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Work vs. Kinetic Friction

  1. Oct 20, 2008 #1
    1. The problem statement, all variables and given/known data
    A 1.20 x 102 kg crate is being pushed across a horizontal floor by a force P that makes an angle of 30.0° below the horizontal. The coefficient of kinetic friction is 0.450. What should be the magnitude of P, so that the net work done by it and the kinetic frictional force is zero?

    2. Relevant equations
    W = FD

    3. The attempt at a solution
    I set the work done by friction 1200 x 9.8 x .450 x D equal to the work done by P which is P x cos 30 x D. I then canceled D out of both sides, and solved for P, and got 611.068 N. What did I do wrong?
  2. jcsd
  3. Oct 20, 2008 #2
    1.20 x 10^2 isn't 1200.
  4. Oct 20, 2008 #3


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    There is not only the weight but also the Sin30 component of the downward force that goes into calculating the normal force that determines frictional resistance.

    Cos30*P = u*m*g + u*Sin30*P is what you need to solve.

    Oh and 1.2 x 102 is 120 as already noted by JoAuSc.
  5. Oct 20, 2008 #4
    Oops, sorry about that. That was only a mistake in my typing; in my calculations I used 120.
  6. Oct 20, 2008 #5
    Ohhhh...I get it. Thank you so much for the help.

    Edit: So if the 30 degree angle was pulling above the horizontal, I would subtract that from the normal force? In this case it's the opposite, but that's the general concept that I was missing, right?
  7. Oct 20, 2008 #6


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  8. Oct 20, 2008 #7
    Thank you for your help.
  9. Oct 21, 2008 #8
    Edit: Nevermind.
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