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Blocks on a Ramp Question

  1. Sep 30, 2007 #1
    1. The problem statement, all variables and given/known data
    In Fig. 6-35, blocks A and B have weights of 51 N and 23 N, respectively. (a) Determine the minimum weight of block C to keep A from sliding if μs between A and the table is 0.23. (b) Block C suddenly is lifted off A. What is the acceleration of block A if μk between A and the table is 0.12?
    **image is attached**

    2. Relevant equations
    fxnet= T-fs=ma
    fymet= FN-Wc-Wa=0


    3. The attempt at a solution

    I completed part (a) and got 4.8 to be the answer, however, I didnt get part (b), I tried to use the tension I got from part (a) and put it in the 3rd equation above and got -14,472 which is obviously not correct. Can anyone tell me what I'm doing wrong?

    Attached Files:

  2. jcsd
  3. Sep 30, 2007 #2

    Doc Al

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    Staff: Mentor

    The tension is not the same once things start accelerating. You'll have to set up equations for both masses and solve for the acceleration (and new tension).
  4. Sep 30, 2007 #3
    Okay, I did that... the second two equations were Fxnet= T-Uk(WA)=ma and Fynet= Wb-T=ma and when I added them together I got Wb - Uk(Wa)=(ma + mb)a and then solved for a and got 0.23 for the answer, and it was yet again wrong. What am I doing that's not right again?
  5. Sep 30, 2007 #4

    Doc Al

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    Staff: Mentor

    Be sure when calculating ma + mb that you are adding the masses and not the weights.
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