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Homework Help: M=rFsinθ Cannot find θ

  1. Oct 17, 2015 #1
    • Thread moved to HH from the technical forums, so no HH Template is shown
    I failed this quiz, and I'm trying to understand b. I need to calculate the magnitude of force P which is necessary to create the same moment (I think it's torque in English?) about point B, knowing that alpha is 10 degrees. The moment in part a was -80Nm.

    I now understand that I should have used the following equation:
    M (about point B) = P * r * sin θ
    80 Nm = P (0.5m) sin θ

    However, I don't understand where theta is. How do I find theta in general for this equation? My TA wrote 120°, but I don't understand how to get this number. My FBD is below, and I wrote my guess at where θ is, but my guess can't be correct. Help me find theta, please.



    Attached Files:

  2. jcsd
  3. Oct 17, 2015 #2


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    It doesn't matter if you use 60° (which you indicated in the sketch as angle between AB and the force) or 180°-60° = 120°, the sine of the angle is the same in both cases.
  4. Oct 17, 2015 #3
    Thank you for your reply, mfb.

    So in the equation
    Moment (in French) = F * r * sin θ
    θ is the angle between F and r?
  5. Oct 17, 2015 #4


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    Staff: Mentor


    Torque is the English word.
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