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Homework Help: Revised Friction Question

  1. Oct 17, 2005 #1
    Hi everyone. I just finished 2 questions and corrected some mistakes 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[backward]
    V2^2=V1^2+2ad
    0=v1^2+(2*6.37*126)
    v1=40m/s
    So the car was travelling 40m/s before it crashed

    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:
    v2^2=v1^2+2ad
    0=15^2+2a*1.1
    2.2a=225
    a=102m/s^2[backward]
    Fbrake(the force of the brake)
    Fbrake=ma
    =60*102
    =6240N
    So the force acted upon the person is 6240N
     
  2. jcsd
  3. Oct 17, 2005 #2
    All correct.
     
  4. Oct 17, 2005 #3
    Thank you sooo much
     
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