Question

A circular ring of mass $m$ is rotating in horizontal plane about its natural axis at the rate of $5rps$. Three point size bodies each of mass $m$ are gently attached to the rim of this ring at equal distances, then the angular velocity of ring is:

• A. $150rpm$
• B. $100rpm$
• C. $75rpm$
• D. $50rpm$

Answer 1

The correct option is C $75rpm$

Final rps of ring and and point mass will be same

So $m{r}^{2}N=m{r}^2{N}_1+3m{r}^2{N}_1$ (by conservation of angular momentum)

So $4N_1=N=5\Rightarrow N_{=}1.25rpsor1.25\times 60=75rpm$