# A trolley started and stopped by catapults

##### 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).

The runway must be adapted, as described in the experiment Energy carried by a moving trolley using a runway with a catapult at each end.

#### Procedure

a The runway should be compensated for friction. (See the Compensating for Friction experiment.)

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.

Repeat this calibration for the second catapult.

b 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 in each catapult.

c An extension experiment is to calculate the kinetic energy of the trolley, 1/2 mv2. Do this by measuring the trolley's velocity (using a stop watch, light gates or ticker-timer) and mass, and comparing this with the strain energies.

#### Teaching notes

1 When a catapult is pulled back by the trolley and released, the energy stored in the rubber band is carried by the kinetic energy of the moving trolley. When the trolley stretches the opposite band, the energy carried by the trolley becomes strain energy 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 strain energy of the first rubber band has been carried to the second.

2 If the rubber bands are not identical, the energy stored 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: an elastic store of energy.

This experiment was safety-checked in November 2005