Forces and Motion

Inverse explosion: trolley practical

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

Trolleys colliding inelastically.

Apparatus and Materials

  • Dynamics trolleys, 2
  • Runway
  • Elastic cords, 2
  • Corks
  • Needle or large pin

Health & Safety and Technical Notes

The runways are heavy and long. They need to be handled with care to avoid damage to students or nearby equipment. Runways are best lifted into position by two people. Place a barrier to ensure the trolleys do not roll off the end of the bench.

Read our standard health & safety guidance

The method by which the cork and pins are attached to the trolley will depend on the particular make used. With most, it is easy to wedge a pin so that it sticks out from the trolley. It may be easier to fix a pin on both trolleys and stick a cork on one of them rather than to fix the cork directly on the trolley.

Another way to get the trolleys to stick together after collision is to fix a block of Plasticine onto one trolley with strong insulating tape at the edges. For the other trolley, another piece of Plasticine can be used or a series of drawing pins can be poked through a strip of insulating tape, before that too is attached to the trolley.

Yet another method is to attach double-sided adhesive pads to the trolleys.


  1. Hold a pair of trolleys some distance apart with two elastic cords stretched between them, trying to pull them together.
  2. Release the trolleys simultaneously and watch what motion is left after impact.
  3. Repeat the experiment with one trolley made twice as massive, by placing an extra trolley on top.

Teaching Notes

  • When two equal masses with the same momentum but in opposite directions collide then after the collision the combined masses will be stationary.
  • Two unequal masses, colliding inelastically, will continue to move in the direction that the mass with the greatest momentum was moving before the collision.
appears in the relation p=mv F=dp/dt λ=h/p ΔxΔp>ℏ/2
has the special case Angular Momentum
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