Rotational Motion

JEE Main level, numerical answer, medium

167 of 280

Question 167 of 280

A person of 80kg80kg mass is standing on the rim of a circular platform of mass 200kg200kg rotating about its axis at 55 revolutions per minute (rpm). The person now starts moving towards the centre of the platform. What will be the rotational speed (in rpmrpm ) of the platform when the person reaches its centre....

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Answer: 9

Applying conservation of angular momentum,
(mR2+MR22)ω=MR22ω′\left({mR}^{2}+\dfrac{{MR}^{2}}{2}\right)\omega =\dfrac{{MR}^{2}}{2}\omega '
(80+2002)×5=2002ω′\left(80+\dfrac{200}{2}\right)\times 5=\dfrac{200}{2}\omega '
ω′=9rev/sec⁡.\omega '=9rev/\sec .

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