School Science Lessons
UNPhysics2, Atmospheric pressure, buoyancy, force and motion, friction, liquid pressure, machines
Please send comments to: J.Elfick@uq.edu.au
2012-05-05c SPP
Table of contents
4.220 Atmospheric pressure
4.200 Buoyancy, flotation
4.160 Force and motion
4.180 Friction
4.190 Liquid pressure
4.170 Machines
4.220 Atmospheric pressure
4.242 Air streams, Bernoulli theorem
4.230 Aneroid barometer, barograph
4.243 Cold air is heavier than warm air, inverted paper bag balance
4.223 Plastic syringes and air pressure, Boyle's Law
4.229 Mercury barometer, barometric pressure, atmospheric pressure
4.229.1 Mountain sickness and hyperventilation
4.240 Model lungs
4.241 Oxidation and air pressure, steel wool over water
4.244 Scuba diving and Boyle's law
4.238 Volume and pressure of air, Boyle's Law
4.200 Buoyancy, flotation
4.200 Buoyancy of water
4.201 Cartesian diver
4.210 Diving bell
4.202 Density of irregular solid, overflow can
4.208 Drinking straw hydrometer
4.205 Float different kinds of wood
4.206 Float eggs in water
4.207 Float grapes in water
4.209 Float in different density liquids
4.204 Float lighted candles
4.211 Float metal boats, Plimsoll line
4.203 Weight of a floating body
4.160 Force and motion
4.168 Action and reaction, pulling forces
4.164 Action and reaction, pushing forces
4.165 Action and reaction, when stepping forward
4.166 Action and reaction, with balloons
4.169 Electric fan on a sailing boat
4.163 Equal forces from spring clothes pegs
4.162 Equal forces on light and heavy bodies
4.167 Thrust from a hose, rifle
4.180 Friction
4.183 Mount a box on wheels
4.185 Reduce friction with ball bearings
4.186 Reduce friction with air stream
4.190 Liquid pressure
4.192 Water pressure does not depend on the size of the container
4.199 Water wheel
4.170 Machines
4.179 Belt drives
4.178 Different inclined planes
4.180 Gear wheel
4.183 Mount a box on wheels
4.187 Propeller
4.186 Reduce friction with air stream
4.185 Reduce friction with ball-bearings
4.175 Simple pulley
4.176 Single fixed pulley
4.177 Single movable pulley
4.170 Three orders of levers, machines, mechanical advantage, velocity ratio
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
4.155 Inertia of a stone
See diagram 16.240: Inertia of 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 of two drink-can pendulums
See diagram 16.241: Two bucket 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.
6. Place a light beer mat ("coaster") over an empty glass. Place a match stick, with head cut off, over the
beer mat. Place a 10 cents coin over the match stick. Using your first finger and thumb, flick the beer mat
forward. The coin drops into the glass with a tinkling sound. Some people can repeat this experiment with
an egg balanced on a matchbox.
4.162 Equal forces on light and heavy bodies
See diagram 16.247: 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 16.248: 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 bow and arrow, catapult, hose, rifle
Order online: Da Vinci Catapult
See 16.6.3.2 Catapult a ball from cart to cart
See 16.6.3.6 Vertical catapult from a moving cart
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. Hold a hose in your hand then turn of the tap. Feel the backwards force on your hand. Drop the hose
on the ground and see it move backwards in a snake-like motion.
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, mechanical advantage, velocity ratio
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 output force (load) to the
input force (effort), load / effort, L / E, output force / input force.
The velocity ratio (distance ratio, gear ratio) VR, of a machine is the ratio of the distance travelled by the
point of application effort to the distance travelled by the point of application load, distance effort applied
 / distance load moved.
The efficiency of a machine is the ratio of the work done by the machine to the work supplied to the
machine. Work = force × distance. For a machine with efficiency = 1, i.e. 800px, MA = VR. However,
the efficiency of a machine is less than 800px 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 × length of load arm =
effort × length of effort arm. Each side of this equation is a moment, i.e. force × 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 21.252.1: 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 21.252.2: 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 21.252.3: 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 21.253: 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 21.254: 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 21.255: 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 21.256: Single movable pulley
See diagram 21.256.1: Wire coat hanger 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) MA = 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,
VR = 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 Different inclined planes
See diagram 21.257.1: Inclined planes
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 drives
See diagram 21.258: Simple 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 21.260: Gear wheels
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.
4.183 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.
4.185 Reduce friction with ball bearings
See diagram 17.264: 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 air stream
See diagram 17.265: Reduce friction with 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 21.266: Propeller
The two parallel propellers of a ship turn outward when the ship moves "ahead'". The propellers in the fore
and aft side of a modern passenger ship are called thrusters. They can be used to turn a ship in a circle,
but are used to speed up docking and undocking without the need for assistance from  tug boats. However,
the authorities in some ports require tug assistance to be available for all ships entering or leaving the
harbour.
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 × 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.
4.192 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.
4.199 Water wheel
See diagram 12.278: Water wheel
See 5.8: Water wheel (Primary)
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 11.200.1: 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 × d
V × d = v × 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 increase with depth.
See diagram 11.279: 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 11.280: Cartesian diver
Order online: Cartesian Diver, density, buoyancy, encapsulated air bubble
See pdf file: Sushi Soy Sauce Diver
Cartesian diver was invented by René Descates, (1596 - 1650), France.
1. A simple Cartesian diver consists of a test-tube containing a bubble of air and which floats mouth
downwards in a partially-filled cylinder of water closed by an elastic cover, e.g. a rubber sheet. The diver
floats in the water because the total mass of the glass of the test tube and the enclosed air is equal to the
weight of the water displaced by the glass and air, (Archimedes' principle). When pressure is applied to
the cover, it is transmitted undiminished through the air to the water surface and through the water to the
air bubble, (Pascal's principle). When the pressure of the air is increased, the volume is decreased, (Boyle's
law). So the volume of air in the test tube is diminished and so is the volume of water displaced with the
result that the weight of water displaced. by the diver is diminished. The downward force equal to the
weight of the diver is greater than the upward force equal to the weight of water displaced. So the diver
sinks.
2. 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 or rubber from a
rubber balloon 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.
3. Repeat the experiment using a very small glass tube or small medicine bottle 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. Use a big bottle of water and an inverted open vial or small test-tube as a diver. Slightly inflate a rubber
balloon by lowing in it and attach it to the mouth of the bottle. Squeeze the balloon and diver sinks.
10. Use a big plastic drink bottle as a submarine. Pierce a hole in the cap and in the bottom of the drink
bottle. Push a plastic tube through the hole in the lid. Fill the drink bottle with water and let it sink to the
bottom of a big tub of water. Blow into the plastic tube and the submarine rises to the surface.
4.202 Density of irregular solid, overflow can
See diagram 11.281: 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 lighted candles
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 11.284: 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 eggs in water
See diagram 11.285: Floating egg
After the egg is laid and it starts to cool, the air cell forms. In a fresh egg the air cell is quite small and the
egg sinks to the bottom of a container of clean water. A fresh egg has a thick white that does nor spread
out far in the pan and the yolk stands up. As the egg gets older it loses water by evaporation. The water is
replaced by air so the egg decreases in weight and starts to stand up, smaller end down. Later the egg
starts to rise to the surface of the water. A floating egg may not be bad but it should be opened in a
separate container and discarded if any bad smell can be detected. The bad smell comes from hydrogen
sulfide (rotten egg gas) produced by the decomposition of proteins in a rotten egg. Such an egg may
contain the dangerous Salmonella bacteria that can cause sickness and death. A fresh egg can be made to
float if you add cooking salt to the water to increase the relative density of the water. Ships float higher in
salt water than in fresh water because salt water is more dense than fresh water.
4.207 Float grapes  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 Float in different density liquids
See diagram 11.287: Floating in different density liquids
A measuring cylinder contains 4 liquids of different densities. Density of liquid D > liquid C > liquid B >
liquid A. Solids A, B, C < have different densities and float at different levels in the measuring cylinder.
For example where kerosene floats over water, a piece of heavy wood may float in the water but below
the kerosene but a cork may float on the kerosene.
4.210 Diving bell
See diagram 11.288: Model diving bell
1. 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.
2, Crumple newspaper and push it  into the bottom of a drinking glass. Invert the glass and check that the
newspaper will not fall out of the glass. Push the inverted glass down into a container of water. Water rises
slightly in the glass but does not wet the newspaper fixed in the bottom of the glass. A diver can swim into
a diving bell and breath in some of the compressed air stored in it.
4.211 Float metal boats, Plimsoll line
See diagram 11.211: Plimsoll line
See 3.5.4.1: Draft (draught) of a ship
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), introduced under the Merchant Shipping Act of 1876, 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.  The water line is the line formed by the
surface of the water against the hull of a ship. The 6 load lines on the Plimsoll mark show the depth to
which the ship can be loaded, i.e. the water-line should not be above this line in different conditions, e.g.
summer or winter, (freshwater or salt water), to allow sufficient reserve buoyancy. In all countries the load
lines show the legal limit of submersion of a ship as administered by various government recognized
authorities, e.g. LR, Lloyd's Register of Shipping. The freeboard is the height of a ship's side between the
water line and lowest part of the deck, the line of the weather deck. So a ship fully loaded in the salty
ocean could become dangerously low in the water when it travels up a freshwater river. Shoes with a
canvass upper and rubber sole are called "plimsolls" because the line where the rubber and canvas meet
reminds people of the Plimsoll line.
4.223 Plastic syringes and air pressure, Boyle's Law
See diagram 12.301: Syringes and air pressure
Order online: Vacuum Container & Pump
Order online: Vacuum Stoppers, creates near vacuum in plastic syringe
[Some school systems do not allow the use of syringes in the classroom.]
1. 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.
2. Fill the syringe with a small amount of air and hang it inverted to serve as a "spring type" balance.
3. Compress moist air within a syringe to cause water condensation and make "artificial rain".
4. Attach a length piece of plastic tubing to make a simple syringe pump.
5. Put water in the tube to make an air thermometer or use 12 m of tubing to make a water barometer.
6. Couple two syringes with a piece of tubing to show pressure changes within closed systems.
4.229 Mercury barometer, barometric pressure, atmospheric pressure
See diagram 12.307: Mercury barometer | See 12.1.01: Pressure definitions
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). One atmosphere = approximately 100
kilopascals (100 kPa).
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.
Correction of barometer readings to 0oC temperature for a mercury barometer with a brass scale
The value of dh should be subtracted from the observed height of the mercury column to give the true
pressure in mm Hg (1 mm Hg = 133.322 Pa).
dh = -0.0001634 ht / (1+0.0001818 t), where h = observed column height in mm and t = the
temperature in degrees Celsius
Thermal expansion coefficient for mercury (volume) = 181 × 10-8K-1.
Thermal expansion coefficient for brass (linear) = 20.3 × 10-8K-1.
4.229.1 Mountain sickness and hyperventilation
Persons climbing above 2500 m may experience headaches, nausea and rapid breathing caused by
hyperventilation to compensate for the low concentration of oxygen above that height. The condition can
be treated by breathing pure oxygen, rest and return to lower altitudes. After about 7 days the symptoms
may disappear.
4.230 Aneroid barometer, barograph
See diagram 12.308: Aneroid barometer
A barograph keeps a continuous record of pressure with a pen attached to an aneroid barometer
recording on paper on a rotating drum.
1. 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.
2. Cut the neck off a balloon then stretch the rubber over the mouth of a large wide mouth jar to form an
air-tight seal. Tie a string tightly around the mouth of the container to keep the rubber in place. Make a
pointer by attaching one end of a light stick or straw to the centre of the rubber with adhesive tape. The
other end of the stick or straw can point to a scale to show changes in atmospheric pressure. The pointer
moves up or down as atmospheric pressure changes.
3. Blow and suck on a chamber containing an aneroid barometer. Put an aneroid barometer in a sealable
chamber with a tap and evacuate the jar with an electric pump.
4.238 Volume and pressure of air, Boyle's Law
See diagram 20.238: Volume and pressure of air
1. 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 × V = a constant. This is called Boyle's Law.
4.240 Model lungs
See diagram 9.242: Model of the lungs
1. 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 12.318: 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 12.319: Funnel, Spool | See diagram 13.242.2: Atomizer
See diagram 13.242.3: 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 × 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. Float a pea and pin in the air. Soak a dried pea in water until it is just soft enough to pass a pin through
the centre of it. Cut a 5 cm length of a drinking straw. Lie on your back and blow gently through the piece
of drinking straw held vertically from your lips. Stop blowing and place the pea on the end of the drinking
straw with the pin vertical so that one end of the pin is pointing down inside the drinking straw. Gently blow
through the drinking straw to lift the pea and later maintain a suspended constant position. The pin will also
revolve when the blown air hits the ends of the pin. Be careful! Do not open your mouth and swallow the
pea and pin!
8. 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.
9. 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.
10. Lift an egg by blowing. Put a boiled egg in a small cup. Blow strongly into the cup to make the boiled
egg jump out. Some people with strong lungs can blow the boiled egg from one cup into another cup.
11. 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, inverted paper bag balance
See diagram 37.118: Balanced flasks
1. 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.
2. Open two same size paper bags. Attach identical pieces of string to the bottom of each bag with an
identical pieces of adhesive tape. Make a loop in the other end of each piece of string. Put the loops over
each end of a balanced rod. Adjust the positions of the loops until the rod is horizontal. Heat the air below
one paper bag. The end of the rod supporting that paper bag rises. Leave the balance to stand without
heating a bag. The rod becomes horizontal again. Heat the air below the other bag. The other end of the
rod rises. This experiment shows that a volume of warm air weighs slightly less than a volume of cool air.
However, the experiment does not give any information about the weight of a volume of air. The flame
under the paper bag heats the air in it and it expands, following Charles' law. Some heated air spills out of
the paper bag leaving less air and less dense air in the paper bag. The air in the heated paper bag weighs
less than the air it displaces so by Archimedes' principle there is an upthrust greater than its weight that
causes the paper bag to rise. When you remove the flame, the warm air in the paper bag cools and
contracts drawing in air at atmospheric pressure. The weight of a paper bag full of air and the bag crunched
together, with all the air squeezed out, is the same. Air in a hot air balloon is heated, it expands and
becomes lighter and the balloon is pushed up because the air left in the balloon is less dense than the
surrounding atmospheric air.
4.244 Scuba diving and Boyle's law
Swimmers descending more than one metre experience increase in pressure caused by the weight of the
overlying water. The resulting decrease in volume of air spaces causes mild to acute pain in the middle ear,
collapse of the auditory tubes and the forcing inwards of the tympanic membranes. The swimmer can
overcome this discomfort by closing the mouth, pinching the nose and exhaling. This action increases the
pressure in the nasopharynx, forces air through the auditory tubes to the middle ear to return the tympanic
membrane to its normal position. When the swimmer returns to the surface, the pressure drops and the air
in the middle ear expands, forcing its way out through the auditory tube and into the nasopharynx. So the
swimmer back on the surface normally feels no discomfort unless a blockage occurs in these passages.
Scuba divers breathe air under the same pressure as the level in the water. Descending more than 10
metres doubles the pressure on a diver. If the diver takes a full breath of air at this depth and quickly
returns to the surface, the volume of air in the lungs will double. This expansion may cause a tear in the lung
wall and even air bubbles in the blood vessels leading to heart attack or stroke. So scuba divers must
avoid holding their breath and must exhale when returning to the surface to avoid this over expansion
syndrome.