Sometimes, we can't measure the moment of inertia directly. For example if the object is oddly shaped. In such cases, we can use an indirect way to measure the moment of inertia - this is described in the Introduction of the lab book here (we recommend you read/watch this before answering the questions).

The general principle is that a falling block causes the disc to rotate, such that at any time

gravitational PE lost by block-earth =     linear KE gained by block        +    rotational KE gained by disc

If is the moment of inertia of the disc and is its angular velocity, which expression allows you to calculate the rotational energy of the disc?