Question

A circular plate is rotating in horizontal plane without friction about its own axis with a time period $0.1s$. Another identical circular plate is gently placed on the first plate such that combination of two plates becomes a thick circular plate. The angular velocity of thick plate is:

• A. $120rpm$
• B. $180rpm$
• C. $240rpm$
• D. $300rpm$

Answer 1

The correct option is D $300rpm$

Let N be the initial rpm ,

Also $T=60/N$ so $N=600rpm$

Let the initial MI =$I$ and final will be $2I$

Using Angular Momentum Conservation,

We have $I\times 600=2I\times N_f$

$N_f=300rpm$