Physics Friction question

1. Oct 17, 2005

PhysicBeginner

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

Staff: Mentor

One thing you're doing wrong is assuming that $d = v t$ applies here. But that only applies when the speed is constant, which is not true here. You can use it, but you only if v is the average speed.

3. Oct 17, 2005

PhysicBeginner

So are the answers and calculations correct if V really is constant?

4. Oct 17, 2005

Staff: Mentor

If V really is constant, then the questions make no sense.

I recommend that you look up (or derive) the various kinematic formulas for uniformly accelerated motion. The formula that you need connects v, a, and d.

5. Oct 17, 2005

PhysicBeginner

So does what you're saying apply to both question or only one of them?
Also is the formula i need v2^2=V1^2+2ad?

Last edited: Oct 17, 2005
6. Oct 18, 2005

Staff: Mentor

Both problems involve accelerated motion, not constant speed. And, yes, that's the formula you need.

7. Oct 18, 2005

Koyuki

Hey, PhysicsBeginner, by any chance do you go to school in Scarborough, Canada? I came here to ask about this same exact question that's in the booklet our teacher gave.

edit: After reading your other posts, I think you go to the same school as I do!! Haha, how cheap.

edit2: Also, are his/her answers correct? Thank you!

Last edited: Oct 18, 2005