3- D Kinematics
Consider two pints A and B on the body shown below. If rA is the position vector of A, rB is the position vector of B, and rA/B is the position vector of A with respect to B, then
Differentiating this expression with respect to time,
and 
as 
will be zero. This is because the magnitude of rA/B remains constant with time, because the body is rigid and the direction of rA/B also remains constant with time, because body is only translating.
Thus, in the translation of a rigid body, everypoint in the body has same velocity and acceleration. Thus, the velocity and acceleration of the whole rigid body can be specified by specifying the velocity and acceleration of one point, say mass center.