1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    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!

B Newton's Third Law textbook -- question about the sign of a force

  1. Sep 5, 2016 #1
    Imagine that I am pushing on a wall. Then my textbook says that by Newton's third law, ##F_{AB} = F_{BA}##, where I am B and the wall is A. Isn't this wrong? Shouldn't it be that ##F_{AB} = -F_{BA}##?
     
  2. jcsd
  3. Sep 5, 2016 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Sure, if they are using ##F## to represent the forces as vectors. Perhaps they were just talking about the magnitudes of the forces, which are equal.
     
  4. Sep 5, 2016 #3
    Ah, I see. I guess it can get kind of ambiguous.
     
  5. Sep 5, 2016 #4

    jtbell

    User Avatar

    Staff: Mentor

    Most introductory textbooks use boldface or arrows on top to denote vectors: $$\mathbf{F}_{AB} = -\mathbf{F}_{BA}\\{\vec F}_{AB} = - {\vec F}_{BA}$$ versus unadorned italics for magnitudes (scalars): $$F_{AB} = F_{BA}$$
     
  6. Sep 5, 2016 #5

    PeterO

    User Avatar
    Homework Helper

    Newton's Third Law: For every force acting, there is an equal and opposite force acting. Equal in magnitude, opposite in direction.
    Since the "opposite in direction" is often taken as a given, other descriptions generally concentrate on the magnitude, and so FAB = FBA.
     
  7. Sep 6, 2016 #6

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    If there is a diagram showing the opposite arrows, they are often meant to indicate the convention for positive direction of each force individually. So the later math doesn't use vectors in one coordinate system, but factors for the indicated unit vectors. A negative result then indicates that the force is opposite to its arrow in the diagram.
     
  8. Sep 6, 2016 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The thing is, for a given object (free body diagram) that force only applies once. If you stick with the data you've been given in any question or practical situation, the sign of any force is either given or is calculable. You can give an 'unknown' an arbitrary sign and the calculation will produce the correct sign eventually.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Newton's Third Law textbook -- question about the sign of a force
Loading...