Demonstration

Introducing the concept of conservation of energy and a definition of the joule.

Apparatus and Materials

Steps 1,2 and 4:
• Masses, 1kg, 2
• Wooden lath, 1 m
• Shelves, 3, equally-spaced, e.g. on the lab wall
• Motor/generator unit
• Line shaft unit and string
• Low voltage power supply, set at 6 V
• G-clamps, 5cm, 2
Step 3:
• Bricks, 2
• Strong cardboard box
• Retort stand
• Single pulley on clamp
• G-clamp, 10cm
• Length of strong cord
• Metre rule
Step 5:
• Retort stands and bosses, 2
• Nail, 15cm, 2
• S-hook
• Compression spring
• G-clamps,10cm, 2
• Single pulley on clamp (If it is difficult to clamp the stands on the bench, a large board can be placed on the bench to carry everything).
• Slotted masses, 6 x 100 g

Health & Safety and Technical Notes

The hazard presented by these activities is that of working at heights. No one must stand on a stool or bench to set up the apparatus on a high shelf. If apparatus (e.g. motor and line shaft unit) can be clamped onto the shelf at bench level, it can be lifted into place by two tall persons standing on the floor. The shelf supports must be rigid. Otherwise, the demonstration(s) must be done from the floor level.

Read our standard health & safety guidance

The 1-m wooden lath should be marked in centimetres, like a ruler. A metre rule with very clearly marked centimetre lines is an alternative.

For step 4 you should fit the three shelves approximately 1/3 metre apart, so that the top shelf is at the same height (about 1 metre) as in step 1.

Procedure

1. Fix one shelf approximately 1 metre above the bench top. Lift the 1kg load from the bench to the 1m shelf. Refer to the teaching notes below for a possible script.

2. Clamp the dynamo unit to the shelf, with the line shaft unit clamped beside it. Secure one end of a length of cord to the axle of the line shaft. Attach the other end to the 1kg load on the bench-top. When you connect the motor to the supply, it will wind up the 1-kg to the shelf height.

3. You can use a brick or a block of wood to represent a lift. Alternatively, a more realistic model can be made by using a cardboard box strong enough to take a brick, or two bricks on end, side by side. The retort stand base should be fixed to the bench with a G-clamp. Fix the pulley to the top of the stand, pass the cord over the pulley, and tie it around the cardboard box to form the lift. Raise the load from the bench top by pulling on the cord and raising the model lift.

4. Lift the 1kg from one shelf to the next, reaching 1 metre high, but in three equal steps. (See the teaching notes on adding energies.) NB Clamp the stands to the bench.

5. Use the G-clamps to clamp two retort stands about 50 cm apart on the edge of the bench. Use a boss to anchor one end of the compression spring to one of the retort stands. Use a second boss to attach the nail to the other retort stand as an anchoring pin.
6. Clamp a pulley to the end of the bench. Pass the string over the pulley, tying one end to the brick resting on the floor, and the other to the S-hook attached to the end of the spring held by the nail. Stretch the spring horizontally and anchor the free end to the nail.
7. Connect the spring to the string, using the S-hook. When you release the spring, the load will be pulled up by some distance. The spring and load must be such that the tension produced when the spring is stretched is sufficient to lift the load.

Teaching Notes

• Teachers and students often take it for granted that energies can be added together like masses. It is a basic property at the root of all ideas of conservation of energy. These demonstrations reinforce this principle.
• Step 1 is the starting point. This is defining the job we want to get done (lifting the load).
• Step 2 emphasizes the need for a fuel of some sort by using an electric motor to do the same job.
• In step 2 the electric motor transfers the same amount of energy as in step 1 when it winds the kilogram up to the shelf. (The speed might drop as the layers of string increase the effective winch diameter). The energy is stored gravitationally in the load.
• Instead of lifting the load by hand you could use an electric motor for the job. You would then have to pay for the fuel that the power station uses. If a person is used to raise the bricks, then they will need to be given extra food in order to do the job.
In step 3 students may be distracted by the very obvious manual method of step 1. Emphasize that whatever way we raise the load, fuel is needed from somewhere. If by hand, fuel comes from the food we eat.
• "When I haul up this model of a lift, do I use fuel? I certainly don't take in petrol or coal but I do eat food. If I had to do a lot of hauling I should get very hungry and have to eat more."
• "As you are learning in biology, the food you eat goes to provide many things that you need: keeping you warm, mending damaged tissue, producing growth and providing energy for movement. Some chemicals in food provide fuel to run your muscles like an engine. When you haul up a load, your muscles burn some of those chemicals hauling up the load and heating your body. When your muscles work you are drawing on fuel, (chemicals), which came from your food."
• In step 4 introduce the concept of adding energies. Show that the same job is done whether it is achieved in one step, or broken down into stages. This is the point at which to introduce the term joule.
• "Suppose I now lift one kilogram up a 1/3 metre step, then a second 1/3 metre step and finally a third 1/3 metre step. How will this compare with the amount of fuel needed to raise the one kilogram through 1 metre in one go? The same amount of food would be needed for both jobs."
• In step 5, show that the energy can be stored in a spring, just as it is stored in the food we eat. The spring does not pull all the way back to its original unstretched length because the load is applying a force to the spring. Some of the energy ends up stored elastically in the stretched spring and some stored gravitationally in the raised load.
• Introducing the joule: "At the Earth's surface one kilogram has a force on it of about 10 N. When it is raised one metre we say that 10 J ( 10 joules) of energy have been transferred, or shifted, so that it is stored gravitationally. Two kilograms raised through one metre means that 20 J of energy is stored gravitationally. Two kilograms raised three metres will store 60 J gravitationally."

This experiment was safety-tested in November 2005

Demonstration

Students ‘experience’ forces and reinforce their understanding of force and energy units.

Apparatus and Materials

• Forces and energy demonstration box

Health & Safety and Technical Notes

Read our standard health & safety guidance

If you do not have a forces demonstration box, simply use a 100 g bag of beans (weight about 1 N) on a string over a pulley. Students pull the string out by 1 m to experience 1 joule of work.

Procedure

1. Let students pull each of the masses in turn and ‘feel’ the pull of the Earth on masses of one pound, one kilogram and 102 grams. (1 lb-wt, 1 kg -wt and 1 newton.)
2. If you leave the box available in the lab over a period of weeks, students can return to it to reinforce their knowledge. It also avoids a queue, which would be wasteful if all the class had to try the forces in one session.

Teaching Notes

• This piece of equipment is very useful to have around the laboratory so that students get a feel for force and energy units. The Imperial units are included as a few students may be more familiar with a pound mass, but you can omit if it is more likely to confuse.
• The pull of the Earth on any body is a force. This is known as the weight of that body. The pull of the Earth on a mass of 102 g is 1 newton and on a mass of 1 kg is about 10 N (9.81 N). The weight of an apple is about 1 N.
• The joule is a secondary unit. It depends on the units of force and distance, and so it is a difficult concept for beginners to grasp. Joulemeters, which read in joules directly, have been developed for measuring the transfer of energy from an electrical supply to a heater, just like the electricity meters in the home. But there is nothing so convenient for measuring transfers from mechanical work.

This experiment was safety-tested in November 2005.

Demonstration

Using a dynamics trolley on a steep ramp, where the energy transfer is not easily measured.

Apparatus and Materials

• Dynamics trolley
• Pulley, single, on clamp
• G-clamps, 4
• Cord
• Length of angle iron, 2
• Hardboard sheet, approx 3 m x 300 mm
• Mass, 1 kg

Health & Safety and Technical Notes

It is important to have the board supports very stable so that the system cannot collapse as students gather round it to watch. A trolley catcher might also be required.

Read our standard health & safety guidance

Clamp one end of the strip of hardboard to the bench-top. Raise the other end about 0.75 cm and rest it on two 1.25 lengths of angle iron (Handy Angle or Dexion, used in rack storage systems). Clamp the board to the bench again about halfway along its length as in the diagram. This arrangement needs to be very secure.

Cut a vertical slit, not more than 1 cm wide, in the middle of the board. Pass a cord through the slit without touching the hardboard and, keeping it parallel to the bench-top, pass it over a pulley secured to the edge of the bench. Tie a loop (about 1/2 m long) in the end of the cord that goes through the slit. Pass the other end over the pulley and fasten it to a 1 kg mass. Hang the loop loosely over two bent posts as shown. The part of the loop that is not suspended between the posts should lie flat on the board (so that the trolley can run over it).

A dynamics trolley with a single peg in the top is released from the top of the ramp, with the single wheel end leading, and the single peg at the rear. The peg engages the loop after the trolley has run over those parts of the loop that are on the board. The trolley is brought to rest, raising the 1 kg mass.

Before you release the trolley, check that its leading end will pass under the raised part of the loop, so that the loop may be engaged by the peg.

Have the minimum of slack string between the slit and the mass, so that the mass starts to rise as soon as the loop is engaged.

Procedure

1. Hold the trolley at the top of the slope and let it run down the board. The peg should catch the loop of cord so that the trolley is brought to a halt by the tension in the cord connected to the 1 kg mass.

Teaching Notes

The energy of the trolley stored kinetically is transferred to the the rising mass, increasing the energy stored gravitationallly. Use E=mgΔh to calculate the energy stored gravitationallly by the trolley at the start of its fall and compare it with the energy stored gravitationallly that was transferred to the mass. Typically, only 50% of the energy of the trolley stored kinetically will transfer to the mass. Point out to students that energy must have been dissipated (stored thermally in the surroundings) because of conservation of energy.

This experiment was safety-tested in January 2006

Class practical

Measuring the energy stored gravitationally transferred so as to be stored kinetically in a dynamics trolley.

Apparatus and Materials

• Ticker-tape
• Pulley, single, on clamp
• Dynamics trolley
• Runway
• Ticker-timer
• Mass hanger and slotted masses (100 g)
• Balance, 5 kg
• Thread
• Knitting needle or length of glass tube

Health & Safety and Technical Notes

Long runways or heavy shorter ones should be handled by two persons. Ensure that a buffer is tied across the bottom of the runway, to prevent the trolley falling onto anyone.

Read our standard health & safety guidance

Procedure

1. Clamp the pulley to the end of the runway. Compensate it for friction in the normal way. Do this by raising one end of the board so that the trolley, once started, will continue at constant speed as judged by the equal spacing between ticker-tape dots. See the experiment:

   Compensating for friction

2. Attach a length of thread to the trolley. Thread it over the pulley and tie it to the mass hanger. The length of thread must be long enough for the mass hanger to hit the ground well before the trolley reaches the end of the runway.
3. Attach a length of ticker-tape to the trolley. Pass the other end through the ticker-timer. See also the experiment:

   Using the ticker-timer to measure time

4. Release the trolley, so that the falling mass accelerates it until the mass hits the ground. After this, the trolley will move with constant velocity v.
5. Estimate the value of v from the ticker-tape. Measure the mass of the trolley on the balance.
Calculate 1/2 mv².
6. Compare this with the change in energy of the load stored gravitationally, Δ E = m × g{Δ{h}

Teaching Notes

• The energy stored gravitationally in the load end up being stored kinetically in the trolley. Students should consider the start point of the load at its lowest point and end point of the load at its highest point. At the end point the load itself will be moving, so there will be some energy stored kinetically in the load and in the trolley. However, if the mass of the load is very small compared to the mass of the trolley then the energy stored kinetically will be (almost) negligble.
• Students are likely to be disappointed when they find that the values for 1/2 mv² and Δ E = m × g{Δ{h} do not agree. You will need to discuss the results. How reliable is the experiment? Where do students think that the energy might have been transferred to? Some energy will be stored thermally s, e.g. warming the pulley, and the wheel bearings, the air.
• It is impossible to demonstrate the principle of conservation of energy for thermodynamic reasons. As a result of friction, energy will always be stored thermally in the surroundings. However, if the demonstration is carried out slowly, and as many energy losses as possible are accounted for, then the two values should be close enough to satisfy the students.
• The experiment can be repeated but in reverse, using the trolley to raise the load.
• Compensate the runway for friction the opposite way round. The thread should be attached to the trolley and carry a 100 g weight hanger as before. This time start the trolley at the end of the trolley board nearest the pulley, with the thread slack. Because the thread is slack it will not stay in a pulley groove, and so a length of glass tube or knitting needle should be used as a roller. The ticker-tape is also fixed to the reverse end of the trolley so that it runs out behind the trolley and passes through the ticker-timer.
• This time when the trolley is given a push, it travels with constant velocity v down the compensated runway until halfway down. Then the thread goes taut and the load is raised a distance d as the trolley comes to rest. The decrease in the energy stored kinetically is compared to energy stored gravitationally.
• This experiment could be extended into a series of readings for different loads and different distances of fall. This would provide different values of v in 1/2 mv².
• The mass of the trolley, m , could also be changed. Students could plot graphs to show relationships between d and v², and graphs for different masses.

This experiment was safety-tested in August 2007

Class practical

Comparing the energy stored in a stretched elastic band with the energy stored kinetically in a dynamics trolley.

Apparatus and Materials

• Dynamics trolley
• Runway
• Ticker-tape
• Ticker-timer
• Elastic band
• Forcemeter

Health & Safety and Technical Notes

Long runways or heavy shorter ones should be handled by two persons. Ensure that a buffer is tied across the bottom of the runway, to prevent the trolley falling into anyone.

Read our standard health & safety guidance

The runway must be adapted, as described in the experiment:

Energy stored by a moving trolley

...so that there is a catapult at one end. This is made with an elastic band stretched between dowel rods.

Procedure

1. The runway should be compensated for friction. See the experiment:

   Compensating for friction

2. Set up the catapult towards the upper end of the runway. Do this by stretching a 10 cm elastic band taut between two dowel rods. This should be at a height that will engage the vertical rod on the trolley firmly.
3. Pull the trolley back so that the rubber band stretches. Measure the distance that the trolley has been pulled back from its initial point in increments of 1.0 cm. Energy will be stored elastically in the catapult. Release the trolley and obtain a ticker-tape record of its constant velocity. Calculate the energy stored kinetically = 1/2 mv².
4. Repeat the experiment, obtaining ticker-tape records for the trolley after it has been pulled back through various known distances. See also the experiment:

   Using the ticker-timer to measure time


5. Pull the trolley back from the initial mark with the forcemeter. Note the forcemeter reading at 1 cm intervals, for a range of distances covering the distances used to obtain the ticker-tape records.

6. Plot a graph of ‘force’ against ‘distance pulled back’ as shown in the diagram.

Teaching Notes

• This experiment is not easy. Students should not expect more than general agreement between the energy stored elastically and energy stored kinetically.
• Stopwatches and metre rules, or light gates, could be used instead of tape and timers.
• On the graph of force against distance pulled back, a small part of the distance pulled back is selected and vertical lines drawn up to meet the curve. The area of that small section gives the force x distance pulled back. This is the energy stored chemically (fuel + oxygen) in muscles that is transferred to the elastic band, in stretching the band that small distance.
• Count the squares to work out the total area under the graph from the beginning of the loading process, when the forcemeter begins to extend the rubber band, to the total distance pulled back. This gives the energy stored elastically in the rubber band for that particular distance pulled back. When the elastic band is extended by the trolley pushing into it and then the trolley is released, this energy stored elastically in the elastic band will be stored kinetically in the trolley.

• If a spring were used instead of a rubber band, the shape of the graph would be a straight line. Instead of working out the area under the graph by counting squares, it could be calculated as 1/2 x force x distance pulled back because the area under the graph would be a triangle.
• Compare the values for the energy stored elastically and energy stored kinetically. Discuss why they might not be equal (e.g. energy is stored thermally as the rubber band heats up).

This experiment was safety-tested in November 2005

Class practical

Comparing the energy stored by two rubber bands.

Apparatus and Materials

• Dynamics trolley
• Runway
• Forcemeter
• Rubber bands
• Light gates and electronic timer (optional extension)
• Balance (optional - to find mass of trolley)

Health & Safety and Technical Notes

Long runways or heavy shorter ones should be handled by two persons. Ensure that a buffer is tied across the ends of the runway, to prevent the trolley falling onto anyone (if necessary).

Read our standard health & safety guidance

The runway must be adapted, as described in the experiment...

Energy stored by a moving trolley

...using a runway with a catapult at each end.

Procedure

1. The runway should be compensated for friction. See the experiment:

   Compensating for Friction

2. Pull one of the catapults back from its rest position with the forcemeter. Note the forcemeter reading at 1 cm intervals for the range of distances that will be used in catapulting the trolley.
3. Repeat this calibration for the second catapult.
4. Project a trolley from the one catapult to the other. Note exactly how far the trolley is pulled back before release. Note how far the second catapult is pushed back to stop the trolley. Compare the energies stored elastically in each catapult.
5. An extension experiment is to calculate the energy stored kinetically by the trolley, 1/2 mv²
6. Do this by measuring the trolley's velocity (using a stop watch, light gates or ticker-timer) and mass, and comparing this with the energies stored elastically.

Teaching Notes

• When a catapult is pulled back by the trolley and released, the energy stored elastically in the rubber band is then stored kinetically in the moving trolley. When the trolley stretches the opposite band, the energy stored kinetically is then stored elastically in the second band, and the trolley comes to a stop briefly. If both rubber bands are similar then the two ‘distances pulled back’ will be the same, showing that all the energy stored elastically in the first rubber band is now stored elastically the second.
• If the rubber bands are not identical, the energy stored elastically in the bands will have to be calculated from the area under a force / ‘distance pulled back’ graph as in the experiment:

  Stretched elastic band: energy stored elastically

This experiment was safety-tested in November 2005

Demonstration

To move along a horizontal plane there is no net change to the way energy is stored.

Apparatus and Materials

• Runway
• Dynamics trolley
• Pulley, single, on clamp
• Bosses, 8
• Retort rods, 3
• Retort stands, 2
• Hanger, 10 g mass
• Platform, wooden, approx 5 cm x 25 cm
• G-clamp
• Thread

Health & Safety and Technical Notes

Long runways or heavy shorter ones should be handled by two persons. Ensure that a string is tied across the bottom of the runway, to prevent the trolley falling onto anyone. At least one retort stand should be clamped to the bench.

Read our standard health & safety guidance

Procedure

1. Compensate the runway for friction. See the experiment:

   Compensating for friction

2. Half-way along the runway, place a retort stand (preferably 100 cm high) on each side. Connect a single rod between the tops of the retort stands. Suspend a pulley on a clamp at the centre, so that the pulley runs freely, with the plane of the pulley parallel to the runway. Fix a simple wooden platform between the retort stands, about 30 cm below the pulley.
3. Lower down the retort stands, fix two rods parallel to each other and close together across the runway as illustrated. These must be at such a height that a trolley moving along the runway will just clear them. At least one of the retort stands should be clamped firmly to the bench with a G-clamp.
4. Join a length of thread to the 10 g mass hanger on the wooden platform and over the pulley, down between the double rods, and fasten it to the trolley. The thread should be just long enough for the trolley to be about 30 cm from the parallel rods.
5. Pull the trolley back so that the weight hanger rises to the pulley. Release the trolley. It is pulled by the descending mass for the first 30 cm of its travel and then proceeds at constant velocity. Towards the end of its run, the thread tightens and the mass is hauled up again. Ideally, it should rise to its original position.

Teaching Notes

• It is essential to arrange this so that energy dissipated is reduced to a minimum. The mass of the load should be quite small by comparison with the mass of the trolley, so that the energy dissipated when the load hits the platform is small.
• Living in a world where there is plenty of friction, students are likely to think that work must always be done to move an object from one place to another at the same horizontal level. A force is needed to keep an object moving with a steady speed to overcome the frictional force. Demonstrations with CO₂ pucks on a glass plate refute that, as the contact is almost friction-free.
• Other experiments should support the story of no work being needed to move across a table. Students can accept that the energy of a brick stored gravitationally at one end of the table is the same as its energy of a brick stored gravitationally near the other end. They will agree that, if the brick is at rest in the first position and again at rest in the second position there is no energy in the stored kinetically, and that the energy stored gravitationally has not changed. Yet they will feel uneasy if you say that there has been no change to the way or ways that energy is stored due to the journey from one position to the other.
• In fact, unless you are prepared to allow infinite time for it, the journey does require the shifting of some energy, temporarily. To get across, in reasonable time, the brick must move quite fast and energy must be stored kinetically in the brick. So to transport it so energy has been shifted from somewhere; but at the end of the trip that energy can be shifted back.
• Ideally the load would be hauled up just as far at the end as it fell at the beginning. This would demonstrate that the same initial energy stored gravitationally is transferred to be stored kinetically, and then is stored gravitationally at the end. Unfortunately there is not only friction but an unavoidable inelastic collision when the thread pulls taut. Also, unless the mass of the falling load is a very small fraction of the mass of the trolley, energy is stored kinetically in the load itself, which will be stored thermally in the platform when it hits it. The platform heats up. Altogether there will be a much smaller rise than the original fall of the load.
• If the load rises to a height that is as low as 60 per cent of the original height, the experiment is not worth doing.

This experiment was safety-tested in November 2005

Demonstration

An electric motor lifts a load.

Apparatus and Materials

• Switch unit (very useful to break circuit quickly)
• Line shaft unit
• Motor unit
• Demonstration meter, 0 - 5 amp DC
• Rubber band or driving belt
• Cord
• Mass, 1 kg
• Power supply, 0–12 V

Health & Safety and Technical Notes

Read our standard health & safety guidance

The mass you select as the load must be suitable for the size of motor you have available.

Procedure

1. Clamp the motor unit next to the line shaft unit and connect their pulleys with a rubber band or driving belt.
2. Secure a length of cord to the axle of the line shaft and attach the other end to the chosen mass.
3. Connect the motor to the power supply in series with the demonstration meter.

4. Read the ammeter when the load is being lifted and compare this with the reading obtained when the motor is running light (that is, when disconnected from the line shaft}.

Teaching Notes

• In this demonstration the current flowing in the motor produces a force that lifts the load.
• When analysing the situation in terms of energy it is helpful to be clear about start and end points.
• Before the motor is turned on, there is more energy stored chemically in the battery or the fuel (and oxygen) in the power station.
• When the load has been lifted and is stationary, there is more energy stored gravitationally in the load, and a small amount is stored thermally in the surroundings due to heating in the wires, heating of the air, heating due to the effects of friction.
• At a point, say halfway, when the load is moving at a steady speed, then there is energy is stored kinetically and gravitationally in the load, stored kinetically in the motor, and stored thermally in the surroundings.
• It is important to point out that these changes are happening concurrently and not consecutively.
• The energy stored kinetically only increases or decreases when there is a change of speed.
• The current increases as the load increases showing that a bigger force needs to be used to raise greater loads. More work is done. More energy is stored gravitationally.
• Even when the motor is running light with no load connected to it, a current flows and the motor turns. The energy stored chemically in the battery or fuel (and oxygen) is decreasing. You might ask students: "Where does this energy get transferred to?" The answer is that it warms up the motor, and the surroundings. Energy is now stored thermally.
• You can analyse the energy transferred using the following equations:
• Energy transferred by the electric current, E = I x V x t
• Change in energy stored gravitationally, E = m x g x change in h
• If you measure these, it is very unlikely that the two will be equal enough to satisfy students because of the energy losses (dissipated) in the system, but they could be used to measure the efficiency of the energy transfer.
• Efficiency = (m x g x change in h /I x V x t) x 100%

This experiment was safety-tested in August 2007

Demonstration

Collisions between trolleys that result in energy being dissipated to the surroundings.

Apparatus and Materials

• Dynamics trolleys, 2
• Elastic cords, 2
• Large pin and cork

Health & Safety and Technical Notes

In place of the elastic cords, a weak spiral spring of good steel wire can be used.

Read our standard health & safety guidance

Procedure

1. Place the two trolleys on the bench with one or two elastic cords held between them, secured over a wooden dowel post on each of the trolleys. Start with the trolleys close together and the elastic slack. With a hand on each trolley, give them outward motions and let go so that they move apart. The elastic will stretch and bring them to rest for an instant. At this point, hold the trolleys in that position and ask where the energy is stored. Release the trolleys so they move towards each other and collide.

2. Now start with the trolleys well apart and the elastic stretched. Release them so that the trolleys meet with a bang. Again ask where the energy is stored.
3. Repeat with a large pin sticking out of the end of one trolley and a cork attached to the other so that the trolleys stick together on collision. (To fix the cork, it may be easier to fix a pin on each trolley and then fix a cork on one of the pins beforehand.}
4. Repeat 3 using the trolleys with the sprung buffer-rods protruding from them. When the trolleys collide, they will now rebound apart, be stopped by the elastic, return and collide again. This process will be repeated several times before they finally come to rest. Again, discuss where the energy is stored.

Teaching Notes

• When the trolleys are moved apart energy stored kinetically is transferred to the cords connecting them, storing energy elastically. Releasing the trolleys, the energy in the stretched cord is transferred to the trolleys, storing energy kinetically.
• When the two trolleys collide they come together with a bang; energy stored kinetically is now stored thermally in the surroundings. The energy stored kinetically is not conserved. Collisions of this type are called inelastic. The energy has been transferred by sound waves and dissipated by warming up, and maybe deforming, the colliding trolleys.
• Spring buffers between the trolleys: As the trolleys collide energy stored kinetically is transferred to the springs as the trolleys briefly come to rest, and energy is stored elastically. The process reverses and the energy stored elastically in the compressed spring is transferred to the trolleys, storing energy kinetically. The trolleys will oscillate backwards and forwards a few times. Ultimately, the trolleys will come to rest as all the energy stored kinetically is transferred to the environment, storing energy thermally so as to warm it up.
• In some collisions, the energy stored kinetically before and after is exactly the same. These are called elastic collisions. Collisions between molecules in air are elastic, for example.

This experiment was safety-tested in November 2005

Class practical

Considering the energy stored in foodstuffs and the energy demanded by human activity.

Apparatus and Materials

Data in the tables below.

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. You should display or provide the data from table A: energy stored in food. Information given on the packet tells you the energy stored in different foods. Students can be asked to compare the energy stored in different foodstuffs.
2. Display or provide the data from the next two tables, Table B (human energy demands) and Table C (data for a coal miner). The miner was 32 years old, 1.75 m tall and had a mass of 67 kg.

Table A: Energy stored in food

Table B: Human energy demands

Table B: Data for a coal miner

Teaching Notes

• From Table A, you might ask students to calculate the energy stored in a recent day's food. They will need to know the approximate mass of a normal serving.
• From Table B you might ask students how much energy they need each 24 hours. The answer depends very much on the sort of person, particularly age and occupation. Table C gives some data for a very physical job.
• The data in these tables could be displayed by a data projector, issued as worksheets, or prepared as wall charts to be left in the classroom or laboratory.

This experiment was safety-tested in January 2006