Discussion Forum



1)

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 ω  rad/s about the vertical support, About  the point of suspension


A) angular momentum is conserved

B) angular momentum changes magnitude but not in direction

C) angular momentum changes direction but not in magnitude

D) angular momentum changes both in direction and magnitude

Answer:

Option C

Explanation:

Angular momentum  of the pendulum  about the suspension  point O is

1532021479_m9.JPG

$L=m(r\times v)$

 Then, v can be resolved into two components, radial component rrad and tangential component rtan. Due to vrad, L  will be tangential due to vtan, L will be radially outwards as shown.

  So, net angular momentum will be as shown in figure whose magnitude will be constant  (|L|=mvl). But its direction will changes as shown in the figure.

              $L=m(r\times v)$

  where, r= radus of circle.