mastering physics :/

[RhysticGenesis]

Ok here is the problem A hollow spherical shell with mass 2.50kg rolls without slipping down a slope that makes an angle of 30.0 degrees with the horizontal.
> Find the acceleration a_cm of the center of mass of the spherical shell.
> Take the free-fall acceleration to be g=9.80m/s^2.
> I got 24.2068 but again I was wrong

- I though it was mgsin(theta) was it not?


> Find the frictional force acting on the spherical shell.
Im not sure but wouldnt it be coefficient of friction (which I do not know) times the normal force (which would be the answer to the above I think?)


Another problem is :
A circular hoop of mass m, radius r, and infinitesimal thickness rolls without slipping down a ramp inclined at an angle theta with the horizontal. View Figure
> http://session.masteringphysics.com/problemAsset/1010918/22/MAD_ia_2_v1.jpg
> What is the acceleration a of the center of the hoop?
> Express the acceleration in terms of physical constants and all or some of the quantities m, r, and theta.

and to be honest I have no clue whatsoever on how to do that problem :/ man Im beginning to hate physics I dont understand it at all
:uhh:

[Erienion]

Well, you know that the mass is 2.5 Kg, now find an equation to find I, the rotational inertia, given that all of the mass is in the spherical shell.