A ball of mass m is tied to a vertical (cyclinder) post of radius R. The mass slides on a frictionless, horizontal surface. As the ball travels around the post the string wraps around the post, shortening the length. At t = 0s, its speed is v(initial) and the distance from the axis of rotation is r(initial). Assume the ball is a point particle when calculating its inertial mass.

What is the angular momentum of the ball after the string has wrapped once around the post?

Please help me thank you.

Question

A ball of mass m is tied to a vertical (cyclinder) post of radius R. The mass slides on a frictionless, horizontal surface. As the ball travels around the post the string wraps around the post, shortening the length. At t = 0s, its speed is v(initial) and the distance from the axis of rotation is r(initial). Assume the ball is a point particle when calculating its inertial mass. 

What is the angular momentum of the ball after the string has wrapped once around the post?

Please help me thank you.

anonymous · Answer

The angular momentum is conserved in this situation as there is no net force acting on the ball  perpendicular to the axis of rotation(direction of pole). Hence no torque acts on the ball so angular momentum is conserved.
Hence final angular momentum = Initial angular momentum = mvr

anonymous · Answer

thank you