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**1. Homework Statement**

Find the moment of intertia of a pendulum, consisting of a disc free to spin attached to a rod that is hinged at one end.

**2. Homework Equations**

Moment of intertia of rod hinged at end = (1/3)Ml

^{2}

Moment of intertia of disc = (1/2)mR

^{2}+ ml

^{2}

**3. The Attempt at a Solution**

Why does the answer disregard the

**part**moment of intertia of the disc (1/2)mR

^{2}that is spinning on its own axis, and only taking into account the ml

^{2}?

Here's what the answer wrote:

"If the disk is not fixed to the rod, then it will not rotate as the pendulum oscillates.

Therefore it does not contribute to the moment of inertia. Notice that the pendulum is no

longer a rigid body. So the total moment of inertia is only due to the rod and the disk

treated as a point like object."

I got completely lost by the first sentence. Why does the disc not contribute to the moment of inertia when it is spinning? I thought the idea behind moment of inertia is linked to the rotational kinetic energy it possesses? Oscillating a spinning disc does not rob it of its kinetic energy!

**1. Homework Statement**

**2. Homework Equations**

**3. The Attempt at a Solution**