Newton's Third Law
Forces and Motion

Stacks of trolleys

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

Seeing a collision from a different point of view.

This demonstration brings to life the observation that physics ought to be consistent, no matter how fast you pass it by. That is, Charlie ought to agree with Bob and Alice, who are moving past at their own different velocities, about the underlying rules of the situation.

As a result it's bringing out the underlying rule of the conservation of momentum that ought to be the focus.

What to Prepare

  • two interacting vehicles on a track (blown apart by magnets, or drawn together by thin elastic)
  • a means of altering the mass of these vehicles
  • a means of trundling the whole track along at different constant velocities
  • possibly some small video cameras (so as to be able to help in taking different points of view) and a means of displaying the resulting video clips two or three at a time

What Happens During this Activity

Perform a series of explosions and collisions with the track stationary. Rehearse the expected outcomes, using the kind of symmetry arguments presented in the Physics Narrative, and show that these do indeed happen.

Then mount the track, so that the same explosions and collisions can be performed with the whole apparatus trundling along. Ask if Alice, firmly seated on the original track, will notice any difference? What about Bob, seated on one of the vehicles, or even Charlie, seated in our laboratory? (As you seek to answer these questions, you may want to place plastic people at these locations, to keep in mind what the different points of view are.) Small video cameras at each location may help. It depends on how well you tell the story, the difficulties your students have in switching from one point of view to another, and the slickness within which you can operate the technology. Some prepared clips may also help.

You may also like to use the interactive on seeing collisions from different perspectives presented in the Physics Narrative.

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