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Homework Help: Motion in one dimension physics problem

  1. Sep 19, 2009 #1
    1. The problem statement, all variables and given/known data

    Determine the stopping distances for an automobile with an initial speed of 90 and human reaction time of 1.0 : (a) for an acceleration = -5.0 , (b) for = -7.5 .

    2. Relevant equations

    vf = vo + at
    avg velocity = (vf + vo) /2
    d = vo)t + (1/2) at2
    vf2 = vo2 + 2ad

    3. The attempt at a solution

    For part A)I tried using the formula s=ut+1/2at^2, I had that
    u=25m/s, t=1.0s, a=-5.0,and then solved for s, which was =23m

    For part B) I used the same formula and everything else was
    the same, except for this part I let a=-7.5, which was = 21m
  2. jcsd
  3. Sep 19, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The 1.0 seconds is the time before the driver steps on the brakes and starts accelerating. During that time he travels at constant speed. How far? Add that to the distance that he travels once he starts braking.
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