Q 11-06-192JEE MainJEE Main 2020 (3 Sep, Shift 1)Medium
A person of $80\ \text{kg}$ mass is standing on the rim of a circular platform of mass $200\ \text{kg}$ 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 ______.
Numerical value type. Enter your answer.
Answer: 9
Treat the platform as a uniform disc. Angular momentum is conserved:
$$\left(\frac12\times200R^{2} + 80R^{2}\right)\times5 = \frac12\times200R^{2}\times n$$
$$180\times5 = 100\,n \Rightarrow n = 9\ \text{rpm}$$
Solution by Sreeraj P, M.Sc Physics