A thin circular ring of mass M and radius r is rotating about its axis with a constant angular velocity . Four objects each of mass m, are kept gently to the opposite ends of two perpendicular diameters of the ring. The angular velocity of the ring will be :
- M 4m
- M M + 4m
- (M + 4m) M
- (M - 4m) M + 4m
Answer
(2) M M + 4m
Solution
No external torque acts ⇒ angular momentum is conserved: I1 = I2 '
I1 = Mr2, I2 = Mr2 + 4mr2 = (M + 4m)r2
⇒ Mr2 = (M + 4m)r2 '
⇒ ' = M M + 4m
Answer: (2)