# Physics - Moments of Inertia, Angular Momentums etc HELP!

1. Apr 16, 2007

### thebigbang

1. The problem statement, all variables and given/known data

A large disk of mass M and radius R is spinning like a CD of merry-go-round - that is, about an axis perpendicular to the plane of the disk. It is rotating at an angular velocity (omega initial). At some instant, a sphere of mass M/4 , which is initially not rotating, is dropped onto the disk at a distance of 3R/ 4 from the center. The sphere sticks to the disk and begins rotating with it. Find the final velocity of the disk-sphere combination.

2. Relevant equations

solid cylinder = I = 1/2 mr squared

solid sphere - I = 2/5 mr squared

(not sure if these are the correct inertia formulas)

3. The attempt at a solution

i have no idea...i don't even know how to start the problem....should i find inertia of disk first?

2. Apr 16, 2007

### Dick

Total angular momentum I*angular velocity is conserved. You should treat the sphere as a point mass. You aren't given its radius - so you can't truly treat it as a sphere.