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Homework Help: I need help with the formula pls

  1. Apr 17, 2007 #1
    1. The problem statement, all variables and given/known data
    When a car is being tested, it also has a braking test done/ In one such test a car took 3.0 s to stop from 40 m/s. Calculate a) the braking distance b) the deceleration

    3. The attempt at a solution
    first off i'm not sure how to find the braking distance and what formula to use, and also for deceleration is there a different formula than d=vxt..v=d/t...etc? If so please show me it thanks for taking your time!:smile:
  2. jcsd
  3. Apr 17, 2007 #2
    Do you know of the Uniform Acceleration formulas?

    [itex] a = v / t[/itex]

    [itex] d = [(V1+V2) / 2 ]t[/itex]

    [itex] d = (V1)(t) + 1/2(a)(t^2)[/itex]

    [itex] d = (V2)(t) - 1/2(a)(t^2) [/itex]

    [itex] (V2)^2 = (V1)^2 + 2(a)(d)[/itex]


    a = Acceleration
    d = distance
    V1 = Initial velocity
    V2 = Final velocity
    t = time
  4. Apr 17, 2007 #3

    yes , i know those formula but thanks for pointing those out:tongue:
    But i'm a little confused on how to find the deceleration!
  5. Apr 17, 2007 #4
    well once you find 'd' you will know be able to use either the 3rd or 4th and get accelleration, which should be negative
  6. Apr 17, 2007 #5
    umm ok so to calculate a) the braking distance...would i use the a= delta v/delta t formula? If so then what do i use next?
  7. Apr 17, 2007 #6
    not quite, try using one of those five .. maybe #2. remember final velocity is 0 m/s
  8. Apr 17, 2007 #7
    once you find distance you can find acceleration, which in this case is deceleration.
  9. Apr 17, 2007 #8
    if so, im not sure how i would get the v1 and v2
  10. Apr 17, 2007 #9


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    Homework Helper

    They are given in the question.
  11. Apr 17, 2007 #10
    v1 and v2 : velocity theres only seconds and one velocity...
  12. Apr 17, 2007 #11


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    Homework Helper

    The question says the car comes to a stop. That means that v2=0.
  13. Apr 17, 2007 #12
    ohh, yes i forgot about that! thanks !
  14. Apr 17, 2007 #13
    but do u kno how i would do deceleration?
  15. Apr 17, 2007 #14


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    Homework Helper

    I'm not sure what you mean. You just use

    [tex]a_{ave} = \Delta v / \Delta t[/tex]

    where [tex]\Delta v[/tex] = v2-v1. You were given the time interval in the question. Since it is deceleration, you will get a negative number.
  16. Apr 17, 2007 #15
    deceleration = negative acceleration, that's it
  17. Apr 17, 2007 #16
    oh ok thnx:tongue:
  18. Apr 17, 2007 #17
    oh i never knew that lol! thnx:eek:
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