Question

Assertion: In circular motion, work done by centripetal force is not zero always.
Reason: If the speed of a particle increases or decreases in circular motion, net force acting on the particle does not remain towards the centre.

A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C. Assertion is correct but Reason is incorrect
D. Assertion is incorrect but Reason is correct.

Answer 1

Hint: Work is defined as force multiplied by displacement in the direction of force. In circular motion centripetal force is perpendicular to velocity. Also if the particles decrease or increase their speeds in circular motion then acceleration is generated which deviates the net acceleration from pointing towards the centre.

Complete step-by-step answer:
We know that circular motion is a movement of an object along the circumference of a circle. It is also defined as rotation of an object along a circular path. It can be of two forms, uniform circular motion, with constant angular rate of rotation and constant speed, or non-uniform circular motion with a changing rate of rotation. Since the direction of object's velocity is constantly changing, the moving object undergoes acceleration by a centripetal force in the direction towards the centre of rotation. Without this, the object would move in a straight line.

We also know that centripetal force is the force that acts on a body moving in a circular path and is directed towards the centre around which the body is moving. Thus in uniform circular motion, the work done by centripetal force is always zero as it is perpendicular to the displacement. Also if the particles decrease or increase their speeds in circular motion then acceleration is generated which deviates the particle from the centre.
Hence the answer is D. Assertion is incorrect but Reason is correct.

Additional Information:
A few examples of circular motion include: an artificial satellite orbiting the Earth at a constant height, a ceiling fan's blades rotating around a hub, a stone which is tied to a rope and is being swung in circles, a car turning through a curve in a race track and so on.

Note: Centripetal force is the force that acts on a body moving in a circular path and is directed towards the centre around which the body is moving. If the particles decrease or increase their speeds during the circular motion the net force deviates the particle from its initial path.