Newton's Third Law
Forces and Motion

Action and reaction: trolleys

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

Convincing kinaesthetic experience that forces come in pairs.

Apparatus and Materials

  • Large demonstration trolleys or skateboards or roller skates, 2
  • Rope, 6 m in length

Health & Safety and Technical Notes

If a level section of playground is not available, this activity is better done in the school hall.

Read our standard health & safety guidance


Since the motions of the two trolleys are essentially the same in all three cases, it is not necessary to make friction negligible – nor is that possible with large trolleys or roller skates. However, it is important to make the friction drags as regular and repeatable as possible. And it is good to make them relatively small.

Procedure

  1. Arrange two students, A and B, on trolleys or roller skates, far apart at the sides of the room facing each other so that they can move towards each other on their wheels as freely as possible. Mark definite starting-points on the floor.
  2. Get the students to hold the rope taut between them. Tell them not to ‘skate’ but just use the rope as follows:
    • Student A merely holds the rope whilst B hauls the rope in until the trolleys collide. Both come to rest and their positions are noted.
    • Repeat this, but with student A hauling whilst student B merely holds the rope.
    • Finally, both students haul on the rope.

Teaching Notes

  • Take care to see that both trolleys start each time from the same position. It will then be found that the collision occurs at the same place however the pulling is done.
  • The experiment could be tried with two students on each trolley, or with two on one trolley and one on the other.

This experiment was safety-tested in April 2006

Newton's Third Law
is used in analyses relating to Collisions

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