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Homework Help: Translational and rotational kinetic energy-Mass Unknown

  1. Nov 6, 2013 #1
    A hollow, thin-walled sphere (ICM = 2MR2/3) of mass 20 kg is completely filled with a liquid of unknown mass. The sphere is released at the top of a plane inclined at 30° to the horizontal, and it rolls 20 m to the bottom in 3.6 s. What is the mass of the liquid?

    2.My approach is translational and rotational kinetic energy, first off. I know that for rolling spheres the v-cm is not dependent upon the mass nor radius of the objects. So I come up with a translational speed but I'm not certain where to go from there. But the problem wants the mass of the liquid. I have two different sphere's in theory, a solid and a thin-walled version.

    3. Looking for a lead into the next step...
  2. jcsd
  3. Nov 6, 2013 #2


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    Hi Crusader711! :smile:
    for a particular shape, yes
    But you're given the moment of inertia anyway.

    Since we're not told the radius of the sphere, I think we're supposed to assume that the liquid does not rotate.

    So find the acceleration, call the mass of the liquid "m", and carry on from there. :smile:
  4. Nov 6, 2013 #3
    Rolling verses Frictionless

    Would we treat the spheres differently?

    thin-walled sphere rolling...

    ...then solid sphere with liquid, liquid sphere moving down incline (not rolling), but we have to account for the ICM of the thin-walled shell too?

    Any thoughts?
  5. Nov 7, 2013 #4


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    (just got up :zzz:)

    i assume "ICM" means moment of inertia?

    you are given the moment of inertia and the mass, why do you need to know anything else?
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