A person of mass 80 kg stands on the rim of a circular platform of mass 200 kg rotating at 5 rpm about its axis. The person walks to the centre of the platform. The rotational speed (in rpm) of the platform then is .

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9. Platform (disc) inertia Iₚ=1/2(200)R²=100R²; person at rim 80R²; person at centre 0. Conserve angular momentum: (100R²+80R²)(5)=(100R²) n. n=(180×5)/(100)=9 rpm.

A person of mass 80 kg stands on the rim of a circular platform of mass 200 kg rotating at 5 rpm about its axis. The person walks to the centre of the platform. The rotational speed (in rpm) of the platform then is ______.

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