# Moment of inertia of a solid sphere

1. Feb 14, 2015

### henry3369

1. The problem statement, all variables and given/known data
Find the moment of inertia of a solid sphere of uniform mass density (like a billiard ball) about an axis through its center

2. Relevant equations
I = ∫rρdV

3. The attempt at a solution
I =ρ ∫r4πr2dr = ρ4π∫r4
Then I integrate this from 0 (the center) to R, so I = (ρ4π)*(R5/5)
And ρ = mv so ρ = M/(4/3)πR3 = 3M/4πR3. Put ρ into the equation for moment of inertia to get I = 3MR2/5.

My book tells me the answer is (2/5)MR^2. Where did I go wrong?

2. Feb 14, 2015

### PeroK

Can you explain how you are setting up the integral? What shape are you integrating from 0 to R?

3. Feb 14, 2015

### henry3369

The surface area of a spherical shell from the center of the sphere to the outer shell of radius R.

4. Feb 14, 2015

### henry3369

I figured it out. If I use spherical shells, the points from the shell aren't all the same distance form the axis of rotation. I was thinking about the distance from the center.

5. Feb 14, 2015

Exactly!