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Work on an object moving downards

  1. Oct 19, 2015 #1
    1. The problem statement, all variables and given/known data
    You move a textbook from a high shelf to a low shelf. Is the work you do on it positive or negative?

    2. Relevant equations
    W = Fcosd

    3. The attempt at a solution
    My understanding is that if you are doing a positive applied force on the textbook, over some distance, the work should be positive. But the answer is negative. Why?
     
  2. jcsd
  3. Oct 19, 2015 #2
    In what direction is the force YOU exert? Compare this with the direction of motion.
     
  4. Oct 19, 2015 #3
    The motion is upwards, the direction is downwards.
     
  5. Oct 19, 2015 #4
    That's not how you phrased the question:

    From a high to a low shelf means the motion is downward. The force you exert is upward. Therefore the work you do is negative (force and displacement are in the opposite direction i.e. θ = 180)
     
  6. Oct 19, 2015 #5
    Right, sorry! I meant the force is upwards. Yes, I understand now. Thanks!
     
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