A horizontal disc rotates freely about a vertical axis through its centre. A ring, having…

Physics · JEE Advanced · NTA ExamsRotational Motion

A horizontal disc rotates freely about a vertical axis through its centre. A ring, having the same mass and radius as the disc, is now gently placed on the disc. After some time, the two rotate with a common angular velocity, then:
  1. some friction exists between the disc and the ring
  2. the angular momentum of the “disc plus ring” is conserved
  3. the final common angular velocity is 2/3rd of the initial angular velocity of the disc
  4. (2/3)rd of the initial kinetic energy changes to heat

Answer

(A) some friction exists between the disc and the ring, (B) the angular momentum of the “disc plus ring” is conserved, (D) (2/3)rd of the initial kinetic energy changes to heat

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