Causes for Work Done and Work not Done
(i) Earth is rotating around the sun, the direction of displacement of the earth at any time is along the tangent of the arc [Fig]. But the gravitational force by which the sun attracts the earth always points towards the sun from the earth; that means, acts along the radius of the arc towards the center. So, the directions of the attractive force of the sun and the displacement of the earth are mutually perpendicular to each other, hence during rotation of the earth, no work is done by the gravitational force of the sun.
(ii) Walking along a plane road carrying a bag in hand, the weight of the bag i.e., gravitational force does not do any work. This is because, that while walking along the road, displacement of the bag is along the horizontal line, i.e., along with the perpendicular direction of the gravitational force. So, although there is displacement of the bag gravitational force does not do any work. Hence, in this case, gravitational force is workless. But while walking in uneven or undulated road work is done by the gravitational force.
(iii) If a stone is made to rotate by holding it by hand through a rope, then the stone moves in a circular path. Here the tension of the rope is a centripetal force. So, during the rotation of the stone, the tension of the rope does not do any work.
