# Homework Help: Help tangent axis

1. Oct 27, 2004

### envscigrl

Help!!! tangent axis

Use the parallel-axis theorem to find the moment of inertia of a solid sphere of mass M=3.80kg and radius R=2.30m about an axis that is tangent to the sphere.
I am being thrown off this problem because the axis is TANGENT to the sphere and not through the center. According to the equations in my book i thought the equation I would use would be:
I= Icm +Mr^2= 3/2 Mr^2
but it didnt work! am i not using the parallel axis right??
Thanks for helping!

2. Oct 28, 2004

### maverick280857

If memory serves me, the moment of inertia of a solid sphere about an axis passing through the center of mass is

$$I_{cm} = \frac{2}{5}MR^2$$

Add to it $$MR^2$$ to get the answer. How are you getting $$\frac{3}{2}MR^2$$??

Remember an axis parallel to one passing through the sphere is clearly a tangent to the sphere from geometry. The parallel axis transformation changes your axes all right but the new axis of rotation is parallel to the one you started out with. So that shouldn't be a problem.

Cheers
Vivek

Last edited: Oct 28, 2004