Question

This question is for ICSE students who are going to give Physics exam tomorrow...

why no work is being done when a body covers even half of the path in circular motion...???

Answer 1

That's because the displacement traced by the object is zero every time the object or the body does even path of the circle

Answer 2

During circular motion, the displacement of the body is *always* along the tangent of the circle, i.e, displacement in circular motion is tangential . And the force acting on the body, in it's motion, is always acting towards the center of the circle. Hence the body possesses centripetal force that pulls it towards the center . Now , if we measure the angle formed by this centripetal force and the displacement of the body, we see that they make a right angle at any point during the motion of the body in it's circular motion. 
Now we know that,
   work done = F . s (dot product of force and displacement)
=> work done = F s cos theta. (theta is the angle between force and displacement)
=> work done = F s cos 90 degrees. ( since force and displacement make a right angle, theta = 90 degrees.)
=> work done = F s x 0. ( cos 90 = 0)
=> work done = 0.
Thus work done in circular motion is zero. 
And yes, the sole reason for this outcome is because force and displacement of a body in circular motion are mutually perpendicular to each other .

Hope it will help you