Efficiency
Energy and Thermal Physics

Trolley collisions

Practical Activity for 14-16 PRACTICAL PHYISCS

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

Efficiency
is used in analyses relating to Engines
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