Instant centre of rotation

Summary

The instant centre of rotation of a rigid body moving in a plane is the point whose velocity is zero at a given instant. At that instant every point of the body moves as if the body were turning about this point. The velocity of each point of the body is perpendicular to the line joining it to the centre and proportional to its distance from it. It exists whenever the angular velocity is not zero, and it moves as time goes on. It is a basic tool for finding velocities in planar mechanisms, together with Kennedy's theorem.