# Investigators measuring a car skid mark to find velocity.

1. Jul 17, 2009

### kavipach

1. The problem statement, all variables and given/known data
At an accident scene on a level road, investigators measure a car's skid mark to be 93m long. It was a rainy day and the coefficient of friction was estimated to be 0.32.
Use these data to determine the speed of the car when the driver slammed on (and locked) the brakes.

2. Relevant equations
Fd=-m*vinitial^2/2

3. The attempt at a solution

2. Jul 18, 2009

### rock.freak667

So what's the problem then?

3. Jul 18, 2009

### kavipach

4. Jul 18, 2009

### kavipach

it gave me -12..and aparently that isnt the right answer

5. Jul 18, 2009

### rock.freak667

What exactly did you do to get -12m/s?

6. Jul 18, 2009

### kavipach

well the F here is the force which comes from Fn*CoFr=Ff where Fn equals Normal Force CoFr is co. of friction and Fr is force of friction. So if we subsitute F for Fn*CoFf, the masses on both sides cancel out. leaving us with CoFr*g*d=-vinitial^2/2. The by pluging in the values, i got -23 acctually which isnt the answer either.

7. Jul 18, 2009

### rock.freak667

Really? Calculate that again.

8. Jul 18, 2009

### kavipach

oh lord..k its 24. thanks alot

9. Aug 13, 2009

### y2kkmac

I got, by using Vf2 = Vi2 + 2ad and using the fact that F/m = (coefficient of friction)*g, 24.15 m/s.

EDIT: Whoops, I'm sorry for the bump!

Last edited: Aug 13, 2009
10. Aug 13, 2009

### pgardn

Bolded fine as long as the road is horizontal. Normal Force is not always = mg
The original poster was using work change in kinetic energy idea I think. No kinematics necessary.