# Revised Friction Question

1. Oct 17, 2005

### PhysicBeginner

Hi everyone. I just finished 2 questions and corrected some mistakes but I'm not sure if the answer is right. So can anyone help me check it?

The first question is:
"Police find the tire tracks of a car that lost control and caused an accident. They want to know if the car was speeding. They determine that μs=0.65 for it's tire on this road. The tire marks are 126m long. How fast was the car traveling?"

Fcar(the force of the car)
Fcar= μsFn Fn=mg
Fcar= μsmg
Fcar=ma
ma=μsmg
cross out the m
a=μsg
a=0.65*9.8
=6.37m/s^2[backward]
0=v1^2+(2*6.37*126)
v1=40m/s
So the car was travelling 40m/s before it crashed

The second question is "Calculate the force acting on the body of a 60kg person who is involved in an accident. His car hits a tree while going 15m/s. The car comes to a stop with the tree embedded 1.1m into it."

0=15^2+2a*1.1
2.2a=225
a=102m/s^2[backward]
Fbrake(the force of the brake)
Fbrake=ma
=60*102
=6240N
So the force acted upon the person is 6240N

2. Oct 17, 2005

### whozum

All correct.

3. Oct 17, 2005

### PhysicBeginner

Thank you sooo much