HRWP6p77

The velocity vector is the time derivative of the position vector. For this type of derivative, the unit vectors (such as (ihat)) are constants. Only the t parts get their derivatives taken. Thus if r = (5t, 3) m, v = (5, 0) m/s since the derivative of 5t is 5 and the derivative of 3 is zero.
The acceleration vector is derived from the velocity vector in the same manner.