A spherical shell and thin cylindrical shell with the same mass and radius roll at the same velocity side by side on a level surface without slipping. If the two objects subsequently approach and roll up an inclined plane without slipping, how much higher will the cylindrical shell go before they both stop?
By conservation of energy
At the bottom of the ramp, both objects will have translational and rotational KE
At the height, they will both have PE
Ke rotational = .5Iw2
I for the cylindrical shell = MR2 and w = v/r, so...
KE rotational = .5(mr2)v2/r2 which simplifies to .5mv2
Thus for the cylindrical shell by conservation of energy...
mgh = .5mv2 + .5mv2 (mass cancels)
gh = v2
For the spherical shell, I = (2/3)(mr2), so
KE = .5(2/3)(mr2)v2/r2) = (1/3)(mv2)
So for the conservation of energy here, we have...
mgh = (1/3)(mv2) + .5mv2 (mass cancels)
gh' = 5/6(v2)
Set the ratio...
gh/gh' = v2/(5/6)v2
h/h' = 6/5
Thus the cylinderical shell will go 6/5 ths higher (that is 1.2 times higher)
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