JEE MAIN - Physics (2014 (Offline) - No. 26)
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:
angular momentum is conserved
angular momentum changes in magnitude but not in direction.
angular momentum changes in direction but not in magnitude.
angular momentum changes both in direction and magnitude.
Forklaring
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Torque working on the bob of mass $$m$$ is, $$\tau = mg \times \ell \,\sin \,\theta .$$ (Direction of torque by the weight is parallel to plane of rotation of the particle)
As $$\tau $$ is perpendicular to the angular momentum $$\overrightarrow L$$ of the bob, so the direction of $$L$$ changes but magnitude remains same.