An Olympic track runner starts from rest and has an acceleration of 2.1 m/s2 for 3.5 s, then has zero acceleration for the remainder of the race. Find the runner's speed at the following times. (a) at time t = 1.3 s
m/s
(b) the end of the race
m/s

a) = 2.73 m/s

b) 7.35 m/s

Explanation:

runner starts from rest, initial velocity  u = 0 m/s

acceleration a = 2.1 m/s^2

time = 3.5 s

after the 3.5 s, his acceleration = 0

a) the time t = 1.3 s falls within the time frame where he was accelerating.

using the equation

v = 0 + (2.1 x 1.3)

v = 2.73 m/s

b) the end of the race fall outside the time frame in which the runner is accelerating.

This means that his speed at the end of the race will be equal to his speed at 3.5 s

we use the formula as before

v = 0 + (2.1 x 3.5)

v = 7.35 m/s

power out to power in less than 100%