Question

A student driver is travelling at 22.2 m/s and sees a school zone ahead. Using her brakes, she accelerates at -2.8 m/s2 to reduce her velocity to 8.2 m/s. How much time did it take to change her velocity?

Answer 1

(deceleration rate)*(time) = (speed change)

She reduces her speed by 14 m/s, so the change is -14 m/s.

time = (-14 m/s)/(-2.8 m/s^2)
= 5 seconds

Answer 2

u=22.2m/s
a=-2.8m/s2
v=8.2m/s
t=?
v=u+at (equation)
8.2=22.2+(-2.8)t
8.2-22.2=-2.8t
-14=-2.8t ('-' sign is cancelled from both the sides)
14/2.8=t
5=t
hence time taken is 5 seconds