Calculating Forces in Car Accidents

[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?"

Answer:
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."

Answer:
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.

 

[Doc Al]

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.

 

[PhysicBeginner]

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

 

[Doc Al]

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.

 

[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?

 

[Doc Al]

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

 

[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!