Question

A particle of mass m is attached to an inextensible massless string, and moves on the horizontal surface of a frictionless tabletop. The other end of the string passes through a hole in the tabletop, and is held taut by me. Initially the particle is moving in a circle with radius r0 and angular speed !0. I start to pull the string through the hole at a steady rate. Answer the following questions: a) Of kinetic energy and angular momentum, which one is conserved and which one is not? Explain how the tension force on the particle acts to change one but not the other

Answer 1

KE is not conserved because it takes work to pull on the string

(KE is a scalar and not a vector quantity)

Momentum is conserved because any force acting on the particle is perpendicular to its direction of motion