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Homework Help: Physics Friction question

  1. Oct 17, 2005 #1
    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

    Doc Al

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    Staff: Mentor

    One thing you're doing wrong is assuming that [itex]d = v t[/itex] 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.
     
  4. Oct 17, 2005 #3
    So are the answers and calculations correct if V really is constant?
     
  5. Oct 17, 2005 #4

    Doc Al

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    Staff: Mentor

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

    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.
     
  6. Oct 17, 2005 #5
    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
  7. Oct 18, 2005 #6

    Doc Al

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    Staff: Mentor

    Both problems involve accelerated motion, not constant speed. And, yes, that's the formula you need.
     
  8. Oct 18, 2005 #7
    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
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