A stone tied to a string is whirled in a uniform circular path in a horizontal plane. What is the work done by the tension in the string after the stone completes half a revolution?
a) Positive and equal to the change in kinetic energy
b) Zero because the tension is always perpendicular to the displacement
c) Tπr where T is the tension and r is the radius of the circle
d) Cannot be determined without knowing the speed