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.