Final answer:
Work is not done on the toy by the string during each revolution as the force is perpendicular to the motion, which means the force does not cause displacement in the direction of the force.
Step-by-step explanation:
The question asks whether work is done on a toy by the string during each revolution as it is swung in a horizontal circle. In physics, for work to be done on an object, a force must be applied to the object and the object must move in the direction of the force. When a girl swings the toy in a circular path, the tension in the string provides a centripetal force that is directed towards the center of the circle. This force keeps the toy moving in a circular path.
However, even though the tension is necessary to maintain the circular motion, it does no work on the toy because the force is always perpendicular to the direction of the toy's motion. Work must be done to rotate objects such as grindstones or merry-go-rounds, but since there is no displacement in the direction of the centripetal force, the work done on the toy by the string each revolution is zero. The significance of this understanding in rotational motion is analogous to work and energy in translational motion, as previously explored in Uniform Circular Motion and Gravitation.