Question

At the scene of an accident, there are skid marks which are 200 feet long showing where a car put on the brakes. The driver swears that he was obeying the speed limit. If we assume that the car braked with a constant deceleration of 16 feet per second squared, determine if the driver was telling the truth.

Answer 1

average speed * time = 200 ft
average speed = (V-0)/2 = V/2
so
t = 200/(V/2) =400/V
V = a t
V = 16 t = 16 (400/V)
V^2 = 16 * 400
V = 4 * 20 = 80 ft/second

Answer 2

80 ft/s ( 3600 s/hr / 5280 ft/mi) = 54.5 miles/hr