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Homework Help: A Moment of Inertia/physical pendulum Problem

  1. Mar 18, 2009 #1
    1. The problem statement, all variables and given/known data
    The 14-cm-long wrench in the figure swings on its hook with a period of 0.90 s. When the wrench hangs from a spring of spring constant 380 , it stretches the spring 2.8 cm
    What is the wrench's moment of inertia about the hook?

    cm of wrench is also 14 cm according to pic

    2. Relevant equations
    I = m*g*L*T^2/2pi

    (How I found the mass)
    Fsp = -ks
    Fg = mg

    3. The attempt at a solution
    I=1.08571428*9.8*.14*.028^2/2 pi
    Not the right answer not sure what to do or where to go
    Last edited: Mar 18, 2009
  2. jcsd
  3. Mar 18, 2009 #2
    It would really help if we had the figure; that has a lot of the information in it.

    From the second sentence you should be able to compute the mass of the wrench.

    It is pretty hard to understand how the CM can be 14 cm from the hook when the wrench is 14 cm long. That is why we really need to see the figure.
  4. Mar 18, 2009 #3
  5. Mar 18, 2009 #4
    According to the figure you posted, that 14 cm is the distance from the pivot to the CM, so this is a very, very strange "14 cm long wrench."

    Now write the equation of motion, and use the definition of the radius of gyration in order to express the mass moment of inertia. From the period information you should be able to find the radius of gyration. From there, you can finally go back to the MMOI itself.
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