Plane Kinematics of rigid bodies
Let us assume that the direction of the absolute velocities of any two points A and B on the body are known and are not parellel. If there is a point about which A has absolute circular motion at the instant considered, this point must lie on the normal to VA through A. Similar reasoning applies to B, and the intersection C of these two perpendicular fulfills the requirement for an absolute center of rotation at the instant considered.
If the magnitude of the velocity of one of the points, say, VA is known, the angular velocity 
of the body and the linear velocity of every point in the body are easily obtained. Thus,
As the body changes its position, the instantaneous center also changes its position both in space and on the body.The locus of the instantaneous centers in space is known as the space centrode and the locus of the position of the instantaneous centers on the body is known as the body centrode.
For the rolling motion, the instantaneous center is at the point of contact of the roller with the ground. The velocities of point A, B, C are 0, wr and 2wr in the horizontal direction.
