How exactly does Newton’s third law work?

Question

So, from what I understand, Newton’s third law says that for every action, there is an equal and opposite reaction.

I’m having a very hard time wrapping my head around it, as it seems as though if his law worked the way I interprete it to, pushing an object or denting an object would be impossible.

For example,

If someone were to push a book on a table, the force they applied on the book to push it, should be countered by an equal and opposite reaction force, therefore not allowing them to move the book.

In another example, if someone were to push a piece of metal, there should be an equal and opposite reaction force (normal contact force?) which would not allow the person to move the metal (can’t dent it), no matter how much force they use.

I guess what I would like to know would be why this is not the case in the two above examples, and what is it that I am not interpreting correctly with Newton’s third law.

Thank you!!

In: Physics

Answer 1

You are correct to a point. Remember that these laws apply to rigid bodies in idealized conditions and simplified scenarios.

The scenario is describe is sitting in a chair. You sit in a chair, the chair holds you up, the chair provides and equal force to support you.

Your first example – pushing on a book. In this case the force resisting motion is friction. As long as the force you apply to the book is less than the frictional force, the book stays put. Once you get past the frictional force the book starts to move, part of the force you apply gets expended on the friction and the rest gets expended on the book’s acceleration. The two forces are still equal. You should be able to draw a free body diagram that will zero out the forces.

Last example: Newton’s laws apply to rigid bodies. This law is a simplification of reality. And in many cases this simplification does the trick. This law does not hold up once you start applying enough force that the bodies you are applying a force to is no longer rigid (ie it bends or breaks). At that point you are getting into more advanced dynamics and structural mechanics questions.