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What is the acceleration of the car

  1. Sep 13, 2011 #1
    1. The problem statement, all variables and given/known data

    A car slows down from 25 to rest in a distance of 75 .
    What is the acceleration of the car.

    I've been fiddling with this problem pretty much all night but can't seem to get to a correct answer, help would be apprechiated.
    2. Relevant equations

    x = x0 + v0t + 1/2at^2
    d = 1/2(v0 +v)t

    3. The attempt at a solution

    x = x0 + v0t + 1/2at^2

    75m = (25m/s)(60.5s) + 1/2a(60.5s)^2

    75m = 1512.5m + 1/2a(60.5s)(60.5s)
    75m = 1512.5m + 1/2a(3660.25s^2)

    75m - 1512.5m = 1512.4m - 1512.4m 1/2a(3660.25s^2)
    -1437.5m = .5a(3660.25s^2)
    ------ ----------------

    .5 .5


    -2875 = a(3660.25s^2)
    ------ -------------
    3660.25 3660.25s^2


    -.79m/s2

    AND

    -25m/s
    ------
    60.5s = -.42m/s2
     
    Last edited: Sep 13, 2011
  2. jcsd
  3. Sep 13, 2011 #2
    Re: Acceleration

    em i don't understand how you got your time? anyway

    notice this question doesn't give you time.

    so you use the equation of motion that doesn't involve time, which is

    v2 = u2 + 2as

    now , v2 = 0 , because final car velocity = 0
    u2 = 25 as stated in question
    s = 75

    so 0 = 25 + 2a(75)
    a = -25/(2x75) = -0.16

    so the negative sign tells you the acceleration is opposite to the direction of the +25 m/s velocity , i.e, the car is decelerating .
     
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