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

When a stone tied to a string is whirled in a circle, the tension in the string provides the necessary centripetal force to maintain the circular path. Work is defined as the dot product of force and displacement (W = F·d cosθ). In uniform circular motion, the centripetal force (tension) is always directed radially inward toward the center of the circle [1]. Conversely, the instantaneous displacement of the stone is always tangential to the circular path. Since the radial force and tangential displacement are perpendicular to each other (θ = 90°), the cosine of the angle is zero. Consequently, the work done by the string on the stone is zero because the force does not have a component in the direction of motion.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 23: Pressure Systems and Wind System > Centripetal Acceleration > p. 309