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Homework Help: Double your speed. (True/False)

  1. Feb 2, 2009 #1
    1. The problem statement, all variables and given/known data

    Assume that the brakes in your car create a constant deceleration, independent of how fast you are driving. If you double your driving speed, which of the following statements are true?
    Your stopping time will quadruple.
    Your stopping time will not change.
    Your stopping distance will double.
    Your stopping time will double.
    Your stopping distance will quadruple.
    Your stopping distance will not change.

    2. Relevant equations

    3. The attempt at a solution

    Well, mainly I'm not sure which formula to use, and I'm also confused cause I can't find one which contains speed. And I know velocity is different from that.

    So maybe to start off with a formula? Thanks
  2. jcsd
  3. Feb 2, 2009 #2


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    You first! :wink:

    What formulas do you know? :smile:
  4. Feb 2, 2009 #3
    Um, vf - vi = at

    d = vi*t + 1/2at^2

    vf^2 - vi^2 = 2ad
  5. Feb 2, 2009 #4
    I'd use the first one of those, if I were you and from that you can set up a ratio between t1 and t2 (;
  6. Feb 2, 2009 #5
    I'm guessing false for all of them but idno thats probably wrong
  7. Feb 2, 2009 #6
    Yeah i still dont get this
  8. Feb 2, 2009 #7
    If you start with the formula:

    vf - vi = at

    If you treat them as different cases you have:


    You know the relationship between the initial velocities, try to work it out from there..
  9. Feb 2, 2009 #8


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    For stopping time you need an equation with t in it.

    For stopping distance you need an equation with d in it.

    Let's start with the stopping time, t … how do you think t depends on vf?
  10. Feb 2, 2009 #9
    Well as the velocity increases, the time usually decreases, but isn't this different? Also this is due in 10 minutes lol
  11. Feb 2, 2009 #10


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    uhh? look at the equation … that's what equations are for! :smile:
  12. Feb 2, 2009 #11
    woo got it at the last second thanks haha
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