Question

9. What will be the work done by the string, when a stone is tied to a string and whirled in a circle?

Answer 1

Work is said to be done when a force produces a displacement in the body and acts in any direction except perpendicular to the direction of displacement.
Work = Force×Displacement
Consider a body tied to one end of a string is rotating in a circle, work done by the centripetal force applied along the string is zero because at every point during the motion force is always perpendicular to the the displacement vector of the body.
θ = 90.
Work done is zero.