How fast was the car traveling?

[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?"
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.

 

[PhysicBeginner]

So can anyone check this? Thanks

 

[quicknote]

Hi there,

I would like to provide some feedback on your answers to these questions.

For the first question, the calculation you have done to find the velocity of the car is correct. However, I would like to point out that the value of the coefficient of friction (μs) may vary depending on the road surface and other factors. Therefore, this answer is an estimate and may not be entirely accurate.

For the second question, your calculation of the force acting on the person is incorrect. The force acting on the person would be the force of the impact between the person and the car, not the force of the braking. To calculate this, you would need to use the equation F=ma, where m is the mass of the person and a is the acceleration due to the impact (which you have correctly calculated as 214m/s^2). So the force acting on the person in this scenario would be 60kg * 214m/s^2 = 12840N, which is a significant force.

I hope this helps and keep up the good work with your calculations!