**CIRCULAR MOTION:** When a particle follows a circular path, its motion is called circular motion.

This circular motion can be ‘uniform’ or ‘non-uniform’.

**NON-UNIFORM CIRCULAR MOTION**: When the speed of the particle is not constant along the circular path, it is called non-uniform circular motion.

[Hint*: In the NCERT syllabus for class 11, only a little knowledge of non-uniform circular motion is needed. So. don't worry too much about this sub-topic*.]

**UNIFORM CIRCULAR MOTION:** When an object follows a circular path *at a constant speed*, the motion of the object is called ‘uniform circular motion’.

**Remember** that here ‘uniform’ means the ‘speed’ is constant, not ‘velocity’. The velocity of the object keeps on changing as its direction keeps on changing. So, this motion is always accelerated motion.

Say, an object is moving along a circular path of radius ‘r’ in an anti-clockwise direction on the plane of the paper, with centre at O.

A line passing through O perpendicularly to the plane of the paper is called the ‘**axis of rotation**’.

Let the particle moves from P to Q in time t, where __/__ POQ is θ.

From geometry we know that θ = arc/radius = PQ/r

**ANGULAR DISPLACEMENT:** Angular displacement of an object in circular motion is defined as the angle swept by the radius at the centre of the circular path. It is measured in radians.

It is a vector quantity. Its direction is given by the right hand rule, which states that if the curvature of the fingers of the right hand represents the sense of rotation of the object, then the thumb represents the direction of angular displacement vector.

**[Hint**:* Remember that nothing actually moves in the direction of angular displacement.*]

**ANGULAR VELOCITY:** The rate of change of angular displacement with time is called angular velocity. Its direction is same as angular displacement. Mathematically, angular velocity, ω = dθ/dt .

**ANGULAR ACCELERATION**: The rate of change of angular velocity with time is called angular acceleration.