When a stone tied to a string is whirled in a circle, the work done on it by the string:

The correct answer is C, is zero.. For a stone whirled in a circle, the string pulls it towards the centre, while the stone moves along the circle, tangent to it at every instant. The tension and the displacement are therefore always at 90 degrees to each other. Work done equals force times displacement times the cosine of the angle between them, and cos 90° is zero, so the string does no work at all. That is why, in uniform circular motion, the speed and kinetic energy of the stone stay the same; the string only keeps changing the direction of motion. Option A is wrong because positive work would raise the stone's kinetic energy. Option B is wrong because negative work would slow the stone down. Option D is wrong because zero multiplied by any mass is still zero, so the mass of the stone does not matter. Exam tip: a centripetal force is always perpendicular to the motion, so it does no work; gravity on a satellite in a circular orbit is the same case.