A small block on a frictionless horizontal surface has a mass of 0.0215 kg. It is attached to a massless cord passing through a hole in the surface. (See the figure below (Figure 1).) The block is originally revolving at a distance of 0.280 m from the hole with an angular speed of 1.85 rad/s. The cord is then pulled from below, shortening the radius of the circle in which the block revolves to 0.200 m. You may treat the block as a particle.

Question

A.) Is angular momentum conserved?
B.) How do you know? Match the words in the left column to the appropriate blanks in the sentence on the right. Make certain the sentence is complete before submitting your answer.
C.) What is the new angular speed?
Express your answer in radians per second.
D.) Find the change in kinetic energy of the block.
Express your answer in joules.
E.) How much work was done in pulling the cord?
Express your answer in joules.

I need help solving this problem. I would want a step-by-step explanation so that in the future, I would be able to understand it.

Answer 1

A) Yes
B) The angular momentum is conserved because the torque by the acting
force is 0 as r=0.
C) mvr = conserved
So find the new v and divide by new r to get the answer
D) Change in K.E = (1/2)m(v.v-u.u)
E) Work Done=Change in K.E

Answer 2

Thanks for the solution!

Answer 3

I only need the answers for D and E.