A thin horizontal disc rotates about a vertical axis through its centre. An insect, at rest near the rim, walks along a diameter to the other end. During the journey, the angular speed of the disc

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(4) first increases and then decreases. Angular momentum is conserved. As the insect moves from the rim towards the centre, the system's moment of inertia falls, so ω rises. Past the centre it moves outward again, raising the moment of inertia, so ω falls. Thus ω first increases, then decreases.

A thin horizontal disc rotates about a vertical axis through its centre. An insect, at rest near the rim, walks along a diameter to the other end. During the journey, the angular speed of the disc

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