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Average acceleration?

  1. May 26, 2004 #1
    A sports car is advertised to be able to stop in a distance of 44 m from a speed of 95 km/h. What is its [average] acceleration in m/s2? How many g's is this (g = 9.80 m/s2)?

    I have 7.9 m/s2, as my answer for average acceleration, but webassign isn't accepting my answer. Can someone help me out?
     
  2. jcsd
  3. May 26, 2004 #2

    arildno

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    How did you arrive at that answer?
     
  4. May 26, 2004 #3
    I used v^2-vo^2=2a(x-xo

    Have to convert to m/s so vo=26.38 m/s, vf=0
     
  5. May 26, 2004 #4

    arildno

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    Looks right to me; however:
    Did you remember the sign on the acceleration?

    That's about what I can think of, unless you were supposed to enter the number of g's the acceleration is in
     
  6. May 26, 2004 #5

    arildno

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    Not 26.39???
     
  7. May 26, 2004 #6
    yea its 26.38888888888 m/s, no it asks for g's in another questions and when I do 7.9/9.8 it says that answer is correct. I wonder if this method is average acceleration or is it instantaneous?
     
  8. May 26, 2004 #7

    arildno

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    It absolutely cannot be the inst. acceleration they're after; in order to know this, you would need a lot more information (effect of brakes, frictional coefficient etc.)

    Since your second answer is correct, I can see only 2 possibilities:
    a) The machine is wrong
    b) It requires a sign for the acceleration

    On second thought: Have you tried using velocity 26.4 and rounding off to one significant decimal?
     
  9. May 26, 2004 #8
    Still average. You have no knowledge of the interior, only the endpoints.

    cookiemonster
     
  10. May 26, 2004 #9
    Okay, thanks mucho. It was the sign :rolleyes:
     
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