School Science Lessons
UNPhysics2, Atmospheric pressure, buoyancy, force and motion, friction, liquid
pressure, machines
Please send comments to: J.Elfick@uq.edu.au
2012-01-28 SP
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
4.180 Gear wheel
4.178 Inclined planes
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
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 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, 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 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 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 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
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.
4.192 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 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 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. The 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 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 12.1.0: Pressure conversion, Statics of
fluids, static pressure, the pascal (Pa)
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 X 10-8K-1.
Thermal expansion coefficient for brass (linear) = 20.3 X 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 X 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 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. 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.