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  4. A person of 80 kg mass is standing on the rim of a circular platform of mass 200 kg rotating about its axis as 5 revolutions per minute (rpm). The person now starts moving towards the centre of the platform. What will be the rotational speed (in rpm) of the platform when the person reaches its centre .

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Q. A person of $80\, kg$ mass is standing on the rim of a circular platform of mass $200 \,kg$ rotating about its axis as $5$ revolutions per minute (rpm). The person now starts moving towards the centre of the platform. What will be the rotational speed (in rpm) of the platform when the person reaches its centre __________.

JEE MainJEE Main 2020System of Particles and Rotational Motion

Solution:

$L _{ i }= L _{ f }$
$\left(80 R ^{2}+\frac{200 R ^{2}}{2}\right) \omega$
$=\left(0+\frac{200 R ^{2}}{2}\right) \omega_{1}$
$180 \omega_{0}=100 \omega_{1}$
$\omega_{1}=1.8 \omega_{0}=1.8 \times 5$
$=9$ rpm