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Homework Help: Homework check

  1. Oct 17, 2005 #1
    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.
     
    Last edited: Oct 17, 2005
  2. jcsd
  3. Oct 17, 2005 #2
    So can anyone check this? Thanks
     
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