acceleration?

Question

with the average acceleration of -2.5m/s2 how long will it take a cyclist t bring a bicycle with an initial speed of 13.1m/s to a complete stop?

Answer 1

Time = Velocity / Acceleration

13.1 m/s / 2.5 m/s^2 = 5.24 seconds

The 'negative sign' in front of the acceleration only implies that he's slowing down, so don't pay attention to it right now.

Answer 2

v= at +vo
0=-2.5t+13.1 so t = 13.1/2.5=5.24 s
sorry you must pay attention to the sign of acceleration if you want to find out v at any time before stop