The difficulty in finding the velocity after 3 s is that we are dealing with a single instant of time ( t = 3 ), so no time interval is involved. However, we can approximate the desired quantity by computing the average velocity over the brief time interval of a tenth of a second from t = 3 to t = 3.1 : average velocity = change in position time elapsed = s ( 3.1 ) s ( 3 ) 0.1