Gujarati
Hindi
6.System of Particles and Rotational Motion
hard

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

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

Solution

The direction of $\vec{\omega}$ vector

is always $\perp^f$ to the plane

containing $\quad \overrightarrow{\mathrm{r}} \& \overrightarrow{\mathrm{v}}, \quad$ as

shown in fig. (at any instant).

Angular momentum

$\overrightarrow{\mathrm{L}}=\mathrm{I} \vec{\omega}$

Obviously, $\overrightarrow{\mathrm{L}}$ changes, as

$\vec{\omega}$ changes in direction but

$\overrightarrow \tau   \Uparrow d\overrightarrow \theta  $

$\therefore$ No work is done by torque due to weight of bob.

So $|\vec{\omega}|=$ constant

or $|\overrightarrow{\mathrm{L}}|=$ constant

Standard 11
Physics

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