out of an enclosed space to create a perfect vacuum. "Nature abhors a vacuum" (Spinosa, 1677, in Ethics).
4.228 Push a drinking straw through a potato
See diagram 4.228: Push a drinking straw through a potato
Put the index finger over one end of a drinking straw. Hold a potato in the other hand. Push the drinking straw quickly through the potato. The air in the drinking straw is trapped between the index finger and the potato when the end of the drinking straw strikes the potato. This compressed air gives the drinking straw enough strength to prevent its bending.
4.229 Mercury barometer, barometric pressure
See diagram 4.229: Mercury barometer
Do NOT construct a mercury barometer. However, if you have access to a mercury barometer you can note how it works. The barometer is manufactured by filling a strong glass tube sealed at one end with mercury, then inverting the open end of the tube in a reservoir of mercury. The mercury in the tube drops down to a steady level leaving above it a vacuum with some mercury vapour. The vertical distance between the level of mercury in the tube and the reservoir is the height of mercury with the same pressure as the atmosphere, the atmospheric pressure. The average atmospheric pressure is about 760 mm of mercury, mmHg (29.9 inches, 1013.2 millibars). The height of the mercury drops with increase in altitude, about 4.5 cm for every 270 m. To read the barometer, tap the side of the tube to prevent the mercury sticking to it, adjust the height of mercury with the zero adjustment knob, then read the height of the meniscus with the vernier. You can adjust the reading for temperature and latitude, g is least at the equator. Barometric pressure is the pressure of the atmosphere read from a barometer in millibars, mbar, or hectopascals, hPa. (1 mbar = 1 hPa) (1 pascal, Pa = 1 N / m2) You can construct a water barometer, but you will need a tube 10.3 m long.
In SI units, standard value for atmospheric pressure at sea level is 101 325 pascals, 101.325 kPa.
4.230 Aneroid barometer
See diagram 4.230: Aneroid barometer
Use a corrugated rubber tube from a motor car, or a bicycle handle grip. Compress the rubber tube then insert two corks at the ends so that the tube can function as a vacuum box. Make the tube airtight by sealing the corked closed ends with wax and by tying around the outside with wire. Attach a weight to hang from the lower cork to extend the tube. Attach a pointer to the weight so that it points to a scale. You can read any changes in atmospheric pressure from the scale. The aneroid barometer is not as accurate as the mercury barometer. An altimeter is an aneroid barometer used in aircraft. The pilot can adjust it before takeoff so that the zero on the altimeter scale corresponds to ground level at the aerodrome.
4.231 Measure atmospheric pressure with a bicycle pump
See diagram 4.231: Measure atmospheric pressure with a bicycle pump
Use a bicycle pump with the washer reversed to measure atmospheric pressure. Make the piston airtight by adding thick oil to the barrel of the pump and sealing the hole for the valve connection. Find the weight supported by atmospheric pressure by hanging various loads from a hook attached to the pump handle. Record the weight F. Take apart the pump to measure the inside diameter of the pump. Record the inside diameter, d. Calculate the cross-section area of the pump, Area = pi X (d/2)2. To calculate the pressure, Pressure = F/A where the force is equal to the weight hanging from the pump handle.
The average atmospheric pressure at sea level is 1.033 kilogram per square centimetre, 14.7 pounds per square inch.
4.232 Measure atmospheric pressure with a suction cup
See diagram 4.232: Measure atmospheric pressure with a suction cup
Measure the area of the suction cup by pressing it down onto a sheet of glass above a sheet of graph paper and drawing its outline. Measure the diameter to calculate the area. Record the area A. Attach the hook of the spring balance to the neck of the sucker. Use a spring balance to find the force required to pull the sucker away from a smooth surface, e.g. a sheet of glass. Record the force, F. Calculate the average atmospheric pressure. When there is no air inside the sucker, by fully pressing the sucker on the smooth surface the force shown on the spring balance equals the force because of atmospheric pressure acting on the sucker. Pressure = force / area. Repeat the experiment with different size suckers, then calculate the average atmospheric pressure. The average atmospheric pressure at sea level is 1.033 kilogram per square centimetre, 14.7 pounds per square inch.
4.233 Syringe lift pump
[Some school systems do not allow the use of syringes in the classroom.]
See diagram 4.233: Syringe lift pump
Drill a hole through the centre of a cork, B, that makes a tight fit inside the glass tube of the syringe body, A. Use a piece of hot wire to burn two small holes through the cork, C, on either side of the centre hole. Pass a metal rod through the centre hole in the cork then expand the end after it has passed through the cork. Cut a circular piece of flexible plastic, D, to the exact size of the cross-section area of the glass tube of the syringe body. Cut a hole in the centre of the flexible plastic to allow the metal rod to pass through it. Attach the inner edge of the plastic to the cork with glue. The piston consists of the cork and metal rod. The inlet valve is the piece of plastic. The inlet is the nozzle of the syringe. Push the piston down, then place the nozzle of the syringe under water. Raise the piston. During the upstroke the inlet valve should remain closed and water is drawn into the lower body of the syringe by reduced atmospheric pressure. Lower the piston. During the downstroke water moves up through the side holes while the inlet valve remains open. Raise the piston. During the upstroke the inlet valve should remain closed, the water above the piston is raised and water is drawn into the lower body of the syringe by reduced atmospheric pressure.
4.234 Test-tube force pump
See diagram 4.234: Test-tube force pump
Use a thinner test-tube and a thicker test-tube. Heat the bottom of each of test-tube with a burner then punch a hole with a nail when the glass in the bottom is soft but not melting. When the test-tubes are cool, fit a ball bearing or marble to sit in the holes in the bottom to act as valves. Fit a cork with a bent glass tube passing through it into the end of the thinner test-tube. Wrap string around the thinner test-tube so that it fits tightly in the thicker test-tube but can still slide up and down. Put the thinner test-tube into the thicker test-tube. The thinner test-tube can act as a piston of a force pump. Put the bottom of the thicker test-tube into water. Raise the piston. During the upstroke water moves up through the ball valve of the thicker test-tube because of reduced atmospheric pressure. Lower the piston. During the downstroke water moves up through the ball valve in the thinner piston while the ball valve in the thicker piston remains closed. Continue to raise and lower the piston until water streams out of the bent tube.
4.235 Siphon
See diagram 4.235: Siphon
Use two tall glass bottles and fill each about half full of water. Connect two 30 cm lengths of glass tube with a 30 cm length of rubber or plastic tubing. Fill the tube with coloured water and pinch it. Put a glass tube in each bottle of water. Siphon the water back and forth by varying the height of the bottles.
4.236 Siphon fountain
See diagram 4.236: Siphon fountain
Fit a glass container, or a flask made from a used electric bulb with a two-holes rubber stopper. Through one hole place a jet tube which will extend to about half way to the top of the flask and about 2 cm outside the stopper. Through the other hole push a short length of glass tube so that it is just flush with the bottom of the stopper. Connect a 20 cm length of rubber tube to the jet tube. Connect a 1 m length to the other glass tube. Place some water in the flask and insert the stopper. Put the short rubber tube in a container of water on a table, let the longer rubber tube go into a bucket on the floor and then invert the siphon. Add ink to the water to see the fountain better. Make a double siphon fountain by making another flask unit similar to the first one and connect them.
4.237 Lift water with air pressure
See diagram 4.237: Lift water with air pressure
Fit a test-tube with a one-hole cork and straight glass tube. Drive the air out of the test-tube by boiling a water in it. Invert the test-tube with the open end under the surface of water. Atmospheric pressure will force water up into the test-tube until it almost completely full.
4.238 Volume and pressure of air
See diagram 4.238: Volume and pressure of air
Use a rubber stopper which just fits inside a measuring cylinder or large syringe. Attach it to the lower end of a wooden rod. Fit a lid to the upper end of the rod to act as a scale pan. Lubricate the piston so formed with some petroleum jelly or heavy engine oil. Use the piston to trap air in the container, put different weights on the pan and measure the volume of air inside the glass cylinder for each weight. Note that the volume is in inverse proportion to the pressure. At constant temperature as the volume, V, decreases the pressure, P, increases, so P X V = a constant. This is called Boyle's Law.
4.239 Pressure affects boiling point of water
See diagram 4.239: Pressure affects boiling point of water
A Steam, B Boiling tube, C Water
1. This method does not allow you to measure the pressure or the temperature inside the flask but it is the safest method. Boil water in half full round-bottom flask Stop heating and insert a tight one-hole stopper fitted with a thermometer. Support the inverted flask on a ring stand, thermometer pointing down. Pour cold water on the bottom of the flask. As the steam condenses, the pressure on the water lowers and the water boils again at a lower temperature. Pour more cold water on the bottom of the flask. The water boils again at a still lower temperature. The boiling point, b.p., of water at standard atmospheric pressure, 760 mmHg, 101,325 Pa, is 100oC. Boiling point changes about 1oC for each 28 mmHg change in pressure. At sea level water boils at 100oC.
Height above sea level and boiling point:  600 m 98oC, 1500 m 95oC, 2000 m 93oC, 3000 m 90oC.
2. Go to a high mountain and put a raw egg in an open cooking pot containing cold water. Heat the water in the pot. The water boils and evaporates at a low temperature so you cannot cook the egg. Put a potato in a pressure cooker containing cold water. Be sure that the valve on the lid is in the open position. Heat the water until it boils and steam comes through the valve. Close the valve. The potato cooks very quickly in the high temperature and pressure.
3. Half fill a round-bottom flask with water and insert a two-holes stopper fitted with a thermometer and a glass outlet tube. Support the flask in a ring stand. The bulb of the thermometer should be in the water. Do not heat the flask. Use rubber tubing to connect an exhaust pump to the glass outlet. Record the temperature. Start pumping air, and water vapour, out of the flask. Students can see the air bubbles rise first and can then watch the water boil at room temperature.
4. Fit a three-holes stopper with a thermometer, an open manometer, and an outlet tube for steam. Put plenty of grease around the stopper then insert it firmly but not tightly into a flask that is half full of water. Heat the flask slowly. Bubbles of air, and later larger bubbles of steam, form at the bottom of the flask then rise to the top. The steam condenses on leaving the outlet tube and striking the air. Note the temperature rise to 100oC and then remains steady. Carefully apply a screw clamp and partly close the outlet valve. Be careful! Do not close the flask completely or an explosion can result. With the outlet partly closed, the steam cannot escape as quickly so the manometer shows an increase of pressure inside the flask. The temperature of the water rises and the bubbling stops because the temperature that water boils at which water boils depends on the pressure. Remove the Bunsen burner so that the water begins to cool. Connect an aspirator to the outlet tube to reduce the pressure in the flask. With reduced pressure the water begins to boil again at a lower temperature. Motor car radiators have a pressure cap to keep water in a liquid state at temperatures over 100oC.
4.240 Make a model of the lungs
See diagram 9.240.2: Model of the lungs
Cut the bottom off a large plastic or glass bottle. Fit a cork to the neck with a Y-tube in it. On each of the lower limbs of the Y-tube tie a rubber balloon or some small bladder. Tie a sheet of brown paper or sheet rubber round the bottom of the container, with a piece of string knotted through a hole and sealed with wax. Pulling this string lowers the diaphragm and air enters the neck of the Y-tube causing the balloons to dilate. Pressing the diaphragm upwards has the opposite effect.
4.241 Oxidation and air pressure, steel wool over water
See diagram 4.241: Steel wool over water
1. Wash a small wad of steel wool in petrol to remove any grease. Squeeze it out and then fluff it. When it is dry, place the steel wool in a flask fitted with a one-hole stopper carrying a 40 cm length of glass tube. Stand the flask and tube in a container of water with the end of the tube under water. After a few hours, note that water is slowly drawn up into the glass tube. As the oxygen in the air reacts with the iron to form rust (iron oxides, Fe2O3) the air pressure decreases in the flask. Atmospheric pressure can push water up 1/5 the height of the glass tube.
2. Repeat the experiment with magnesium ribbon rubbed with sandpaper or white phosphorus.
4.242 Air streams, Bernoulli theorem
See diagram 4.242.1: Funnel, Spool | See diagram 4.242.2: Atomizer | See diagram 4.242.3: Bernoulli theorem
1. Put a ping-pong ball inside a funnel. Blow hard through the stem of the funnel to blow the ball out of the funnel. You cannot blow the ball out of the funnel. Air streams behave as fluids. According to the Bernoulli theorem, as the velocity of a fluid increases, its pressure decreases. At any point in a fluid-filled pipe, the kinetic energy and the potential energy of a mass of a flowing fluid is constant. The fast-moving air travelling through the neck of the funnel is at a lower pressure than the slow-moving air in the wide section of the funnel so the ball is pushed towards the neck of the funnel.
2. Invert the funnel and hold the ping-pong ball in the hand. Blow hard through the stem. Remove your hand from under the ping-pong ball. The ping-pong ball does not fall.
3. Put the ping-pong ball on a table. Cover it with the funnel. Blow through the stem and pick the ball up from the table. The pressure in the wide section of the funnel is greater than the pressure in the neck of the funnel so the ball is pushed up towards the neck.
4. Cut a 7 X 7 cm square of thin cardboard. Draw diagonals from each corner and put a pin through the card where the lines cross at the centre. Secure the head of the pin by covering it with adhesive tape. Put the pin in the hole of an empty thread spool and try to blow the card from the spool by blowing through the spool. Turn the spool and card upside down. Hold the card against the spool lightly with a finger. Blow firmly through the spool, then remove the finger. Air moving inside the spool is at a lower pressure than the air outside the spool. So, atmospheric pressure pushes the card against the end of the spool.
5. Repeat the experiment with your clenched fist. The ping-pong ball may rise and stay there. However, when doing the experiment with an empty fist, the flowing speed in not fast enough because of many cracks between your fingers so that not enough low pressure area forms and the ball does not rise. You may need to practice this experiment several times before demonstrating to the class. Some teachers cannot do it.
6. Attach a funnel to a source of compressed air such as a vacuum cleaner. Blow up a balloon and put a piece of copper wire around the neck for a weight. Turn on the compressed air and balance the balloon in the air stream. Try to balance a ping-pong ball between the balloon and the funnel.
7. Display a floating ball. Suspend a ball in an upward jet of air. Support a ping-pong ball on a vertical stream of water, air or steam. Suspend a Styrofoam ball in an air jet from a vacuum cleaner.
8. Make a Venturi tube. Use two glass tubes or two transparent drinking straws. Stand the first tube vertically in a half glass of coloured water. Hold the second tube at a right angle to the top end of the first tube so that the ends of the two tubes are close together. Blow through the second tube and observe the water level in the first tube. Moving air has less pressure than stationary air. Air is moving over the top of the vertical tube so the pressure in this region is less than atmospheric pressure and atmospheric pressure pushes water up the tube.
9. Lift water by blowing. Observe the action of an atomizer by blowing a jet of air across one end of a U-tube half full of water.
4.243 Cold air is heavier than warm air
Use two identical paper bags that are the same size. Inflate each bag by blowing into them as if they are balloons. Tie the openings closed tightly with string. Tie the end of the string into a loop and suspend the bags from the end of a balanced rod. Move the loops along the rod until the inflated bags exactly balance. Gently heat the air beneath one bag with a small candle. The bag containing the heated air moves up and the bag containing the cooler air moves down. Move the candle under the other bag to see the same result. The bags are sealed and so the mass of gas is unchanged when heating or cooling takes place. This experiment shows Archimedes' principle in action, not mass change.