A stone of mass m is attached to a string and moves round in a horizontal circle of radius R at constant?

Question

speed V. The work done by the pull of the string on the stone in one complete revolution is

A. zero.
B. 2πmVsquared
C. 2πmVsquared divided by R
D. 2πmv divided by R

Answer 1

A: Zero.

A simple way to see this:
Work = force × displacement, and displacement is 0 after one complete revolution.

In fact, the work done is 0 at any time, not just after a whole number of revolutions:
At any given point, the force being applied is perpendicular to the direction of motion. So the motion in the direction of the force is 0 at every point along the path, and hence no work is done by the centripetal force. (In calculus terms, the work element is F.ds, and F and ds are always perpendicular so their dot product is 0.)