The driver of a car, who tried to prevent a collision with a tractor standing on the roadway

The driver of a car, who tried to prevent a collision with a tractor standing on the roadway, began braking with a deceleration of 5.5 M / s ^ 2, left sliding marks 38 m long before the collision point. Determine the speed of the car at which the driver attempted to warn the accident if according to the examination, the speed at the time of the collision was 30 km / h

Since the driver of a car, who tried to prevent a collision with a tractor standing on the roadway, began braking with a deceleration a = – 5.5 m / s ^ 2, leaving slipping traces with a length of S = 38 m to the point of collision, then to determine the speed of the car , in which the driver made an attempt to prevent an accident, we use the formula:

S = (v ^ 2 – v₀ ^ 2) / (2 ∙ a), then

v₀ = √ (v ^ 2 – 2 ∙ a ∙ S).

It is known from the condition of the problem that, according to the examination data, the speed at the moment of collision was v = 30 km / h = 25/3 m / s. Substitute the values ​​of physical quantities in the calculation formula:

v₀ = √ ((25/3 m / s) ^ 2 – 2 ∙ (- 5.5 m / s ^ 2) ∙ 38 m);

v₀ ≈ 22 m / s;

v₀ ≈ 79.5 km / h.

Answer: the speed of the car before the start of braking was ≈ 79.5 km / h.