# Rotational Inertia

1. Mar 1, 2017

### AlexH

1. The problem statement, all variables and given/known data
A solid uniform sphere of radius R and mass M has a rotational inertia about a diameter that is given by (2/5)MR^2. A light string of length 3R is attached to the surface and used to suspend the sphere from the ceiling. What is its rotational inertia about the point of attachment at the ceiling?

2. Relevant equations
mr^2 + md^2

3. The attempt at a solution
I used the parallel axis theorem and got 2/5mr^2 + 9mr^2, which simplifies to 47/5mr^2, but that's not the correct answer.

Thanks for any help!

2. Mar 1, 2017

### TSny

Be sure you know the exact meaning of $d$ in the parallel axis formula.