In Exercises 3.13 and 3.14, we have carefully distinguished between average speed and magnitude of average velocity. No such distinction is necessary when we consider instantaneous speed and magnitude of velocity. The instantaneous speed is always equal to the magnitude of instantaneous velocity. Why?
A particle is acted upon by a force of constant magnitude which is always perpendicular to the instantaneous velocity of the particle. The motion of the particle takes place in a plane. It follows that