Rotation 2 Question 4
4. A ring of mass and radius is rotating with angular speed about a fixed vertical axis passing through its centre with two point masses each of mass at rest at . These masses can move radially outwards along two massless rods fixed on the ring as shown in the figure.
At some instant, the angular speed of the system is
(2015 Adv.)
(a)
(b)
(c)
(d)
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Answer:
Correct Answer: 4. (d)
Solution:
- Let the other mass at this instant is at a distance of
from the centre . Applying law of conservation of angular momentum, we have
Solving this equation, we get
NOTE If we take identical situations with both point masses, then answer will be (c). But in that case, angular momentum is not conserved.