Question

A car leaves skid marks 90 long on the highway in coming to a stop.

Assuming a deceleration of 3.5m/s^2, estimate the speed of the car just before braking.

Answer 1

0 = Vi - a t
so
t = Vi/3.5

d = Vi t - (1/2)(3.5) t^2
90 = Vi^2/3.5 - 1.75 Vi^2/12.25

Vi^2 - .5 Vi - 315 = 0

Vi = [ .5 +/- sqrt ( .25 + 1260) ]/2

Vi = [ .5 +/- 35.5 ] / 2

Vi = 18 m/s

Answer 2

15