# Rotational inertia

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1. Oct 18, 2015

### kolua

If there is a disk of mass M and radius R that is already rotating, then someone puts a block with mass m on it a meters away from the center of mass.
What is the rotational inertia then?

Is it I=(M+m)R^2?

2. Oct 18, 2015

### tommyxu3

The moment of inertia is additive. The original moment of inertia of the disk should be $\frac{1}{2}MR^2$ and the block's is $m\cdot (1m)^2$ if it's small enough for us to ignore its volume. The answer should be the sum of above.