In many situations, there is a need to examine the motion of two or more points relative to each other.

Consider two points, A and B, both in motion relative to the origin O. This section will examine the motion of B relative to A.

If r_B/A is the position vector of B relative to A, then it can be stated that

rB = rA + rB/A

This is simply adding two position vectors to get a new total vector. This is illustrated in the diagram at the left.

To find the relative velocities of two points, take the time derivative of the position, giving

     d(r_B)/dt = d(r_A + r_B/A)/dt

Relative acceleration is found by taking the time derivative of the velocity,

     d(v_B)/dt = d(v_A + v_B/A)/dt