Verified Solution • 2025–26 Session moderate

QJEEM20Ph25ec: JEE Main 2020 Physics - Class 11 JEEM20Ph25ec Solution: A person of 80 kg mass is standing on the rim of a circular…

A person of 80 kg mass is standing on the rim of a circular platform of mass 200 kg rotating about its axis at 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 _________.

Step-by-Step Verified Solution

I1ω1=I2ω2{I_1}{\omega _1} = {I_2}{\omega _2}

⇒\Rightarrow (MR22+mR2)ω1=MR22ω2\left( {{{M{R^2}} \over 2} + m{R^2}} \right){\omega _1} = {{M{R^2}} \over 2}{\omega _2}

⇒\Rightarrow (1+2mR2MR2)ω1=ω2\left( {1 + {{2m{R^2}} \over {M{R^2}}}} \right){\omega _1} = {\omega _2}

⇒\Rightarrow (1+2×80200)ω1=ω2\left( {1 + {{2 \times 80} \over {200}}} \right){\omega _1} = {\omega _2}

⇒\Rightarrow ω2=(1.8)ω1{\omega _2} = \left( {1.8} \right){\omega _1}

⇒\Rightarrow 2 π\pi f2 = 2 π\pi f1 ×\times 1.8

⇒\Rightarrow f2 = 5 ×\times 1.8 = 9

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