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

Options

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.

Answer

A

Explanation

Let T is the tension in string.
Two forces are responsible for developing the torque about point of suspension of bob
`vectau_(T)=0,r=lsintheta`
`vectau_(mg)=mglsintheta`
`vectau_("net")=vectau_(T)+vectau_(mg)=vectau_(mg)ne0`
`:.vectau_("net")=(dvecL)/(dt)ne0`
`impliesvec(L)ne` constant
But as `omega` is constant
`|vecL|=mvl` =constant
`vectau` of mg is perpendicular to `vecL`. Hence magnitude of `vecL` is constant but direction will change.