Efficiency
Energy and Thermal Physics

A trolley started and stopped by catapults

Practical Activity for 14-16 PRACTICAL PHYISCS

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

Efficiency
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