Question

If a man standing on the periphery of a stationary
disc starts walking in the anticlockwise direction
along the boundary, then
(1) The disc remains stationary
(2) The disc rotates in the clockwise direction
(3) The disc rotates in the anticlockwise direction
(4) The disc rotates in either clockwise or
anticlockwise direction​

Answer 1

Answer:

Starting from rest it reaches 100 rev/sec in 4 seconds. ... If the angular speed of the disc is at some instant, with what speed is the block going down at that instant .... stationary cylinder.

Answer 2

If a man standing on the periphery of a stationary disc starts walking in the anticlockwise direction along the boundary, then the disc rotates in the clockwise direction. Hence the correct option is (2) The disc rotates in the clockwise direction.

→ Explanation:

→ According to the law of conservation of angular momentum, if the net torque acting on a system about a point is zero, then the angular momentum of the system about the point remains constant.

Therefore it can be said that the change in angular momentum of the system is zero if the net torque acting on the system is equal to zero.

→ In the given question:

Initially, the man, as well as the disc, are at rest, hence the initial angular momentum of the system about the centre of the disc is equal to zero.

The man starts to run along the boundary of the disc in an anticlockwise direction. This produces an angular momentum about the centre of the disc in an upwards direction.

To counter the angular momentum produced due to the running of the man, the disc will start rotating in a clockwise direction thus producing an angular momentum in a downward direction about the centre of the disc.

The angular momentum produced due to the running of the man and the rotating of the disc are equal in magnitude but opposite in direction. Hence the net angular momentum remains equal to zero.

Hence we can conclude that, when the man standing on the periphery of a stationary disc starts walking in the anticlockwise direction along the boundary, then the disc rotates in the clockwise direction.

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