# Homework Help: Homework check

1. Oct 17, 2005

### PhysicBeginner

Homework question

Hi everyone. I just finished 2 questions 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
d=vt
t=v/a
d=v*(v/a)
126=v^2/6.37
v=28m/s
The velocity of the car was 28m/s
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."
t=d/v
=1.1m / 15m/s
=0.07s
a=v/t
a=15m/s / 0.07s
=214m/s^2
Fbrake(The force of the break)
Fbrake=ma
=60*214
=12840N
So the force acted upon the person is 12840N
I'm not really sure of question 2's answer cause it seems a bit too big.

Last edited: Oct 17, 2005
2. Oct 17, 2005

### PhysicBeginner

So can anyone check this? Thanks