1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about Newton's Law

  1. Jul 7, 2010 #1
    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. jcsd
  3. Jul 7, 2010 #2

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Question Newton's Date
Conceptual questions about Newton's Laws... Feb 5, 2018
Basic Inertia/Newton's Laws Questions Nov 1, 2017
Magneto-static force and Newton's 3rd Law Aug 9, 2017