# Finding velocity (deals with coefficient of kinetic energy)

1. Nov 17, 2008

### mandaa123

1. The problem statement, all variables and given/known data

Police lieutenants, examining the scene of an accident involving two cars, measure the skid marks of one of the cars, which nearly came to a stop before colliding, to be 85 m long. The coefficient of kinetic friction between rubber and the pavement is about 0.70. Estimate the initial speed of that car assuming a level road.

2. Relevant equations
Fnet=ma
Ff=(coefficient of kinetic energy) Fn
vf = vi + at

3. The attempt at a solution
i have no idea what to do since none of the formulas work with the knowns.

2. Nov 17, 2008

### LowlyPion

You should also know this formula

V2 = 2*a*x

It among others can be found here:
https://www.physicsforums.com/showpost.php?p=905663&postcount=2

You also know that the force on the car from friction is m*a = u*m*g

Which means that a = u*g

V2 = 2*a*x = 2*u*g*x

3. Nov 17, 2008

### mandaa123

what does x represent in that formula?

4. Nov 17, 2008

### LowlyPion

Doesn't the problem say that it decelerated over a distance of 85 m ?

Wouldn't you want to know how much the car decelerated over that distance to give you an idea of what velocity it slowed from?