Teaching Physics with the Physics Suite Edward F. Redish

When does N3 hold?

Newton's third law says that objects that touch each other exert forces on each other that satisfy:

If object A exerts a force on object B, then object B exerts a force back on object A and the two forces are equal and opposite

Consider the following three situations concerning two identical cars and a much heavier truck.

1. One car is parked and the other car crashes into it.
2. One car is parked and the truck crashes into it.
3. The truck is pushing the car, because the car's engine cannot start. The two are touching and the truck is speeding up.
In which of these situations do you think Newton's 3rd law holds or does not hold? Explain your reasons for saying so.

Note to the instructor: Surpisingly, even when told Newton's third law clearly and with the statement "this always holds," many students will still assume that it only holds in cases of symmetry. The FMCE has a set of nice multiple choice questions probing this. You may be surprised if you ask your students this essay question on an exam.

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Page last modified October 8, 2002: D09