Question

A horizontal circular platform of mass 100 kg is rotating at 5 r.p.m. about vertical axis passing through its centre. A child of mass 20 kg is standing on the edge of platform. If the child comes to the centre of platform then the frequency of rotation will become ______.

Options

• 7 r.p.m.

• 9 r.p.m

• 5 r.p.m

• 12 r.p.m

Answer 1

A horizontal circular platform of mass 100 kg is rotating at 5 r.p.m. about vertical axis passing through its centre. A child of mass 20 kg is standing on the edge of platform. If the child comes to the centre of platform then the frequency of rotation will become 7 r.p.m.

Explanation:

`(1/2 "Mr"^2 + "mr"^2)omega_1 = 1/2 "Mr"^2 omega_2`

`therefore omega_2 = ((1/2 "Mr"^2 + "mr"^2)omega_1)/(1/2 "Mr"^2)`

`= ((1/2 "M + m")omega_1"r"^2)/(1/2 "Mr"^2)`

`= (50 + 20)/50 xx 5`

`= (70 xx 5)/50`

= 7 r.p.m.