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Homework Help: Sports car Acceleration problem

  1. Oct 6, 2007 #1
    1. The problem statement, all variables and given/known data

    A sports car moving at constant speed travels 120 m in 4.7 s. If it then brakes and comes to a stop in 4.0 s, what is its acceleration? Express the answer in terms of g's, where g = 9.80 m/s2. (Take the positive direction to be the direction of travel.)

    2. Relevant equations

    Change in speed/time of change

    3. The attempt at a solution
    120/4.7=25.53m/s
    end speed 0m/s-start speed 25.53m/s divided by time 4s= -6.38m/s^2 divided by 9.8m/s2= -.651 g

    I got this wrong...what did I do wrong?
     
    Last edited: Oct 6, 2007
  2. jcsd
  3. Oct 6, 2007 #2

    Kurdt

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    I think its probably because you forgot the minus sign. The question specifically states which direction is positive.
     
  4. Oct 6, 2007 #3

    learningphysics

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    Looks right to me.
     
  5. Oct 6, 2007 #4
    I used a minus sign...still wrong. It is a web based hw system and it is horrible. we have 4 trys to get it right...so i used a - sign then a + sign then scientific notation then changed my sig figs...so it says I am wrong but I don't see how.
     
  6. Oct 6, 2007 #5

    learningphysics

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    Double check that you posted the question exactly as it appears... you might have mixed up the numbers or something....
     
  7. Oct 6, 2007 #6
    I copy and pasted the question it is exact
     
  8. Oct 6, 2007 #7

    Kurdt

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    So the question gave g as 9.80m/s2? I'd have expected 9.81m/s2 if it was to two decimal places.
     
  9. Oct 6, 2007 #8
    I emailed my teacher And I will see what she says. I really hate these web assignments because they aren't multiple choice (hence wrong sigs, diff. calculators etc... will mess answers up due to rounding)
     
  10. Oct 6, 2007 #9
    A car slows down from 23 m/s to rest in a distance of 91 m. What was its acceleration, assumed constant?

    I only have one more try on this one... is -2.91 right?
     
  11. Oct 6, 2007 #10

    learningphysics

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    Yes, that's what I get rounded to 2 decimal places.
     
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