# Moment of Inertia for Sphere

#### danielatha4

Can someone please tell me the moment of inertia for a solid sphere rotating about an axis tangent to its surface?

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#### zachzach

What does tangent to its surface mean? Where is the axis of rotation?

#### danielatha4

The axis of rotation is touching the surface of the sphere at one point. That is the definition of tangent. Can it be treated as a point particle distance R from the axis?

#### zachzach

Parallel axis theorem?

#### danielatha4

Sorry, never heard of that. It's not the goal of the assignment to figure out the moment of inertia around the axis tangent to the surface, but it would be a useful tool.

#### zachzach

Well look it up, it is not that difficult. It allows you to use the moment of inertia of a sphere (axis through center) to calculate the moment of inertia somewhere else as long as the other axis is parallel to the axis through the center. And finding the moment of inertia with axis through center is in almost every text book and on the internet. http://en.wikipedia.org/wiki/Parallel_axis_theorem

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