Demonstration

Simple demonstrations to introduce the concept of energy stored kinetically.

Apparatus and Materials

• Retort stand
• Pulley, single, on clamp
• Mass hanger and slotted masses (100 g)
• Thread
• Dynamics trolley
• Spring, expendable
• G-clamp, 10 cm

Health & Safety and Technical Notes

In all of these experiments a student should act as a trolley catcher to ensure no trolleys land on toes.

Read our standard health & safety guidance

Procedure

1. Put the trolley on a bench and give it a push. Energy stored chemically (in food + oxygen) is now stored kinetically.

2. Instead of a trolley you could, more impressively, use a student on a skate board (with due consideration to student behaviour and safety).
3. Put a trolley on the bench. Fasten the pulley to the edge of the bench, running a thread over it from the trolley to a 100 g hanger. Let the load fall a short distance to the floor so that the thread falls slack, allowing the trolley to continue moving. Energy stored gravitationally is now stored kinetically.

4. Repeat the demonstration, but in reverse. Start the trolley moving with a push away from the pulley. Let it pull the thread taut and lift the load as it comes to rest. Energy stored kinetically is now stored gravitationally.
5. Before you use the expendable spring, stretch it until it is clearly an open, weak spring. Anchor one end (A) to the bench by slipping the end loop over the rod of a retort stand, which is itself clamped to the bench.

6. Fasten 1 – 2 m of thread to the other end (B) of the spring. Then stretch the spring gently by at least several centimetres. Anchor this end (B) temporarily.
7. Straighten out the thread and attach the free end to a trolley. Position the trolley so that the thread is taut. Energy stored chemically in muscles is now stored elastically in the thread.
8. Release the end B. You need to have a long enough piece of thread for the trolley to travel 30 cm or so at constant speed after the spring has fully contracted.
9. Repeat the experiment, but in reverse. Give the trolley a push so that it causes the spring to stretch. Energy stored kinetically is now stored gravitationally. If you make any compensation for friction, by tilting the board, this will have to be adjusted when you reverse the transfers.

Teaching Notes

• The concept of energy stored kinetically is very important. Energy stored in fuels can be transferred to energy stored kinetically in rockets, gas molecules and anything else which is moving.
• Before students can tackle calculations with energy stored kinetically, they need a clear picture of what energy stored kinetically is, and a good feeling for it. These simple qualitative demonstrations invite discussion. It is useful to use them before attempts to measure and calculate values for energy stored kinetically.

This experiment was safety-tested in November 2005

Demonstration

Forces produce a transfer of energy so it is stored kinetically.  In Demonstration A, elastic bands store energy elastically. Demonstration B introduces energy stored in a magnetic field.

Apparatus and Materials

• Dynamics trolley
• Runway
• Horseshoe magnets, 2
• Dowel rod
• Rubber bands

Health & Safety and Technical Notes

Long runways or heavy shorter ones should be handled by two persons.

Retort stands should not be used to form a catapult unless they can be clamped to the bench.

If a trolley may fall from either end, fit a buffer across the end of the runway to catch it.

Read our standard health & safety guidance

The runway needs to be adapted by stretching a catapult across it. Here are two ways of doing this.

The first way is easy, but clumsy and not very rigid. See safety notes below.

1. Set up two massive retort stands on either side of the runway. Clamp the stands to the bench. Stretch elastic between them.

2. Here is a better arrangement for a satisfactory and simple catapult. Drill the runway with holes to take 15 cm lengths of 1 cm wood dowel. Stretch a 10 cm elastic band (0.3 cm width is satisfactory) between the two dowels.

An alternative to the trolley and ramp in Demonstration B is a train set with the appropriate track and wagons. In addition it is possible to fit compression springs to the wagons as shown below.

Procedure

Demonstration A: Transferring energy between elastic bands
1. Set up a catapult across each end of the runway (about 30 cm from the ends). Do this by stretching large elastic bands between the dowel rods (or retort stands) fixed at the sides of the runway as described above.
2. Firmly fix a single vertical dowel rod on the trolley. Adjust the height of the elastic so that the vertical rod will catch the middle of it. (It is probably better to let the vertical rod engage the rubber band, although it is possible to have the rubber lower and let it engage the actual body of the trolley.)
3. Place the trolley on the runway. Pull it back against one of the catapults so that the rubber is stretched by a measured amount. Then release the trolley so that it is projected by the catapult along the runway and strikes the second catapult. If you have aligned the apparatus well, the trolley will oscillate backwards and forwards for a few cycles, before it comes to rest due to frictional forces and hysteresis in the rubber band (warming it up).

Demonstration B: Transferring energy from a stretched rubber band to a magnetic field

1. Firmly attach two powerful horseshoe magnets to two trolleys. Sellotape can be used for this. The magnets should be orientated so that the trolleys will be repelled as they approach one another.

2. Place each trolley at an end of the runway. The back of the raised front of the trolley, below the magnet, should be touching the catapult. Pull the trolleys back by equal amounts. Release them so that they run towards one another, are brought to rest and are then pushed away again by the magnets, before returning to the catapults.

Teaching Notes

• When the catapult is pushed back by the trolley in Demonstration A, the elastic band stretches. Energy is stored elastically in the elastic band. When the trolley is released the elastic band returns to its original length, and the trolley accelerates. Energy that was stored elastically is now stored kinetically in the trolley.
• The trolley will continue at constant speed if no energy is dissipated. (Note that energy may be transferred by means of frictional forces, warming up surfaces and wheel bearings until it strikes the elastic band at the other end of the board. The elastic band will extend and the trolley will come to a stop for a moment. Energy that was stored kinetically is now stored elastically.
• You might ask students to look at the extensions of the rubber bands to see whether the energy stored elastically in each band is the same. Note that this will only succeed if you have a very free-running or friction compensated arrangement.
• You will need to rehearse Demonstration B carefully to ensure that the trolleys do not slew round as they approach one another.
• Each trolley is pushed into its respective rubber band, so that energy is stored elastically in the band. When the trolleys are released, the elastic bands return to their original length, and the trolleys accelerate. Energy stored elastically is now stored kinetically in the trolleys.
• As the magnets on the trolleys approach each other fhe force between the magnets increases, the trolleys slow down and will come to a stop. All the energy stored kinetically is now stored in the magnetic field.
• The trolleys then bounce back and repeat the process.

This experiment was safety-tested in November 2005

Demonstration

The simple pendulum used as a stimulus to discuss changes in the ways that energy is stored.

Apparatus and Materials

• Metal or wood blocks to hold the pendulum cord, 2
• Simple pendulum bob, 1 - 2 cm lead, iron or brass
• Cord for pendulum, at least 1 m or longer
• Retort stand, boss, and clamp
• G-clamp

Health & Safety and Technical Notes

Any attempt to fix the support to the ceiling requires two persons: one to hold the ladder or steps.

The use of a brick as a pendulum bob would be unwise. The brick may rotate and present a rough edge or corner to the demonstrator.

Read our standard health & safety guidance

This demonstration is most successful when the energy dissipation is kept to a minimum. This can be done by using a massive support and by ensuring that the cord is firmly clamped. Clamp the pendulum cord between a pair of blocks with the G-clamp, keeping the lower edges of the two blocks flush.

If it is possible to clamp the cord from the ceiling, the support will be even better.

Procedure

1. Clamp the pendulum to a rigidly held retort stand.
2. Let the pendulum swing back and forward and encourage students to discuss the transfers taking place.

Teaching Notes

• Do not do any timing of the pendulum, nor discuss the periodicity. Changes to the ways that energy is stored are the focus of this demonstration.
• Students should look for the ways energy is stored. Energy is stored gravitationally at the top of its swing, and kinetically (and gravitationally) at the bottom of its swing. The change in energy stored gravitationally is equal to the energy stored kinetically if there is no energy dissipated.
• Apart from at the top and bottom of the pendulum's swing, the bob has a mixture of energy stored gravitationally and energy stored kinetically. As the bob swings one gradually increases as the other decreases.
• If the pendulum is fixed as described, it should rise to the same height on either side for a good number of swings. As long as this continues, all the energy stored gravitationally is transferred to energy stored kinetically and back again. The total amount of energy stored kinetically plus energy stored gravitationally remains constant.
• This is one of the few demonstrations which, for a few oscillations before energy is transferred to the support, illustrates the Principle of Conservation of Energy.
• An entertaining extension to this experiment is to hold the pendulum bob at nose height and to leave go of the bob, taking care not to push it. The bob will come back to the same point just in front of your nose, as long as you don't flinch. If the bob is more massive the effect is more dramatic, and illustrates that energy cannot be gained from outside the system.

This experiment was safety-tested in November 2005

Demonstration

Roll a ball on a curved track to explain Galileo's idea, which in turn led to Newton's First Law of Motion.

Apparatus and Materials

• Large ball bearing (or large marble)
• Retort stands, 2
• Flexible curtain rail
• Boss
• Clamp
• G-clamps, 2

Health & Safety and Technical Notes

The risks here are trivial provided the instructions are followed. If the ball is turned into a projectile, consider the use of eye protection.

Read our standard health & safety guidance

The flexible rail should be symmetrical and not too flimsy. An alternative to a curtain-rail is the flexible track used with Hot wheels toy cars.

A good way of supporting the curtain rail is to glue a 0.5 m wooden lath (1 cm square) to each end of the underside of the curtain rail. One end can be held with a retort stand and clamp, at a height of about 30 cm above the bench. The other end can be held in another retort stand or you may prefer to hold it by hand.

It is very important for the support to be rigid if the experiment is to be effective. Avoid energy dissipated by the rail moving.

Procedure

1. Hold the ball bearing at the top of one end of the curtain rail and release it so that it rolls down one side and then up the other. Release the ball from each end in turn to see if any difference occurs.
2. Tilt the curtain rail to various slopes, both equal and unequal and repeat the demonstration The experiment may also be tried with a horizontal length between the two slopes. Finish with a slope on one side and the other side horizontal.

Teaching Notes

• When the ball is at the top of the track energy is stored gravitationally. When the ball is at the bottom of the track the energy is now stored kinetically. If no energy is dissipated through friction, the ball would return to its orginal height on the far side of the track, and briefly be at rest. This remains true regardless of the shape and slope of the track. In practise, energy will be dissipated. The ball will warm up, as will the track, and also sound will be produced.
• When the steel ball rolls down the slope and along a horizontal level it continues at uniform speed (as long as there was no frictional force) because there is no force to do work on it. There is no change to the energy stored kinetically. It would go on for ever. This idea led Galileo to what we now call Newton's First Law:
'If a body is at rest it remains at rest or if it is in motion, it moves with uniform velocity until it is acted upon by a resultant force.'
• Galileo also used this experiment as a starting point for his theory of forces and motion on an inclined plane, which gave a hint towards Newton's Second Law F = ma.
• If students are genuinely willing to blame friction for the failure of the ball-bearing to reach its original height, the demonstration will have worked. If they accept the excuse because you tell them to, the demonstration is probably not worth pursuing.
• See also the collection on this site:

  Inertia and Newton's First Law

This experiment was safety-tested in November 2005

Demonstration

An electric current transfers energy.

Apparatus and Materials

• Lamp, 240 V 100 W
• Lamp, 12 V 24 W
• Lamp, 12 V 6 W
• Lamp, 12 V 36 W and power supply
• Stopclock
• Demonstration meter with 0-5 amp DC and 0-5 amp AC ranges, 0-12 volt and 0-240 V AC ranges
• Power supply, 12 V AC
• Power supply, 240V
• Socket complete with wattage and current meter (see Technical notes)

Health & Safety and Technical Notes

Do not build circuits for connection to the mains.

The simplest demonstration just uses a safety pattern BC batten lamp holder wired via a 5 A double-insulated flex to a 13 A plug.

If ammeter and voltmeter(s) are used, all connections must use shrouded 4 mm connectors. The connections to the mains can be bared wires using a safebloc.

Read our standard health & safety guidance

The lamps can be purchased from Beecroft and Partners or other scientific equipment suppliers.

An alternative is a device called cost plug, which is no longer on sale.

The socket with wattage and current meter is available from the supplier: Machine Mart.

Procedure

1. Connect a simple series circuit of a 12 volt 24 watt lamp and demonstration meter to a 12 volt supply. Switch on for 15 seconds. Discuss the ways that energy is being transferred: an electric current is flowing, doing 'electrical work', and the filament is getting hot and emitting radiation.

2. Repeat using the 12 volt 36 watt and again with the 12 volt 6 watt lamp.
3. If an AC ammeter is available, repeat again, using the 12 volt AC terminals of the transformer.
4. Connect the 240 volt 100 watt lamp using the commercial socket with in-built meters.

Teaching Notes

• An electric current transfers energy stored chemically that was stored in the fuel (and oxygen) in a power station to the surroundings where it is stored thermally. When the electric current passes through the filament, the filament warms up. Its temperature rises so that it glows red or white hot. The energy is transferred from the filament as electromagnetic waves of visible light and infrared radiation which spread out into the surroundings, so warming up the environment.
• The 5 W,24 W and 36 W lamps, operating from 12 V, glow with different brightness. The 36 W lamp is the brightest and so radiates most energy per second. The electric current in the circuit with the 36W lamp is also greatest. It appears that the energy radiated may be proportional to the current. (More experiments than this one need to be done before this can be substantiated!)
• There is an advantage in setting up two circuits side by side e.g. 12 V 24 W bulbs, one on DC and one on AC. Students can see that AC makes very little difference.
• The 240 V 100 W lamp and 12 V 6 W lamp carry the same current but more energy is radiated from the 240 V lamp per second, and this is indicated by its 100 W rating.
• You have shown that the energy radiated depends not only on the current but also on the potential difference. It will also depend on how long the current is flowing:
Energy transferred by the electric current, E = V x I x t = VQ
• The volt can be defined as the energy transferred per coulomb passing from a power supply to a component. This definition is frequently used when students are learning electrical concepts in the early stages.

This experiment was safety-tested in December 2005

Demonstration

Simple demonstrations to stimulate discussion about the different ways in which energy can be stored: gravitationally, elastically, kinetically.

Apparatus and Materials

• Large mass or block of wood (tied up with string)
• Single pulleys on clamps, 2
• Large spring
• Dynamics trolley
• G-clamps, 5 cm, 2
• Retort stands, bosses and clamps, 2
• Rough cloth, piece approx 30 cm x 30 cm

Health & Safety and Technical Notes

The retort stand should be clamped to the bench. Any attempt to attach a pulley to the ceiling requires two persons: one to hold the ladder or steps. Ensure no one walks beneath the suspended object.

Steps 2 and 3 require a trolley catcher.

Read our standard health & safety guidance

For demonstration 2, a better alternative to the pulley attached to a retort stand is a hook fixed in the ceiling. A single pulley can be attached to the hook.

Procedure

1. Hang the spring from a retort stand and attach the large mass or block of wood to the lower end. Support the mass or block with your hand raised from the equilibrium position, release it and then catch it at its lowest point. Discuss the changes in the way that energy is stored - as suggested in the teaching notes.
2. Clamp one end of the spring to the end of the table with a G-clamp. The two pulleys are clamped to the retort stand, which itself will need clamping to the bench. The cord is run round the two pulleys as shown in the diagram, but without attaching the free end to the spring. Stretch the spring horizontally and hold it in the stretched position while you discuss the way in which energy is stored. (See teaching notes.) Now attach the free end of the cord to the stretched spring. When the spring is released the unbalanced force from the spring accelerates the mass upwards; in energy terms, the energy stored elastically decreases (as the spring contracts) and the energy stored gravitationally increases (as the mass rises).

3. Keep the spring clamped as in 2 and attach the other end to the dynamics trolley. Stretch the spring and hold it still. Then release the trolley. The unbalanced force will accelerate the trolley. In energy terms, the energy stored elastically decreases (as the spring contracts) and the energy stored kinetically (kinetic energy) increases (as the trolley speeds up). What is more, the increase in energy stored kinetically is the same as the decrease in the energy stored elastically. Energy is conserved.

4. Lay the cloth on the bench as in the diagram. Give the trolley a push so that when it runs over the rough patch of cloth it slows down and stops. You can discuss the transfer of energy to the cloth as suggested in the Teaching notes.

Teaching Notes

• Demonstrations 1 and 2 involve energy stored gravitationally.
• Take care in defining where is the zero of gravitational energy. It is best to refer to changes in energy stored gravitationally rather than absolute values. Students are often mystified when they discover that, if you raise a mass from the floor the energy stored gravitationally will increase, but that if a trap door is opened in the floor then the mass could fall further and the energy stored gravitationally can decrease. When the mass is on the floor, the system stores more energy gravitationally with respect to the basement but less with respect to the ceiling. Before you start, think carefully and avoid confusing statements regarding the gain or loss of gravitational potential energy.
• In 1 the system is storing energy gravitationally at the start. Although the spring is pulling up on the mass, to begin with the gravitational force downwards is bigger than the upwards force from the spring. The net downwards force causes the mass to accelerate downwards. As it moves downwards, the spring will begin to stretch.
• In energy terms, the energy stored gravitationally decreases; the energy stored kinetically increases and the energy stored elastically increases.
• At the equilibrium point, the spring's force balances the gravitational force. Beyond that point, the upwards force from the spring is greater than the downwards gravitaitonal force. So the mass slows down (it is accelerating upwards whilst moving downwards). As it does so, the spring is stretched more. In energy terms, the energy stored kinetically is decreasing (as is the energy stored gravitaionally) as the energy stored elastically is increasing. When it reaches the bottom, the mass is no longer moving and the system is storing energy elastically.
• The stretched spring is still pulling upwards on the mass with a force bigger than the gravtiational force. So there is still a net upwards force on the mass. Therefore it begins to accelerate upwards. It overshoots the equilibrium position, and the process repeats.
• In each cycle, the stretching and compressing of bonds in the spring will result in its temperature going up a little. And, in turn, the spring will raise the temperature of the surroundings (by heating). Therefore, in each cycle, it does not go quite so high because some of the energy has been transferred to the surroundings and there is less energy to be stored gravitationally.
• Over time, the spring will come to rest. At this point, the energy stored thermally by the surroundings has increased by an amount equal to the energy stored gravitationally by the system before the mass was released.
• Initially, you might say: "I have lifted the mass from its rest position so the energy stored gravitationally has increased. What will happen to this energy when I let it drop?"
• When you catch the mass at its lowest point you might ask: "How is the energy stored now? Will the mass stay here when I let it go? If not, why not?" You should then allow the mass to oscillate for a few cycles. Ask again for an explanation of the changes in the way in which energy is stored in the system.
• When you stretch the spring horizontally the energy stored elastically increases. Pause at this stage for students to consider where and how that energy was stored before (stored chemically (food + oxygen) in your muscles).
• When the stretched spring is attached to the mass, around the pulley, the force from the spring lifts the mass. In energy terms, the energy stored elastically has gone down and the energy stored gravitationally has increased.
• In demonstration 3, the energy stored elastically decreases and the energy stored kinetically (due to the moving trolley) increases.
• In demonstration 4 when the trolley is given a push, the energy stored chemically (food and oxygen) in muscles decreases and the energy stored kinetically increases.
• When the trolley moves over a rough surface it is slowed down by friction. There is a very slight rise in temperature as the sufaces rub over each other. When the trolley comes to rest the energy stored thermally by the surroundings has increased and the energy stored kinetically - due to the moving trolley - has decreased.

This experiment was safety-tested in November 2005

Demonstration

A fun demonstration of the relationship between energy stored gravitationally, and energy stored kinetically; it can also be used to consider the forces involved during a complete loop through a vertical circle.

Apparatus and Materials

• Flexible curtain rail, 3m to 4m long
• Ball bearing
• Toy hot wheels kit (OPTIONAL)

Health & Safety and Technical Notes

Be very careful over working at heights when setting up this apparatus and when using it. No one should climb on stools or benches.

Read our standard health & safety guidance

Toys such as 'Hot Wheels' allow these demonstrations to be set up easily at floor level.

It is rewarding to construct a version of this apparatus, very well mounted with the top section detachable. A convenient method of mounting is to glue blocks of wood 3 cm x 3 cm x 3 cm at 30 cm intervals around the curtain rail. The blocks are screwed to the track with counter-sunk screws (so that the steel ball does not hit the screw-heads as it goes round the rail). The blocks could be drilled so that the holes fit the ends of clamps attached to retort stands with bosses (see illustration). Alternatively, 10 cm nails can be put through the holes in the blocks.

Bend the curtain rail so that the ball can 'loop the loop'. This is more effective with a 3.5 m length or even a 4 m one. The initial fall should be as steep as possible and the loop needs to be tight.

If you plan to discuss circular motion: the size of the gap should be such that while the ball is in the gap the parabolic motion it gains is approximately equal to the circular motion it would have had if the track was there.

A "Roller Coaster Physics" kit is now (October 2011) available from the supplier: Data Harvest. This allows the user to build a variety of models of roller coaster and investigate several related concepts. It includes a 62-page teachers' guide, with lesson plans and design briefs. Order number KX78880.

Procedure

1. Use a tray of sand, or a good wicket keeper, to catch the ball.
2. Use the apparatus to explore what happens when the ball is released from different positions on the track. (See teaching notes.)
3. Extension for circular motion discussion: remove the top piece and release the ball from different heights until it completes the circle.

Teaching Notes

• It is great fun to challenge students to set the steel ball off at the right height, h (assuming that no energy is dissipated, the ball bearing just goes round the loop when h = 5R/2). You might ask them why you need to store more energy gravitationally at the start than is needed to lift the ball to the height of the top of the loop.
• Ask: "What makes the ball go round a circle? What pushes or pulls the ball with a real force to make it do that?"
• Point out that there must be some inward force towards the centre of the loop

• Ask: "What provides the force at the top of the loop B?" (the track and gravity). After removing a bit of the track in step 3, the ball will still go round the loop, provided the speed is right. Too fast and it flies upwards along a projectile (parabolic) path and too slow and it falls downwards along a projectile (parabolic) path.
• Ask: "What provides the force at the sides halfway up the loop (A and C)?" (The track.) The effect of gravity pulls vertically and only slows the ball down a bit.
• Ask: "What provides the force at the bottom of the track (D)?" (The resultant between the inward push of the track and the downward force of gravity.)