Q 3.15 The instantaneous speed is always equal to the magnitude of instantaneous velocity. Why?
Instantaneous speed is defined as the first derivative of distance travelled with respect to time and magnitude of instantaneous velocity is the first derivative of the magnitude of displacement with respect to time. The time interval considered is so small that it is safe to assume that the particle won't change its direction of motion during it and therefore the magnitude of displacement during this interval will be the same as the distance travelled and therefore instantaneous speed is always equal to the magnitude of instantaneous velocity.