During emergency braking, the car stopped after 4 seconds, having covered a distance of 40 m. Find its starting speed.

Initial data: t (duration of emergency braking) = 4 s; S (distance traveled by the vehicle when braking) = 40 m.

1) Let us express the acceleration of the body from the formula for determining the speed: V = V0 + a * t; 0 = V0 + a * t and a = -V0 / t = -V0 / 4 = -0.25V0.

2) Substitute the initial speed in the path formula and calculate it: S = (V ^ 2 – V0 ^ 2) / 2a = -V0 ^ 2 / (2 * -0.25V0) = V0 / 0.5 = 2V0.

V0 = S / 2 = 40/2 = 20 m / s.

Answer: The initial speed of the vehicle before emergency braking was 20 m / s.