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Homework Help: Hanging Rod - Torque Question

  1. Dec 13, 2016 #1
    Hey, folks! So upon studying for my Physics final tomorrow, I came across this question on the practice exam, which I know the answer is D from the solution key. However, I'm not sure how to get that for the answer.

    1. The problem statement, all variables and given/known data


    2. Relevant equations
    τ = rA × Fnet
    LA = Ltrans, A + Lrot
    Ltrans = rcm × Ptot
    Lrot = r1 × p1 + r2 × p2 + ...

    3. The attempt at a solution
    I personally don't entirely know where to start. I'm going to guess this is a torque-related question, but I'm not sure if that's the right way to go at it.
  2. jcsd
  3. Dec 13, 2016 #2


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    The rod is at rest when the force F is applied. What can you say about what the sum of the torques about the hinge must be for bodies at rest? No need to get involved with momentum
  4. Dec 14, 2016 #3
    The net torque would equal zero.
  5. Dec 14, 2016 #4


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    So what are the torques? What equation do you get for a zero sum?
  6. Dec 14, 2016 #5
    Would it just be gravity and the force pushing the rod upwards?
  7. Dec 14, 2016 #6
    I asked one of the graduate teaching assistants about this and he was able to help me figure this out. I simply forgot that the center of mass is where gravity is pulling down on the rod.
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