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

V0 = 72 km / h = 20 m / s.
V = 0 m / s.
t = 4 s.
S -?
Let’s find the acceleration of the car a during braking by the formula: a = (V – V0) / t.
Since V = 0 m / s, the formula will take the form: a = – V0 / t.
We find the braking distance S by the formula: S = (V ^ 2 – V0 ^ 2) / 2 * a.
Since V = 0 m / s, the formula will take the form S = – V0 ^ 2/2 * a.
Substitute the acceleration expressions into the formula for determining the braking distance:
S = – V0 ^ 2 * t / 2 * (- V0) = V0 * t / 2.
S = 20 m / s * 4 s / 2 = 40 m.
Answer: the braking distance of the car is S = 40 m.