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A composite disc is to be made using equal masses of aluminium and iron so that it has as high a moment of inertia as possible. This is possible when

1. the surfaces of the discs are made of iron with aluminium inside
2. the whole of aluminium is kept in the core and the iron at the outer rim of the disc
3. the whole of the iron is kept in the core and the aluminium at the outer rim of the disc
4. the whole disc is made with alternate sheets of iron and aluminium

Correct answer: the whole of the iron is kept in the core and the aluminium at the outer rim of the disc

Solution

To maximize the moment of inertia, mass should be distributed farther from the axis of rotation. Placing aluminium, which is less dense, at the outer rim and iron, which is denser, at the core achieves this.

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