School Science Lessons
(UNPh16)
2024-09-17a
Force
Contents
16.1.0 Accelerometers
16.2.0 Force, mass, acceleration
16.3.0 Inertia
16.5.0 Newton's laws of motion
16.1.0 Accelerometers
Experiments
16.1.1 Balloon accelerometer
16.1.2 Float accelerometer
16.1.3 Glycerine accelerometer
16.1.4 Iron ball and cork accelerometer
16.1.5 Spirit level accelerometer
16.2.0 Force, mass and acceleration
Experiments
16.2.1 Force, fundamental forces
10.0 Force, (Primary)
16.2.2 Air track cart
16.2.3 Atwood's machine
16.2.4 Ball in a thrown tube
16.2.6 Drop different objects
16.2.7 Elevators
16.2.8 Equal forces from spring clothes pegs
16.2.9 Equal forces on light and heavy bodies
16.2.10 Hold a deflating big balloon
16.2.11 Newton's sailboat, fan on a sailing boat, fan cart
16.2.12 Press fingers together
16.2.13 Push a wheelbarrow
16.2.14 Thrust and recoil
16.4.0 Momentum, seat belts
16.2.15 Vanishing weight
16.2.16 Weight and pressure
16.2.17 Ladder against a wall
16.3.0 Inertia
16.5.1 Newton's first law of motion, inertia
Experiments
Inertia of a solid
16.3.2 Coin keeps moving
16.3.3 Coin snatching
16.3.4 Coin tricks
16.3.5 Inertia in daily life
16.3.6 Inertia of a stone
16.3.7 Inertia tricks, tablecloth pull
16.3.8 Moment of inertia, inertia and mass
Inertia of a fluid
16.3.10 Inertia of a drop of liquid
16.3.11 Inertia of a liquid in an alcohol thermometer
16.3.12 Inertia of a fluid, two bucket pendulums
16.3.13 Inertia of two beverage can pendulums
16.3.14 Cooled hand
16.3.15 Exhaust fan flag
16.3.16 Helium balloon in a motor vehicle
Inertia of motion
16.3.17 Cart on a cart
16.3.18 Persistence of motion
16.3.19 Water hammer
Inertia of rotation
16.1.6 Rotational inertia
16.2.20 Inertia balance to measure inertia
16.3.21 Inertia of rotational solid
16.3.22 Spin dryer for clothes
16.3.23 Spinning ice skater
16.3.24 Turning water can, aeolipile of Hero
16.3.25 Spinning eggs
16.5.0 Newton's laws of motion
16.5.1 Newton's first law of motion, inertia
16.5.2 Newton's second law of motion, force
16.5.3 Newton's third law, action and reaction, normal reaction
16.5.4 Action and reaction, pulling forces
16.1.1 Balloon accelerometer
Suspend a balloon filled with air from the top of a clear box mounted on wheels and suspend a helium balloon from the bottom of a clear box mounted on wheels.
16.1.2 Float accelerometer
Observe a float in a glass of water on an accelerating cart.
Cork on a string in a clear water-filled box.
16.1.3 Glycerine accelerometer
Mount a clear plastic box containing coloured glycerine on a cart and roll it down an incline or give it a push up an incline.
16.1.4 Iron ball and cork accelerometer
Suspend an iron ball from the top and a cork ball from the bottom of a clear box filled with water mounted on wheels.
16.1.5 Spirit level accelerometer
A glass tube with a special shape is nearly filled with methylated spirit, or similar liquid, to leave a visible bubble of air and spirit vapour.
The bubble always rises to the highest level in the glass tube.
The spirit level is used to test whether the surface to which it is applied is horizontal.
It is found in many kinds of surveying instruments, e.g. theodolite and dumpy level, and instruments to assist carpenters, brick layers and builders.
Alcohols are used, because of their low viscosity and surface tension, so the bubble can move without being retarded much by the glass surface.
At low temperatures, water in a spirit level could break the glass by ice expansion.
The spirit usually contains a colorant to increase visibility.
The bubble of a spirit level, moves in the direction of acceleration, so you can use it as an accelerometer.
16.1.6 Rotational inertia
Projectiles, bullets, rockets and footballs, (Rugby football or American football) are "spin stabilized".
They make them to spin about the axis of their direction of motion then do not tumble end over end and be retarded by extra air resistance.
A children's top falls over when place on its tip, but a rotating top remains upright until it loses all its angular momentum to friction between the tip and the ground and some air resistance.
16.2.1 Force, fundamental forces
The four fundamental forces are gravity, electromagnetism, strong and weak nuclear forces.
However, most interactions and phenomena can be explained by considering gravity and electromagnetism.
The forces that act on objects influence their motion, shape, internal energy and state of equilibrium.
When forces act they may be balanced or unbalanced.
Unbalanced forces change the motion of objects.
Gravity is an attractive force that reaches further than the other forces to keep planets in orbit, but it is the weakest in magnitude.
In the general theory of relativity gravity is defined as the curvature of space time around an object with mass.
Electromagnetism is the force interaction of particles with an electrical charge.
Charged particles at rest interact by electrostatic forces, but in motion interact by electrical and magnetic forces.
The weak nuclear force acts on the atomic nucleus t> control the radioactive decay of atomic nuclei.
The strong nuclear force keeps protons and neutrons together, even binding two positively charged protons in the helium nucleus.
This strongest force allows gluons to bind quarks together to form the nucleons.
16.2.2 Air track cart
An air track cart is timed while pulled by a mass on a string over a pulley.
Accelerate an air track cart up an inclined track by the string pulley and mass system.
Fix a scale on the cart to measure the tension in the string directly.
16.2.3 Atwood's machine
(George Atwood, 1784, England)
See diagram 16.4.14: Atwood's machine 1.
See diagram 16.4.12: Atwood's machine 2.
Used to verify laws of motion if constant acceleration.
Two objects mass m1 and m2 connected by an inextensible string over an ideal pulley.
If m1 = m2, the system is in neutral equilibrium regardless of the position of m1 and m2.
If m1 is not equal to m2, both masses have uniform acceleration
weight = m1g or m2g.
If m1 > m2, m2 has two forces acting it, an upward force T exerted by the string and down force m2g, its weight.
If T > m2g, upward acceleration a occurs
(i) T - m2g = m2a
m1 has an upward pull T on it and downward force m1g on it
(ii) m1g -T = m1a
so by combining (i) with (ii), m1g - m2g = m1a + m2a
a = g (m1 -m2) / (m1 + m2)
d = gt2, so find the time taken by m1 or m2 to travel a distance to calculate a.
T = 2gm1m2 / (m1 + m2)
Experiment
1. Hang two equal masses from a light pulley and move one mass to the other side.
Place 1 kg on each side of a light pulley on good bearings then add 2 g to one side.
Measure the distance the mass falls and the time taken to fall through this distance.
2. Place 1 kg on each side.
Add a 2 g mass to the high side.
Measure the distance the mass falls and the time it takes to fall this distance
16.2.4 Ball in a thrown tube
Invert and throw a Plexiglas tube full of water that contains a cork.
The rising cork will remain stationary during the throw.
Throw or drop a long water-filled tube containing a cork or rubber stopper or air bubble.
A rising bubble in a jar remains stationary while you throw the jar.
Join a lead weight and cork with a spring, then put the assembly in a tube of water so the cork just floats and when you drop the tube, the cork sinks.
Drop a ball in a tube from the ceiling so that the ball strikes the bottom of the tube after the tube hits the floor.
16.2.6 Drop different objects
1. Drop a leaking bucket
Punch a hole in the bottom of a bucket and fill it with water so that when you drop the bucket no water will run out.
Drop a can with several vertical holes to show no flow in free fall.
Use a pulley system to accelerate the bucket greater than g then the top hole will issue the longest stream of water.
2. Drop a mass on a spring.
Drop a frame with an oscillating mass on a spring and the mass will be pulled up, but stop oscillating.
3. Drop a slinky spring.
Hold a slinky spring so some of it extends downward then drop it to show the contraction.
4. Drop a pendulum.
Suspend a pendulum from a stick.
Drop the stick when the pendulum is at an extreme and the stick and pendulum will maintain the same relative position.
16.2.7 Elevators
* Quickly raise and lower a spring balance and hanging mass.
* Construct in an elevator, a rope over a ceiling-mounted pulley with a weight on one side and a spring scale and lighter weight on the other side.
* Observe a passenger standing on a spring scale in an elevator.
* A large hydrometer flask in a beaker of water remains at its equilibrium position as you move the beaker up and down.
16.2.8 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 identical marbles 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.
16.2.9 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.
16.2.10 Hold a deflating big balloon
Inflate a big balloon and fasten its mouth.
Lift it over your head with your hands.
Make sure the balloon's mouth upright against your head.
When you untie the string on the balloon's mouth, you may feel the air current spurting out of the balloon.
Besides your hands feel that the balloon tries to move upward.
Repeat the experiment, but hold the balloon horizontally and wear a pair of ice skates.
Observe that the direction of your body's movement is opposite to air current's spurting and the time when your body feels some force.
16.2.11 Newton's sailboat, fan on a sailing boat, fan cart
1. Show that force can be resolved into its individual components.
Newton's cart carries a small fan that can be rotated through 180o.
When the fan as at direction 0o, the fan causes the maximum acceleration and the cart moves straight ahead with high speed.
When the fan is at direction 90o, the fan causes the maximum acceleration and the cart does not move.
When the fan is at direction 45o, the fan causes some acceleration and the cart moves slowly.
If you attach a vertical barrier to the cart in front of the fan, the cart does not move, even when the fan is at direction 90o.
Place masses on the cart to vary the acceleration and demonstrate Newton's Second Law.
2. Fix a sail in front of a battery-powered fan on an air track cart or toy boat, or use a balloon to provide a wind source.
Put a battery-operated fan on a model sailing boat.
When it blows against the sail the boat does not move forward, because an equal and opposite force acts 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.
If the fan is placed on the boat facing the rear there is some forward force.
3. Fix a potable electric fan and six 1.5 v batteries connected in series on a roller skate or on a toy train carriage on rails.
Turn on the fan and the skate moves in the opposite direction.
Attach a strong cardboard sail at right angles to the axis of the skate and direction of the fan.
Turn on the fan and the skate or carriage does not move, because of the equal and opposite forces on it.
4. Cut a beverage can with a ring on the top into two half parts along the diameter.
Use one half as the hull.
Cut two 40 cm wide strips off the other half of the jar.
Use adhesive tape to connect them into a longer strip.
Fold the strip into an L shape and put it into the hull then put a wood block of 30 mm x 30 mm x 10 mm on the bottom of the strip.
This is the sail of the boat.
Place the boat on the water at a large basin.
Blow the sail and observe the direction of the boat moving.
Use a candle according to the height of the sail.
Light the candle then paste it with waxen oil on the block.
Remove the position of the block to adjust the balance of the boat.
Take care to keep a certain distance from candle flame to the top of the sail.
Observe the movement of the boat now.
16.2.12 Press fingers together
Let your middle fingers just touch.
When you press the left middle finger with the right, you may find not only the left middle finder reacted, but also the finger to its right reacts.
Observe the changes in shape of the two fingers and whether they are the same.
Repeat the experiment using your left middle finger to press your right middle finger.
Use your right hand to clap your left hand then observe whether simultaneously the two hands feel pain.
16.2.13 Push a wheelbarrow
See diagram 21.1.2b: Wheelbarrow.
Raise the handles of a wheelbarrow full of sand and push it towards a small hillock.
It may be difficult to push the wheelbarrow over the hillock unless you lower the handles until they are almost parallel to the ground.
By lowering the handles, you are applying a force in the direction of motion, i.e. tangential to the slope.
If you push the wheelbarrow with handles raised, a component of the force you apply is into the hillock, which also increases frictional loss, with only the component of force parallel to the slope
being
utilized to move the wheelbarrow over the hillock.
16.2.14 Thrust and recoil
Thrust and recoil, recoil from a bow and arrow, catapult, hose, rifle
Thrust is force applied on a surface in a direction perpendicular or normal to the surface is called thrust, a reaction force of Newton's laws of motion.
A thrust force tends to increase velocity or momentum.
A drag force tends to decrease velocity or momentum.
The propellor of a ship or plane and a jet or rocket engine exerts this propulsive force.
The recoil of a gun describes how it springs back with the force of discharge.
Some guns have a hydraulic cylinder that compresses and so absorbs the shock of recoil and decelerates the rearward thrust of the firearm.
Experiments
1. Use a bow and arrow or catapult.
The force exerted by one arm to pull back equals the force used to hold steady the bow or the fork of the catapult.
2. Hold a hose in your hand then turn of the tap.
Feel the backwards force on your hand.
Drop the hose on the ground and see it move backwards in a snake-like motion.
3. 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.
16.2.15 Vanishing weight
Pull a strip of paper from between two weights.
It will tear unless dropped.
Drop a mass on a spring scale.
Drop an object with a second object hanging by a rubber band.
Stretch a rubber band over the edge of a container and drop.
16.2.16 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 commercial 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.
16.2.17 Ladder against a wall
See diagram 16.166: Ladder against a wall.
A model ladder leans against a heavy wooden box and a mass is hung from a rung.
Move the mass higher and higher or adjust the angle smaller and smaller until the ladder slips.
Different materials can be used for the surfaces to show different static friction.
Set a model ladder against a box and move a weight up a rung at a time.
Forces on a ladder:
Mount a set of wheels at the top of a ladder and place some shoes at the bottom to decrease friction and climb the ladder until you fall down.
16.3.2 Coin keeps moving
Use a wood block with a slippery surface.
Put the wood block on a slippery tabletop.
Put a coin on the wood block.
Keeping your other hand at the front, hit the wood block with your hand so that it moves quickly in a straight path on the table.
When your hand stops the wood block suddenly, the coin continues to move horizontally.
If you place a heavy bag next to the back window of a motorcar and the car stops suddenly, the bag keeps going and hits you on the back of the head!
16.3.3 Coin snatching
1. Bend your arm back so that your forearm is horizontal and you hand is beside your ear.
Place a pile or coins or bottle tops on the edge of the elbow.
Bring the arm forward quickly and catch the coins in mid air.
This trick is used in a children's game to see who can catch the most bottle tops, because of inertia the coins remain in place in space until the force of gravity applies to them.
The distance travelled in a fall, S = gt2.
However, if the vertical component of the acceleration of the hand > g, then the hand can catch the coins.
2. The rules for coin snatching (World series)
* The coins are to be placed on the elbow.
* All coins must be caught in one snatch and the snatch is to be achieved on a downward beat of the same arm and the coins caught palm down.
If not all of the coins that are stacked are caught, the attempt is still valid, but only those that are caught would count.
In this instance, the documentation should mention both the number of coins stacked and the number successfully caught.
* Once they are ready on the elbow, the coins are not be touched by the hand that is not being used to catch them.
* No adhesive of any sort may be used.
* The coins to be used are 10 p coins in Great Britain, or in other countries coins weighting at least 10 g (0.35 oz), and having a diameter of at least 2.8 cm (1 1/2 in).
The latest world record is held by Dean Gould is 328 new 10 p UK coins.
16.3.4 Coin tricks
See diagram 16.4.8: Flick a card under a coin.
1. Use the fingers to flick a coin, e.g. an American one cent piece, at the coin at the bottom of the pile, e.g. American nickels.
The bottom coin leaves the pile.
The bottom coin of the pile must be thicker, or the same thickness, as the coin flicked towards it.
2. Strike the lowest coin sharply with a ruler to dislodge it without the pile falling over.
3. Strike the lowest coin with a tight string held in both hands and pulled quickly against the bottom coin or place loose string against the lowest coin and pull the string tight by pulling at both ends.
4. Stack 10 coins on a plate.
Lift the plate about 10 cm over the table.
Quickly lower the plate and pull it towards you.
The stack of coins falls onto the table with each coin keeping the position it had on the plate.
5. Use two coins with a big mass difference.
Support the playing card on two fingers of your left hand.
Shoot the card off quickly with the index of your right hand and let the coin fall on the fingers of your left hand.
Some people can balance the coin on the card on one fingertip, flick the card and let the coin remained balanced on the finger tip.
6. Put a stiff cardboard playing card on a beaker.
Put a coin on the card.
Flick the card quickly with your forefinger.
The card moves horizontally, but the coin drops vertically into the beaker.
7. Bend a strip of semi-rigid paper into an arc so that the ends are within the rim of the beaker.
Put a coin on the strip of paper.
Flick the strip of paper quickly with your forefinger so that it moves away.
The coin falls down into the beaker.
8. Cut a 1 cm wide hoop from a plastic drink bottle.
Stand it over the mouth of a wide mouth jar.
Put a coin on top of the hoop.
Flick the inside the hoop with your index finger.
The hoop moves away and the coin drops into the jar.
9. 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.
16.3.5 Inertia in daily life
1. An empty bottle on the floor of a bus or train will roll forwards or backwards as the bus or train slows or accelerates.
2. Forcibly shake the dust or water off clothing.
3. A worker digging a drain must stop the shovel suddenly in the air when he throws the shovel from the bottom of the drain to the ground.
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. When a bus or train stops suddenly the passengers may fall forward due to their inertia.
5. Do not leave objects on a shelf below the back window of a car.
If the car stops suddenly, the objects will keep moving due to inertia and perhaps hit the passengers.
6. When a car is towing a trailer or caravan, applying the brakes suddenly is very dangerous while turning.
The car slows or stops, but the trailer or caravan keeps moving by inertia in the direction when you applied the brakes.
The car and trailer will "jack-knife"!
16.3.6 Inertia of a stone
See diagram 16.240: Inertia of a stone.
See diagram 16.155: Inertia of a heavy ball.
1.0 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.
* 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.
* 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.
2.0 Suspend a stone with a piece of thin cotton thread just able to withstand the weight of the stone.
Tie another piece of the same thread around the middle of the stone and let the end of the thread hang down.
Suspend the stone from a firm support.
Quickly pull the lower thread hanging down.
The thread hanging down breaks, but the thread suspending the stone does not break.
The stone remains suspended.
The inertia of the stone slows the transfer of downward force to the upper suspending thread, so the lower thread breaks.
* Suspend the stone again.
Slowly pull the lower thread hanging down.
The thread suspending the stone breaks and the stone falls down.
You evenly distribute the downward force in the two threads.
So this force and the weight of the stone break the upper suspending thread.
16.3.7 Inertia tricks, tablecloth pull
See diagram 16.4.9: Tablecloth pull.
1. Lay a tablecloth on a table with a smooth surface, e.g. glass top table.
The tablecloth should itself be smooth and have no bumps or embroidery in it.
Place heavy objects on the tablecloth, e.g. a heavy plate.
Pull the tablecloth quickly towards you as fast as possible.
The heavy object remains on the bare table.
It does not move towards you, because of its inertia and insufficient lateral force is acting on it.
The heavy object is at a state of rest and will remain at rest unless a force acts on it.
The horizontal force on the dishes is due to kinetic friction between the dishes and the tablecloth as you pull the tablecloth horizontally.
If you pull the tablecloth very quickly, friction between the dishes and the table surface rapidly removes any horizontal velocity of the dishes.
This experiment shows Newton's first law and impulse momentum.
Both the force and the time during which it acts are small resulting in a small change in momentum of the dishes.
2. Drape a smooth cloth over the edge of a table.
Put a cup of water on the cloth.
Quickly pull the away cloth leaving the cup of water on the table.
(Sir Isaac Newton is supposed to have done this experiment to demonstrate inertia.)
3. Put dining table objects, e.g. plates, bottles, cups, bottle of water, on a silky smooth cloth.
The objects remain standing if the cloth is pulled out sharply from beneath, thus demonstrating the inertia of the objects involved.
Always place the smooth silky surface down facing the table.
Some physics lecturers ask students to repeat the demonstration after secretly placing a small ball of blue tack between the table and tablecloth.
The objects do not remain standing.
4. 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.
Snap a playing card out from under a tall object.
Snap a playing card from under a steel ball.
5. Place a bottle on a strip of paper.
Pull the paper quickly from under the bottle with no motion of the bottle.
Jerk a sheet of paper out from under a thin steel cylinder.
6. Cut a strip of paper the size of a ruler.
Place the strip at right angles half over the side of a table.
Put an object, e.g. a pencil, on the part of the strip over the table.
Hold the end of the strip out so that the part of the strip not over the table is almost horizontal.
Use your pointing finger of the other hand to hit the middle of the strip not over the table.
The pencil does not move and the strip falls down.
7. Stand an empty soft drink bottle, open end down, on one end of a bank note, e.g. a dollar bill.
Roll a pencil over the other end of the banknote.
The drink bottle has a high centre of gravity.
Jerk the pencil very quickly and the bottle remains in place.
Pull the pencil slowly and the bottle moves with it.
Pull the pencil more quickly and the bottle falls over.
8. 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.
9. 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.
10. Make a pile of coins or checkers.
Strike the lower most coin sharply with a ruler to dislodge it without the pile falling over.
Repeat the experiment by dislodging the lowermost coin with a tight string held in both hands and pulled quickly against the bottom coin.
11. 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.
12. 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.
13. Cut a round potato half way through with a knife.
Hold the knife horizontally with the potato stuck to it.
Hit the back of the knife with the back of another knife.
You cut the potato in halves, because it stayed at rest when you hit the knife.
Put a round potato on a cutting board and stab it down the middle with a sharp knife.
The potato stays in place with the end of the knife in it.
Hold the knife and potato with the knife handle down and hit the cutting board with the end of the knife handle.
The potato moves down the blade of the knife.
The potato stayed in motion when hitting the cutting board stopped the movement of the knife.
14. Make a pile of coins or checkers.
Strike the lower most coin sharply with a ruler to dislodge it without the pile falling over.
Repeat the experiment by dislodging the lowermost coin with a tight string held in both hands and pulled quickly against the bottom coin.
15. 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.
16. Build a tower using children's building blocks.
Hold the back of a ball point pen with a spring clip next to a middle block.
Discharge the spring clip.
The middle block flies out, but the tower does not fall over.
17. Place an egg on a matchbox case and put this on a bread board over a basin of water.
Pick up the breadboard then move it very quickly to the side.
The egg has great inertia so it falls into the water.
The matchbox case has little inertia so it moves to the side.
18. Suspend two heavy iron balls hung separately between lengths of string, pull slowly on one and jerk quickly on the other.
19. Attach a rope between a heavy iron ball and a hammer head, so that a fast swing of the hammer takes up the slack and breaks the rope without moving the ball.
20. Place a lead block or a brick on your hand and hit it with a hammer!
21. Hit nails into a 50 kg wood block placed on the hand.
22. Suspend two large cylinders, 3 kg wood, and 50 kg iron, then compare displacements when struck by a hammer.
16.3.8 Moment of inertia, inertia and mass
1. Put carbon copy paper on a slippery table top with the long edge of the paper parallel and to near the edge of the table.
Separately put three weights of 50 g and 100 g and 500 g on the carbon paper on a line parallel to the long edge of the paper.
Quickly pull the carbon paper off the table.
The weights fall on the table.
Pull the carbon paper with different speeds to find which weight is the easiest and most difficult to move along with the paper.
2. Repeat the experiments with three identical drink cups containing different amounts of water.
3. Some people can pull a table cloth off a table laid with glasses of water and never spill the water or break the glasses.
However, this trick needs a lot of practice.
16.3.9 Throwing a ball
Sit on a stand on a roller cart and throw a heavy medicine ball.
Throw a ball while sitting on a stool mounted on a conveyor.
16.3.10 Inertia of a drop of liquid
1. Use a glass tube with an open-mouthed end and inner diameter more than 10 cm.
Put it on a horizontal plane.
Put coloured water into the horizontal glass tube with a glass tube or a drinking straw with a small rubber ball.
After the water is at rest, forcibly hit the end of the glass tube with a small stick.
Observe the movement of the coloured water when the glass springs out in a straight path.
2. Cut off 1 / 4 of a side wall of a large plastic drink bottle.
Put a big drop of water on the upper edge of the inner wall.
The drop of water runs down to the lowest level then moves some distance up the other side.
The drop continues to move forwards and backwards with decreasing height up the side wall until it settles at the lowest level.
16.3.11 Inertia of a liquid in an alcohol thermometer
Use an alcohol thermometer of 100oC range.
Put it into the boiling water at a cup.
When the alcohol column increases fast to 50oC to 60oC, slowly get it out the water and quickly dry it with a paper towel.
You notice that the alcohol column keeps going up some distance, then stops, then falls back.
16.3.12 Inertia of a fluid, two bucket pendulums
See diagram 16.241: Inertia with two bucket pendulums.
1. To experience the relationship of the inertia of an object to its mass use two buckets and pieces of string.
Tie each bucket to the ceiling.
Fill one bucket full of sand, but let the other bucket remain empty.
Push the two buckets and compare which bucket is easier to push.
Try to stop the buckets moving and compare which bucket is more difficult to stop.
2. Use long strings to suspend from the ceiling two large identical buckets.
Fill one bucket with sand.
Use the hook of a spring balance to push each bucket.
Note what force is necessary to start the buckets moving.
Use your hand to stop the buckets when they are moving.
You can feel the difference in inertia of the two buckets.
16.3.13 Inertia of two beverage can pendulums
See diagram 16.241: Inertia with two bucket pendulums.
Use long strings to suspend from the ceiling two large identical beverage 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.
16.3.14 Cooled hand
Wet the back of your left hand.
Extent horizontally your arms and keep your left palm horizontal.
Move your right hand fast then stop moving it suddenly 10 cm from the back of the left hand.
The back of the left hand feels cool, because the air pushed by the right hand keeps moving after the right hand stops moving.
16.3.15 Exhaust fan flag
Cut out a piece of light and soft paper of 4 cm x 10 cm.
Paste the 4 cm edge of the paper to a stick.
Blow the paper to make it wave nearly horizontally.
The paper falls back when the air current stops.
Hold the stick with your hand and put the part pasted with the paper under an exhaust fan entry in a kitchen.
The paper waves when you turn on the exhaust fan, but does not stop waving at once when you turn the exhaust fan off.
16.3.16 Helium balloon in a motor vehicle
Experiments
1. Tie the end of the string of a helium balloon to the parking brake.
In a motor vehicle travelling at constant speed, the balloon maintains its position relative to the parking brake.
* If the motor vehicle accelerates forward, the balloon moves forward relative to the parking brake.
* If the motor vehicle reduces speed due to braking, the balloon moves backwards relative to the parking brake.
* If the motor vehicle turns a corner, the balloon moves inwards.
* If the motor vehicle turns to the right, the balloon moves to the left.
* If the motor vehicle turns to the left, the balloon moves to the right.
The contents of the motor vehicle, including the air and the driver, have inertia.
The helium balloon floats towards the air of lowest density.
When the motor vehicle accelerates, air tends to accumulate in the back and the drives feels a reverse force.
When the motor vehicle brakes, air tends to accumulate in the front and the driver feels a forward force.
In a high speed turn, the driver feels an outward force as the balloon moves inwards.
2. A science teacher purchased a remote-controlled toy helicopter small enough to fly under remote control in his car.
He reported that the toy helicopter moved in much the same way as the balloon in the experiment above, but other science teachers expressed doubts about the teacher's interpretation of the behaviour of the toy helicopter in a speeding car.
16.3.17 Cart on a cart
Put a smaller roller cart or skateboard on a larger cart so that when you stop the larger cart the smaller cart continues to move.
16.3.18 Persistence of motion
See diagram 16.4.16: Car on dynamics track, cart on air track.
Level a track, give a cart a push and watch it bounce end to end along the track.
An air track works better, but is noisier.
16.3.19 Water hammer
Evacuate a tube except for some water.
When you stop the tube suddenly, the water strikes the end of the tube with a click.
Check for water hammers when you turn off a tap at home.
When you shake a tube partially filled with water and evacuated, the water hits the bottom of the tube with enough force to make an audible sound.
16.3.20 Inertia balance to measure inertia
Load the cups on a torsion pendulum with various masses.
A horizontal leaf spring acts as an inertial balance if you place masses on a platform supported by horizontal leaf springs.
You can use an inertia balance to measure mass independent of the earth's gravitational force.
It has two platforms connected by two horizontal spring steel blades.
A cylinder can rest in the hole in one platform or be suspended by a hook.
Calibrate the apparatus by determining the vibration frequency for known platform loads using the platform with the hole.
Calculate the period in seconds for each load and plot period against it.
Find the mass of the unknown from the calibration curve, and compare the value with the weight using a balance.
16.3.21 Inertia of rotational solid
Observe the inertia of an object at the state of rotational motion.
Rotate a coin with your middle finger and thumb on a slippery tabletop.
When leave the fingers off the coin, it does not stop rotating at once.
It keeps moving for a long time then falls down.
Use a 25 cm piece of string.
Fasten one end of the string to a clip.
Hold the other end of the string to quickly rotate the clip differently at upright, horizontal and inclined planes.
When your hand holding the string stops suddenly, the string and clasp always keep rotating several circles before they stop.
16.3.22 Spin dryer for clothes
Half fill a spin dryer with wet clothing.
Turn on then turn off the spin dryer and count the number of rotations and record the time until it stops.
Observe the arrangement of the clothing in the stopped spin dryer.
Repeat the experiment with the spin dryer 3 / 4 full of wet clothing and make the same observations.
The more wet clothing in the spin dryer, the more rotations when you turn it off, because of the greater rotational inertia.
The clothing dried off is always distributed equably with more on the outside.
If the spin dryer contains only pieces of wet clothing, the spin dryer barrel does not rotate normally, because of unequal distribution of mass.
16.3.23 Spinning ice skater
1. Go to an ice stadium or watch on television the actions of an athlete rotating at high speed.
Observe the positions of the body, arms and legs of the athlete starting to rotate, rotating, stopping rotating, the changes in position, the relationship of the changes to the velocity and time of the rotation.
The positions of the body, arms and legs of athletes affect their rotational inertia through affecting the distribution of their mass.
An ice skater can start a spin on one toe with one leg extended and both arms extended (I is large and ω is small), but when the ice skater brings both legs and both arms together (now I is small and ω is large), the moment of inertia decreases and speed of spin increases, the skater spins much faster due to conservation of angular momentum.
2. Spinning ice skater, angular momentum
Angular momentum, L = I × ω.
When there is no net torque on an object its angular velocity and angular momentum are constant.
Conservation of angular momentum:
When the sum of torques acting on an object = 0, the angular momentum is constant.
The greater the value of angular momentum, the more torque is needed to change its direction so bullets, rockets and even footballs can be aimed more accurate if spin stabilized.
Similarly, a spinning top stays upright until friction causes it to lose angular momentum.
An ice skater can start a spin on one toe with one leg extended and both arms extended (I is large and ω is small), but when the ice skater brings both legs and both arms together (now I is small and ω is large) the skater spins much faster due to conservation of angular momentum.
Angular momentum of the earth = moment of inertia × angular velocity = (1 × 1038) × (7 × 10-5) = 7 × 1033 newton
metre (Nm).
16.3.24 Turning water can, aeolipile of Hero
Ae aeolipile of Hero, steam ball of Hero of Alexandria, Hero's steam turbine
See diagram: 16.4.5: Aeolipile.
Be careful! Do not scald yourself!
Drill two holes to fit two one-hole stoppers in the opposite sides of around metal can, a short distance from the top.
Insert short glass tubes bent at right angles through the holes in the one-hole stoppers.
Turn the ends of the glass tubes to point in opposite directions.
Fill the can of water.
Heat the water in the can with a Bunsen burner.
When the water in the can is boiling and steam is coming out of the glass tubes in opposite directions, observe the movement of the can.
The can turns in the opposite direction to the steam coming out of the glass tubes.
If you apply more heat the temperature of the boiling water is not raised, but the spin velocity increases.
16.3.25 Spinning eggs
Spinning eggs, forces with a fresh egg and hard-boiled egg
1. 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.
2. Use your fingers to spin a fresh egg and a hard-boiled egg end-on, i.e. about its short axis.
The hard-boiled egg spins faster and longer.
The raw egg soon slows, because of the inertia of liquids and friction within the egg.
The spinning raw egg damps more quickly than a boiled egg due to internal friction.
The inertia of its liquid contents exerts a drag effect that kills the spin.
3. If a spinning hard-boiled egg is touched it, stops spinning, but if a spinning raw egg is touched, it stops spinning, then weakly continues to spin when the finger is removed.
The raw egg starts spinning again, because internal liquids are still moving when the outer shell has been stopped by the finger.
Centripetal forces are different with spinning fresh and hard-boiled eggs.
4. Centripetal forces with a fresh and hard-boiled egg
Use your fingers to spin a fresh egg and a hard-boiled egg end-on, i.e. spin it about its short axis.
The hard-boiled egg spins longer, because the inertia of the fluid contents of the fresh egg brings it to rest sooner.
The fresh egg slows, because of internal motion and viscous friction.
16.5.1 Newton's first law of motion, inertia
Bottle Jet Drag Racer Lab Pack (Pack of 12), Newton's laws (toy product).
See diagram 16.4.0: Forces.
Newton's first law (Isaac Newton, 1642 -1727, England), the law of inertia
A body continues in a state of rest or uniform motion in a straight line unless it is acted upon by an external force
Inertia is the property of a body, proportional to its mass, in that it continues in a state orest or uniform motion in the absence of an external force.
Inertia is the "reluctance" of a body to accelerate.
Unless a force acts on a body, its velocity remains constant.
The greater the mass, the greater the inertia of the body.
Inertial mass, m, is the measure of an object's reluctance to accelerate, usually measured in kilograms, kg.
Mach's principle (Ernst Mach, 18381916, Austria), states that the inertia of a body is caused by the gravitational interaction between that body and all the bodies in the rest of the universe.
So if a body could be isolated from the gravitational forces from all other bodies in the universe, it would have zero inertia.
16.5.1a Normal reaction
See diagram 16.4.2: Normal reaction.
The mutual actions of two bodies on one another are equal in magnitude and opposite in direction, whether the bodies are moving or at rest.
A body pulled along a surface by a string exerts a force on the string equal and opposite to the force exerted on it by the string.
If an object A exerts a force on object B, B exerts an equal and opposite force on A.
This is Newton's third law of motion.
It shows the fact that forces always interact between objects and thus always occur in pairs.
When one object exerts a force on another, the second object exerts an equal and opposite force on the first object.
To every action, there is an equal and opposite reaction.
When an object is resting on a horizontal surface, the normal reaction perpendicular to the surface balances its weight.
16.5.2 Newton's second law of motion, force
1. The rate of change of a moving body is proportional to the force acting to produce the change
Time rate of change of momentum of a body is proportional to the impressed force and takes place in the direction in which the force acts.
The effect of an unbalanced force on an object is to cause it to accelerate in the direction of this resultant force.
Momentum is the quantity motion of a moving body equal to the product of its mass and velocity, mv.
2. Force is proportional to the time rate of change of momentum it produces, i.e. force x time rate of velocity x mass.
Force mass x acceleration.
However, if the unit of force is defined as equal to that force that will produce unit acceleration in unit mass, F = m x a, F = ma.
3. The SI unit of force is a newton (N) (1 newton = 1 kg m s-2). F = ma, where F = force in newton, N, m = mass in kilograms, kg, and a = acceleration in metre / second, m s-2.
Measure force directly with a spring balance.
4. When a single force replaces a system of forces, this force is called the resultant.
5. If an object acts on another object and causes its acceleration, the measure of this action is a vector quantity, which is called force.
Forces are caused by the interactions of pairs of bodies.
The effect of an unbalanced applied force on an object is to cause it to accelerate in the direction of this resultant force.
Acceleration is directly proportional to this force and inversely proportional to inertial mass of the object.
6. Weight of an object is the gravitational force exerted on it by the Earth, F = mg, F = weight in newton, N, m = mass in kilograms, kg, g = Earth's gravitational field strength at the place =
9.8 N / kg, 9.8 N kg-1.
7. Free fall acceleration due to gravity = 9.8 m / sec2.
8. Impulse is the change in momentum of an object.
F = change in mv / change in t, so impulse, F x change in t = change in mv. Impulse = change in momentum.
For a given change in momentum, the longer time the force acts, the smaller the force needed for a change in momentum.
To reduce effects of motor vehicle accidents, when bringing a vehicle to zero velocity a long impact time reduces the collision force, e.g. crumpling of forward parts of the motor vehicle after a collision.
Experiments
16.2.8 Equal forces from spring clothes pegs
16.2.9 Equal forces on light and heavy bodies
16.2.17 Ladder against a wall
16.2.11 Newton's sailboat, fan on a sailing boat, fan cart
16.5.13 Pull on a spool
16.5.15 Pull the bike pedal
16.2.14 Thrust and recoil
16.5.3 Newton's third law, action and reaction, normal reaction
16.5.1a Normal reaction
Experiments
16.5.7 Acceleration of light and heavy objects
16.5.4 Action and reaction pulling forces
16.5.5 Action and reaction, pushing forces
16.5.6 Action and reaction with balloons
16.5.8 Action reaction engine, balloon-driven boat
16.5.9 Action reaction engine, balloon-powered rocket
16.5.10 Action reaction, air track gliders
16.5.11 Action and reaction, when stepping forward
16.5.12 Cannon car, recoil roller skate
16.2.8 Equal forces from spring clothes pegs
16.5.14 Equal forces on light and heavy objects
16.5.16 Helicopter rotor
16.5.17 Impulsive force, thrust, balloon on a balance
16.5.18 Impulsive force, thrust, garden hose, lawn sprinkler
16.5.19 Liquid nitrogen cannon
16.5.20 Match rocket, match missile
16.5.21 Milk carton sprinkler, spinning cylinder
16.2.11 Newton's sailboat, fan on a sailing boat, fan cart
16.5.22 Pop pop boat
16.5.23 Push me pull me carts
16.5.24 Pulling forces, link two spring balances
16.5.25 Pushing forces, push sponges together
16.5.26 Reaction force from a water jet
16.5.27 Reaction train
16.5.28 Recoil, bow and arrow, catapult, fire a rifle
16.2.14 Thrust and recoil
16.5.4 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.
16.5.5 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.
16.5.6 Action and reaction with balloons
1. Use a rubber balloon as a simple rocket engine.
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.
16.5.7 Acceleration of light and heavy objects
See diagram: 16.4.3a: Blocks on the table.
Place a ruler horizontally on a table.
Screw a ring screw into the centre of the smallest side of a wood block, 5 x 7.5 x 13 cm.
Tie the one end of a piece of elastic to the ring of the screw and put the other end of the elastic on the table and close to the 0 cm mark on the ruler.
Press down this end of the elastic then pull out the elastic 15 cm by pulling the block, i.e. align the front of the block to the 15 cm scale.
Release the block.
Observe the movement of the block.
Repeat the experiment with different elongation of the elastic.
Compare how fast the state of motion of the block changes with different elongation of the elastic.
To repeat the experiment, put another block on this block and secure them together.
Compare how fast the state of motion of the block changes when the mass increases.
16.5.8 Action reaction engine, balloon-driven boat
See diagram 36.103: Balloon-powered boat.
1. 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. Insert a short glass tube into the mouth of the balloon and bind around the balloon mouth and tube with adhesive tape.
Punch a hole the size of the glass tube in the side of a waterproof paper box.
Put the balloon into the box and push the end of the glass tube through the hole.
Tie string around the box so that the balloon cannot jump out.
Inflate the balloon through the glass tube and cover the end of the glass tube tightly with your finger.
Put the balloon in the box on the water.
Remove your finger from the end of the glass tube and observe the movement of the paper box boat.
3. Make a balloon-powered boat.
Cut away one side of a milk carton and make a hole in the bottom near the edge.
Insert a tube into the hole and attach the balloon to it.
Inflate the balloon and place the boat in water.
Note the direction the boat moves as air leaves the balloon.
Repeat the experiment with the open end of the tube under the surface of the water.
Notice any difference in speed if the open end of the glass tube is under the surface of the water.
16.5.9 Action reaction engine, balloon-powered rocket
See diagram 36.103: Balloon-powered rocket.
1. Designs for balloon-powered rockets usually involve placing the balloon in a container, e.g. plastic drink bottle, inflating the balloon, sealing the inlet by twisting, then releasing the balloon.
Air rushes out of the inlet to cause an equal and opposite force forwards.
However, the motion of the rocket is unpredictable, so the container should be harnessed, e.g. attached to a drinking straw threaded on a taut line.
2. Attach a drinking straw to the side of a long balloon.
Pass a thin wire through the drinking straw, pull the wire until it is tight, and attach it to the opposite sides of the room.
Put a cardboard cylinder in the nozzle of the balloon.
Inflate the balloon and then release it suddenly so that the balloon travels along the wire.
3. Inflate a long balloon and seal the mouth by tying tightly with string.
Attach a drinking straw to the balloon along its long axis with adhesive tape.
Attach a long fishing line to a post and pull the other end tight.
Attach something thin and heavy, e.g. a needle, to the end of the fishing line then thread it through the drinking straw.
Pull tight again to the end of the fishing line now with the balloon attached to it.
Cut the string around the mouth of the balloon and watch the balloon move along the string.
4. Make a balloon-powered rocket.
Attach a drinking straw to the side of a long balloon with adhesive tape.
Pass a wire through the drinking straw, attach each end of the wire to fence posts and tighten the wire.
Inflate the balloon then release it.
The balloon travels along the wire.
5. Make a drinking straw rocket.
Fit a one-hole stopper to a plastic drink bottle.
Attach a thin glass tube, open at both ends, through the one-hole stopper.
Seal one end of a drink straw with Plasticine or modelling clay and place it over the thin glass tube, sealed end out.
Squeeze the plastic drink bottle suddenly and the drinking straw shoots out like a rocket.
Air from the plastic drink bottle is forced out through the thin glass tube to increase air pressure in the drinking straw.
Air rushes out through the back open end of the drinking straw so the opposite reaction occurs causing the straw rocket to move forwards.
6. Make a balloon-powered rocket.
Use adhesive plaster to paste several small iron rings to the side of a long shaped balloon.
Use a straight slippery thin iron wire (if existing iron wire is not straight, you may step on it then rotate it several times so that it will became straight.)
Insert the iron wire through the iron rings and fasten the iron wire to any two rings.
Put a small glass's end into the mouth of the balloon.
Use white adhesive plaster air proof the connecting part of the glass and the balloon.
Hang the glass to two rings on the iron wire with a piece of string.
Make sure he glass parallel to the iron wire.
Now this is a balloon-driven "rocket".
The end of the iron wire near the bottom of the balloon is the rocket head.
Draw the balloon backward to the other end of the iron wire.
Inflate the balloon then release it suddenly.
Observe how far the "rocket" can go ahead.
Plot a graph using the times of blowing as x-axis and the distance of the "rocket" gone as y-axis.
Bend up the "rocket heads" a bit then repeat the experiment.
7. Rocket on a wire
Insert a soda bulb into the back of a rocket or a cart.
The soda bulb is filled with compressed carbon dioxide gas.
When pricked, the ejection of the gasses propels the rocket or cart forwards.
8. Use air from a rubber balloon to propel an air cart.
9. Two identical air track gliders are connected with a piece of cotton.
The air track gliders have compression springs or like poles of magnets facing each other.
Burning the cotton releases the air track gliders.
16.5.10 Action reaction air track gliders
See diagram 16.3.2.2: Action-reaction gliders.
Two air track gliders on an air track have compression springs or like poles magnets facing each other and are connected with a length of cotton.
Burn cotton and the air track gliders move apart with equal, but opposite motion.
16.5.11 Action and reaction, when stepping forward
1. Put on a pair of roller skates and throw a large ball over the head to another student.
Note the direction in which the other student moves.
The student moves in the opposite direction to the motion of the ball.
If you then takes a step forward, the other roller skate tends to move backwards.
2. Repeat the experiment with both students on roller skates.
3. Wear a pair of roller skates with idler wheels and stand on a cement floor.
Hold a basketball with hands and forcibly throw the ball ahead overhead.
Experience the force acting on your hands from the ball and feel that you move backwards.
4. 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.
16.5.12 Cannon car, recoil roller skate
"Bottle Jet Drag Racer", Newton's laws (toy product).
1. A small brass cannon mounted on one cart fires a bullet into a wood block on another cart of equal mass.
If string ties the carts together, no motion will result.
2. Attach a heavy rubber band to the front of a roller skate such that it can be used as a catapult.
Pull back the rubber band with a string tied in the middle of it and attach the other end of the string to the back of the roller skate.
Put a marble in the central loop of the rubber band.
Cut the tight string.
The marble shoots forward due to the catapult action and simultaneously the roller skate moves backwards slightly, because the mass
of the marble is much less than the mass of the roller skate.
16.5.13 Pull on a spool
See diagram 16.167: Pull on a spool.
Pull the rope that is wound around the spool.
The angle between the rope and the table determines the direction the spool will roll.
At some angle, the spool will not roll, but slide when you pull it.
Pull on the cord wrapped around the hub of a spool, e.g. a cotton reels, at various angles to make the spool change directions.
Note the angle of pull on the cord when the spool does not roll, but slides in the direction of the pull.
16.5.14 Equal forces on light and heavy objects
See diagram 16.4.11: Equal forces on light and heavy objects.
1. Draw a one metre line on a slippery tabletop with chalk and mark the line every five centimetres.
Attach a wooden spring clothes peg to each end of a one metre piece of elastic.
Pull the elastic out 25 cm along the line then release the ends simultaneously.
Observe how the two clothes pegs meet at the middle of the elastic.
Pull the elastic out 35 cm along the line then release the ends simultaneously.
Repeat the experiment with two attached clothes pegs.
The single clothes pegs move faster than the two attached clothes pegs.
The distance the single clothes pegs move is longer than the distance the two clothes pegs move.
Repeat the experiment with different numbers of clothes pegs attached to the ends of the elastic.
2. Use a wooden clothes peg and several nuts with different masses.
Use string to fasten the back part of the clothes peg to make its front open.
Place the tied clothes peg on the middle of a slippery tabletop.
Place a heave and light nut close to either side of the clothes peg.
Use a lit match to burn off the string fastening the clothes peg.
Observe which nut moves faster.
16.5.15 Pull the bike pedal
Pull backward on a pedal at its lowest point and the bike will move backward.
16.5.16 Helicopter rotor
In a helicopter, the symmetric propeller deflects air down causing upward lift.
16.5.17 Impulsive force, thrust, balloon on a balance
Measure the size of impulsive force.
Use a pan balance, some weights, an inflated balloon.
Put a balloon on the right pan of a pan balance.
Put small weights on the left pan so that the weight on the left pan is more than the weight of the balloon.
Untie the mouth of the balloon a bit and let the air rush out of the balloon hitting the right pan of the balance.
So an impulsive force is exerted on the right pan of the balance.
Adjust the weight on the left pan to balance the force from the balloon on the right pan.
16.5.18 Impulsive force, thrust, garden hose, lawn sprinkler
Hold a garden hose in one hand and turn on the water with the other hand.
Feel the movement just when the flow of water increases suddenly.
Observe the direction of rotation of lawn sprinklers and the direction of the water spurting out.
Observe the change in velocity of a lawn sprinkler when you suddenly increase the flow of water.
16.5.20 Match rocket, match missile
See diagram: 16.4.6: Match rocket.
Be careful! Do not stand in the direction of shooting!
Open a "slide on" paper clip so that the angle between the two arms is 45oC.
Fix a matchstick with the match head in one of the loops of the paper clip.
Wrap around the match stick and paper clip loop tightly with kitchen
aluminium foil or the silver paper used to wrap chocolates.
Enclose the match head, but leave an open tube around the end of the match stick.
Hold the paper clip by one arm so that the other arm with the matchstick is pointing upwards at 45oC.
Heat the match head through the silver paper.
The match head ignites and the matchstick missile shoots out.
16.5.192 Liquid nitrogen cannon
Fire a cork out of a liquid nitrogen cannon mounted freely on a railway track or fixed to the track.
16.5.21 Milk carton sprinkler, spinning cylinder
See diagram: 16.4.4: Milk carton sprinkler, spinning cylinder.
1. Make four identical small holes in the bottom end of the sides of a milk carton.
Fill the milk carton with water.
Tie a string through a hole in the upper lid.
Let the milk carton twist as water rushes out through the holes.
Note the relationship of the position of the holes and the direction of the turn.
If each hole is in the bottom right hand corner of the carton, when the water spurts out an equal and opposite inwards force at each hole occurs, so the milk carton turns anticlockwise.
2. Observe water sprayed from a spinning box.
A paper box spins caused by water spouting from it.
Make four small holes in the sides of a milk carton, each in the right hand lower corner.
Tie a string through a hole in the top of the carton, to hang it or lift it by your hand.
Fill the carton full with water, observe the state of water flowing from the holes.
Then turn string around, remove your hand, observe if the spinning direction of the carton is the same to that of flowing water.
The water sprays due to the centrifugal force.
3. Observe water from a spinning cylinder.
Use a transparent plastic cylinder.
Punch three holes on top of it and four holes at the bottom of it distributed evenly.
Tie thread through the three holes on the top and hang it.
Fill it half full with water.
Cover the four small holes at the bottom with your right hand fingers except the middle finger.
Hold the bottom of the cylinder upward to loosen the thread.
Then rock it along a circular line in horizontal plane in one direction, i.e. in the direction of clockwise, until the surface of water in the cylinder forms a deeper whirlpool.
Place it down rapidly until the thread is pulled tightly, then remove your hand, observe the spinning of the cylinder and if the spinning direction of it is opposite to that of the flowing water.
This is because the cylinder is acted on reaction exerted by both spinning water and sprayed water.
16.5.22 Pop pop boat
See diagram 16.3.19: Pop pop boat.
This toy boat has a simple metal boiler made from a beverage can, or very thin metal or even a coiled copper tube connected to a copper tube or two drinking straws, at a small angle, leading out of the rear of the toy boat.
The boiler and tubes are almost filled with water.
A small lighted candle is placed under the "flash boiler" to convert water in the boiler to steam that pushes out the column of water in the rear tubes or tubes.
This backward force causes the forward movement of the boat in accordance with Newton's third law of motion.
Any remaining steam in the boiler condenses to reduce the pressure and so water enters the boiler and rear tubes again.
However, whereas the water cylinder expelled from the tubes moves in one direction backwards the water re-entering the rear tubes enters from
all directions so there is a net forward force to keep the
boat moving forward.
The "pop pop" noise is caused by water column moving backwards and forwards in the tubes and so expanding and compressing the contents of the boiler.
This toy boat is also called a pop pop boat, putt putt boat, flash steamer, hot air boat, jet boat, pulsating water engine.
16.5.23 Push me pull you carts
See diagram 16.168: Push me pull you carts.
See diagram 16.4.10: Push me pull you carts.
Two people stand on identical roller carts or boats or skateboards.
Both pull on a rope or push with a long stick.
With both carts at rest one person pulls on the rope, but both move.
16.5.24 Pulling forces, link two spring balances
Screw a ring screw into the top and bottom of a cork.
Hook a spring balance in each ring of the ring screw.
Put a finger of your left hand through a ring of a spring balance and put a finger of your right hand through the ring of the other spring balance.
Your left hand does not exert force, but must keep the system steady, just like a wall.
Pull out with your right hand and observe the readings on two spring balances.
Change the direction of the pull and observe the readings on two spring balances.
Repeat the experiment with both hands pulling apart at the same time.
16.5.25 Pushing forces, push sponges together
Put together two blocks of bathroom sponge or artificial sponge, with long sides opposite.
Push them face to face using right hand only, left hand only, both hands pushing.
Observe the change in shape of the two blocks of sponge.
16.5.26 Reaction force from a water jet
Tie one end of a rubber hose to a spring and turn on the water, then cut the string.
16.5.27 Reaction train
An electric train is placed upon a circular track that is free to rotate.
By carefully adjusting the voltage the train can be made to remain stationary while the support rails will rotate beneath.
16.5.28 Recoil, bow and arrow, catapult, fire a rifle
Recoil is the spring back force from the force of a discharge from an object.
It is caused by the conservation of momentum when particle are discharged.
The internal combustion of gunpowder in a gun causes a bullet to leave the gun with its own momentum, mass of bullet x (backwards) velocity of the gun, felt by the hand as recoil.
The hand is much lighter than the gun, so the velocity backwards of the hand is much greater than the velocity backwards of the gun.
Experiments
16.3.9 Throwing a ball
16.3.24 Turning water can, aeolipile of Hero