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Homework Help: Torque about P (Cross Product?)

  1. Feb 11, 2013 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    The book says that

    Torque = the length of the moment arm X the magnitude of the vertical component of F
    = r X F

    3. The attempt at a solution

    My problem is that I don't have any vectors to do a cross product. Instead I tried to just multiply what I know:

    50lbs x 9in x sin60° ≈ 389.71ft-lb

    which is far from the correct answer, 32.5ft-lb. The problem seemed simple enough so am I making some small mistake or going about this completely wrong?
  2. jcsd
  3. Feb 11, 2013 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Just because you aren't given any explicit vectors doesn't mean you can't formulate your own.

    Looking at the problem, suppose that we set up an x-axis along the wrench.
    In this case, vector r is (9,0) inches, with the emphasis on inches

    The force vector F is going to be (50 cos (-60), 50 sin (-60)) lbs.

    You can then evaluate r x F in the usual manner

    Remember: in x lbs is not equal to ft-lbs
  4. Feb 11, 2013 #3
    Ah, you're right! Thank you, that makes perfect sense.
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