Question

Angular momentum of the disc will be conserved about

• A. centre of mass
• B. point of contact
• C. a point at a distance $3R/2$ vertically above the point of contact
• D. a point at a distance $4R/3$ vertically above the point of contact

Answer 1

The correct option is C a point at a distance $3R/2$ vertically above the point of contact

From equation of motion we obtain,

$m{A}_{cm}=F-f$($f$ is the friction)

and $\frac{m{R}^2\alpha }{2}=fR$

$A_{cm}=fR$

Thus $f=F/3$

Angular momentum is conserved about that point about which torque is zero due to these F and f forces.

From above result we conclude that torque is zero about a point at a distance 3R/2 vertically above the point of contact.