# Homework Help: Question about Newton's Law

1. Jul 7, 2010

### llamajuice

Newton's third law says that for every action force there's a simultaneous reaction force equal in magnitude to the action force, but in the opposite direction. So doesn't that make the net force zero? If that's true how does anything move?

2. Jul 7, 2010

### D H

Staff Emeritus
The third law says that the force exerted by object A on object B is equal but opposite to the force exerted by object B on object A. Note very well: Those equal but opposite forces act on different bodies. The acceleration undergone by object A depends only on the forces that act on object A. That object A is also exerting forces on other objects does not affect the acceleration of object A, at least not directly. The connection is indirect.

Suppose A and B are the only objects in an otherwise empty and very large region of space. Whatever other forces are acting on A and B from outside that large region of space is so small that it can be ignored. A will accelerate because B is exerting a force on A, and B will accelerate because A is exerting a force on B. The nature (magnitude and direction) of these forces will change over time, so what A does to B now will change what B does to A in the future.