Question

A man of 80 kg mass is standing on the rim of a circular platform of mass 100 kg rotating about its axis. The mass of the platform with the man on its rotates at 12 r.p.m. If the man moves to the center of the platform, the rotational speed (take platform as a circular ring)

• A. 21.6 rpm
• B. 22.4rpm
• C. 23.6 rpm
• D. 24.2 rpm

Answer 1

The correct option is A

21.6 rpm

According to conservation of angular momentum,
$\Rightarrow L_i=L_f$
$\Rightarrow \left[\right(m_{Man}\left)\right(R^2\left)w\right]+\left[\right(m_{platform}\left)R^{2}w\right]=\left(m_{platform}\right)R^2{w}^1$
$\Rightarrow 80R^{2}w+100R^{2}w=100R^2{w}^1$
$\Rightarrow 180R^{2}w=100R^2{w}^1\Rightarrow w^1=\frac{}{1}80\times 12100=21.6rpm$