Question

A disc of mass ${}^{\prime }{m}^{\prime }$ and radius $R$ is placed on a smooth horizontal floor such that the plane surface of the disc is in contact with the floor. A man of mass $m/2$ stands on the disc at its periphery. The man starts walking along the periphery of the disc. The size of the man is negligible as compared to the size of the disc. Then the centre of disc.

• A. moves along a circle of radius $R$
• B. does not move along a circle
• C. moves along a circle of radius $R/3$
• D. moves along a circle of radius $R/2$

Answer 1

The correct option is C moves along a circle of radius $R/3$

The centre of mass of (man + disc) shall always remain at the rest. Since the man is always at periphery of disc, the centre of disc shall always be at distance $R/3$ from centre of mass of two body system. Hence, centre of disc moves in circle of radius $R/3$.