In coming to a stop a car leaves skid marks 92m long on the highway. Assuming a decceleration of 7.00m/s^2, estimate the speed of the car just before braking.

Answer 1

d = 0.5 * a * t^2,
92 m = 0.5 * 7.0 m/s^2 t^2,
92 = 3.5t^2,
t^2 = 92/3.5 = 26.29,
t = sqrt(26.29) = 5.13 s.

a = (Vt - Vo) / t,
-7.00 m/s^2 = (0 - Vo) / 5.13 s,
-7.00 = -Vo /5.13,
-Vo = - 7.0 * 5.13 = -35.91,
Vo = 35.91 m/s. = velocity Just before braking.

Answer 2

Where did you get the 0.5?