During emergency braking, the car moving at a speed of 72 km / h stopped after 4 seconds, find the braking distance.

Given:

v = 72 km / h = 20 m / s – vehicle speed before braking;

t = 4 seconds – the time interval from the start of braking to the complete stop of the vehicle.

It is required to determine S (meters) – the braking distance of the car.

Let’s find the acceleration with which the car was braking:

a = v / t = 20/4 = 5 m / s ^ 2.

Then the braking distance will be equal to:

S = v * t – a * t ^ 2/2 = 20 * 4 – 5 * 4 ^ 2/2 = 80 – 5 * 16/2 = 80 – 5 * 8 =

= 80 – 40 = 40 meters.

Answer: the braking distance of the car is 40 meters.