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!

Negative Force-potential energy relationship?

  1. May 10, 2014 #1
    I've been wondering for a while why force is the negative derivative of potential energy. In our books, they write that F=-dU/dx, and U=-W
    I don't really understand why it should be negative. Doesn't the force need to be positive in order to increase the potential energy? For example, when you apply a positive force to an object (assuming downward is the negative y direction), Ug goes up, right?
    I'm new to this forum, so sorry if I posted this in the wrong section...
     
  2. jcsd
  3. May 10, 2014 #2

    Doc Al

    User Avatar

    Staff: Mentor

    When you apply a positive force, that means the force associated with the potential is applying a negative force. Using your example, while you exert a positive force to raise an object, the force of gravity is negative.
     
  4. May 10, 2014 #3
    Oh, I see, so the "F" in the equation is referring to the force that causes the potential energy, not the force that changes it?
     
  5. May 10, 2014 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Right. For gravitational PE, the F refers to the gravitational force. Similarly for other conservative forces.
     
  6. May 10, 2014 #5
    Ok, thanks. That cleared up a lot.
     
  7. May 10, 2014 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Welcome to PF! :smile:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Negative Force-potential energy relationship?
Loading...