question_answer

A bob of mass m attached to an inextensible string of length l is suspended from a vertical support. The bob rotates in a horizontal circle with an angular speed \[\omega \]rad/s about the vertical. About the point of suspension:

JEE Main  Solved  Paper-2014

A) angular momentum changes in direction but not in magnitude.

B) angular momentum changes both in direction and magnitude.

C)  angular momentum is conserved.

D) angular momentum changes in magnitude but not in direction.

Correct Answer: A

Solution :

\[{{\vec{\tau }}_{net}}={{\vec{\tau }}_{T}}+{{\vec{\tau }}_{mg}}\]                            \[={{\vec{\tau }}_{mg}}\ne 0\] \[\vec{L}\ne \]constant But as \[\omega \]is constant \[\left| {\vec{L}} \right|=MVL=\]constant