Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Work Sign Conventions

  1. Feb 28, 2009 #1
    1. The problem statement, all variables and given/known data
    Is work done by something always negative and work done on something always positive, or do you determine that by the cos theta portion of the work equation. Also, can potential energy be negative

    2. Relevant equations

    W=Fdcos theta
    3. The attempt at a solution

    I've been generalizing, and sometimes it makes me come out with the wrong sign, so that's why I need clarification.
  2. jcsd
  3. Feb 28, 2009 #2
    I've learned that a negative or a positve merely show direction, and as long as you're consistent throughout the problem, the magnitude of the answer is what matters. For example, the acceleration of gravity is commonly known as -9.8m/s2. However, if you want down to be positive it can be as long as you make up negative. I hope that made sense. :smile:
  4. Feb 28, 2009 #3

    Doc Al

    User Avatar

    Staff: Mentor

    The work done by a force on an object can be positive or negative depending upon the direction of the force with respect to the displacement. If the force and displacement are in the same direction (more generally, if the cosine of the angle between them is positive), then the work done is positive; if they are in opposite directions (if the cosine is negative), the work done is negative.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook