The velocity of the point P along its trajectory P (t) is obtained as the time derivative of this position vector, v P = [ T ˙ ˙ ( t ) ] p = [ v P 0 ] = ( d d t [ A ( t ) d ( t ) 0 1 ] ) [ p 1 ] = [ A ˙ ˙ ( t ) d ˙ ˙ ( t ) 0 0 ] [ p 1 ] . The dot denotes the derivative with respect to time; because p is constant, its derivative is zero.