Question

A car skids to a stop. The length of the skid marks is 50m. What information do you need in order to decide whether the car was speeding before the driver hit the brakes

Answer 1

v = Vi - a t if constant deacceleration a
at stop v = 0
so
a t = Vi , the initial speed
now
d = skid length = average speed during stop * t
so 50 meters = Vi/2 * t
or
Vi *t = 100

now we need some physics
F = m a
so
m a = mu * m g = stopping force
where mu is the coefficient of friction between tires and road
so
a = mu * g
put that back in
a t = Vi = mu * g * t
but we know t = 100/Vi
so
Vi = mu * g * 100 / Vi
Vi^2 = 100 * mu * g
so all we need is that coefficient of friction and g which is about 9.81 m/s^2 on earth

Answer 2

Thanks so much!

Answer 3

You are welcome.

Answer 4

V^2 = Vo^2 + 2a*d;
0 = Vo^2 + 2a*50.
So, to find the initial velocity(Vo); I'll need to know the deacceleration.