# Homework Help: Finding length of skid marks using f k

1. Oct 6, 2009

### physjeff12

Finding length of skid marks using fk

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

A 1000kg car traveling at a speed of 40m/s skids to a halt on wet concrete where µk = 0.60. How long are the skid marks?

(i know this an easy problem, but i am stuck when trying to find the length of the skid marks.)

2. Relevant equations

Fg = mg = weight = normal force

fk = µkFN

3. The attempt at a solution

Fg = 1000(9.8) = 9800 N, therefore the normal force = 9800N
µk = .60

fk = .60(9800)
fk = 5800 N

v0 = 40m/s
fk = 5880 N

Where do i go from here to find the distance of the skid marks?

i was thinking maybe using the formula, x = x0 + vt, but i am not sure

2. Oct 6, 2009

### tiny-tim

Welcome to PF!

Hi physjeff12! Welcome to PF!
From fk, find the acceleration, and then use one of the standard constant acceleration equations.

3. Oct 6, 2009

### physjeff12

Re: Welcome to PF!

thanks

and how do i find the acceleration using fk? i don't know any formulas that connect acceleration with friction force.

EDIT: nvm, i am stupid. i found out how. F = ma

4. Oct 6, 2009

### tiny-tim

Friction is like any other force …

just use good ol' Newton's second law.