Energy and moving trolleys

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