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Homework Help: Force below horizon

  1. Feb 22, 2010 #1
    1. The problem statement, all variables and given/known data

    Find the acceleration of the crate when it is pushed by the same force at an angle of 30 degree below the horizon.

    Given mass = 2.0kg Applied Force = 10 N, and uk = 0.047

    2. Relevant equations

    Fx/y = ma = sum of all force vectors
    Fk = ukN

    3. The attempt at a solution

    So I did a free body diagram. Here is my attempt

    <Fx = ma
    <Fy = 0

    Fx = ma = Fcos(-30) * Fk
    Fy = 0 = Fsin(-30)+N-mg

    Is that it? Anything tricky?

    I just want to make sure all my setups are correct.
  2. jcsd
  3. Feb 22, 2010 #2


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    Homework Helper

    That's right, but to prevent confusing yourself, you might want to express everything in terms of magnitudes. So ma=Fcos(30)*Fk: the magnitude of ma is equal to the magnitude of Fcos(-30)*Fk. N=mg+Fsin(30), because the ground pushes up against both gravity and the y component of the force F. I find that this approach is much more intuitive than using Fcos(-30) and Fsin(-30).

    EDIT: Oops, that's not right. You forgot to account for the contribution of gravity to the normal force.
  4. Feb 22, 2010 #3
    Why are you multiplying the x-component of the applied force by the force of friction?

    didn't he do that here?
  5. Feb 22, 2010 #4
    yeah that was a typo. it was suppose to be minus
    Fx = F - Fk
    and Fk also has x, y components, which is along the x axis
    Last edited: Feb 22, 2010
  6. Feb 22, 2010 #5
    Looks good to me. Just make sure you keep your signs straight -- I'd write F + Fk with Fk being a negative vector to denote direction. I do, however, believe you meant the right thing.
  7. Feb 22, 2010 #6
    Hi, thanks for your reply.

    I am sorry, but why is it F + Fk?

    My thought would be that Fk is moving in the opposite direction of Fx.

    Or you meant let Fk = -(uk*N)
    So in general, the vector sum F + Fk
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