# Investigators measuring a car skid mark to find velocity.

## Homework Statement

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.

## Homework Equations

Fd=-m*vinitial^2/2

rock.freak667
Homework Helper

## Homework Equations

Fd=-m*vinitial^2/2

So what's the problem then?

the answer didnt work.

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

rock.freak667
Homework Helper
it gave me -12..and aparently that isnt the right answer

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

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.

rock.freak667
Homework Helper
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.

Really? Calculate that again.

oh lord..k its 24. thanks alot

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:
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!

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.