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Pivoted rod

  1. May 2, 2013 #1


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

    A uniform rod of length 1.6 m and mass 20 kg is free to rotate about a frictionless pivot at one end in a vertical plane. The rod is released from rest in the horizontal position. The acceleration of gravity is 9.80 m/s2.

    2. Relevant equations

    What is the resultant force in the pivot immediately after the release?

    3. The attempt at a solution

    I know that the Moment of Inertia around such a rod is 1/3*m*L^2 and that the tourqe around that rod is L/2*mg, this gives me the angular acceleration 9.1875. I assume that the normal force in this situation is zero?

    That would only give me the tangental force F_t = m*r_cm*a left (since F_n = m*r*w^2, but w = 0), which gives me the answer 147 N, but the answer should be 49 N
  2. jcsd
  3. May 4, 2013 #2


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    hi fkf! :smile:
    hint: 49 + 147 = 196 :wink:
  4. May 4, 2013 #3


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    What are you calling the normal force here? What is the linear acceleration of the rod's centre?
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