Question

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 a angular speed $\omega \text{rad/s}$ about the vertical. About the point of suspension

• A. Angular momentum changes in magnitude but not in direction
• B. Angular momentum changes both in direction and magnitude.
• C. Angular momentum is conserved
• D. Angular momentum changes in direction but not in magnitude

Answer 1

The correct option is D Angular momentum changes in direction but not in magnitude

Torque of weight about point of suspension. ${\tau }_{weight}=mg\times lsin\theta$
Direction of torque by weight is parallel to the plane of rotation of the particle.

As $\overrightarrow{\tau }$ is perpendicular to the angular momentum of the bob so the magnitude of angular momentum remains the same but direction changes.