Solve Physics Problem: Stopping a Car with Skid Marks

[runningirl]

Homework Statement

Driving in a car on a level road, you hit the brakes hard, lock up the wheels and skid to a stop. You measure the skid marks and find them to be 40 m long. Use this information to determine the speed of the car at the moment you hit the brakes. The coefficient of friction for rubber tires skidding on asphalt is 0.50.

Homework Equations

f=ma

The Attempt at a Solution

a=(9.8)(sin theta)
Ff=coefficient(Fn)
9.8(sintheta)(.5)=Ff

i honestly wasn't sure where to go with this problem

 

[Sourabh N]

Can you write the expression for frictional force?

 

[CarlRozario]

v^2 = u^2 + (2*a*s)
The 3rd kinematical equation...
.
You have the following...
v = 0m/s (the final velocity, since your car has stopped)
s = 40m (distance traveled)
u = initial velocity, (that's what we want to find)...
If we only have a (the acceleration, in our case deceleration)
.
Ff = coefficient(Fn)-------(1)
Fn = mg as you're driving in a car on a level road... --------(2)
Ff = ma --------(3)
.
Substituting 2 and 3 in eq 1
a = (-1*coefficient*g) ...(negative since its a deceleration)
.
Put it all in the first equation...

 

[Sourabh N]

Hi Carl!

That was very well put :-) But what we try to do here on Physics Forums is not give away the formulas and let the person plug and chug and be happy. The idea is to guide the student through the solution, so that next time they have a similar problem (say in the exam), they can solve it themselves.

 

[CarlRozario]

Sure thing...
New here, so still have to get used to things...
In future if you catch me going outta the regular way, let me know...
Thanks
:-)