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Homework Help: Inertia of a Giraffe in Terms of a Monkey (I'm serious)

  1. Oct 28, 2014 #1


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    1. The problem statement, all variables and given/known data
    For my second thread of the day I will give the weirdest problem I've encountered in physics yet.
    In the mobile what is the inertia of the giraffe mgir in terms of the inertia mm of the monkey?
    2. Relevant equations
    I = md2

    3. The attempt at a solution
    In the picture, the giraffe and monkey are on their own rod type thing, each 2-d away. So the inertia of both are mm 4 d2 = Im and mgir 4 d2 = Igir. It says to find the inertia of the giraffe in terms of the inertia of the monkey, but I don't really have any equations that relate the two as far as I know (other than the inertia of the whole system, but I don't know what that is equal to and that seems like it would lead me down a fruitless path). Can I even calculate inertias of objects individually? This isn't too clear to me.

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  2. jcsd
  3. Oct 28, 2014 #2


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    I think you are supposed to assume the mobile will hang balanced if still. This allows you to deduce the relationship between the three masses.
    You don't state explicitly an axis for rotational inertia. There's no physical axis that they are both 2d from. Have you posted the whole question, word for word? Are you sure it's asking about rotational inertia?
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