Homework Help: Translational and rotational kinetic energy-Mass Unknown

1. Nov 6, 2013

Crusader711

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. Nov 6, 2013

tiny-tim

Hi Crusader711!
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.

3. Nov 6, 2013

Crusader711

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?

4. Nov 7, 2013

tiny-tim

(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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