A car travelling at 45 mi/hr (20.1 m/s) comes to rest in 4 seconds. Find the acceleration of the car in m/s2.

Acceleration = (change in speed) / (time for the change)

Change in speed = (speed at the end) minus (speed at the beginning)

(0 - 20.1 m/s) = - 20.1 m/s

Time for the change = 4 sec

Acceleration = (-20.1 m/s) / (4 sec) = - 5.025 m/s²