In Newton’s laws of motion, we have seen that an unbalanced force is necessary for change of motion. No net force is acting on a body, which is undergoing uniform motion (uniform velocity). Then we introduced the concept of friction. We showed that friction is necessary for motion. To overcome kinetic friction (sliding or rolling friction), we need the extra net force even when a body is in motion. For a frictionless motion, no unbalanced force is necessary. But in practical everyday situations, frictionless surfaces do not exist! Thus a net force is a must for all motions.

Initially we have discussed linear motion. If force is necessary for motion, then in linear motion, force and acceleration are in the same direction. Velocity could be parallel or anti–parallel to acceleration, but in one line. We soon upgraded our knowledge and discussed two–dimensional non–linear motion.

In circular motion we encountered a strange fact, strange because it is contrary to our knowledge of direction of force, acceleration and velocity. In circular motion, the force and acceleration are pointed towards the centre of the circle, but the velocity is tangential to the circle. That means acceleration and velocity are at right angles to each other ! This is where the confusion arises in every learner’s mind. Let us try and answer why this happens in case of circular motion (uniform or non–uniform circular motion).