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Help 1D kinematics problem

  1. Jan 14, 2007 #1
    A motorist is traveling at 18 m/s^2 when he sees a deer 38 m ahead. If the maximum negative acceleration is -4.5 m/s^2, what is the reaction time (delta t) of the motorist that will allow him to avoid hitting the deer? Answer in units of 's'.

    I tried using x(t)=x0+v0t+1/2at^2 and it didn't work out for me. I know the answer is supposed to be 0.1111 s and I can't get this no matter what I try. help!
  2. jcsd
  3. Jan 14, 2007 #2


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    trying using v = u +at and s = 0.5( u+v) t
    and 0.1111s actually is 1/9 or 2/18 or ....
  4. Jan 14, 2007 #3
    Mjsd...that makes sense to me except I don't understand why 's' is equal to '1'?? Also, in the s = 0.5 (u+v)t equation I would think that acceleration should be used somewhere in the equation since that is how fast it is decelerating.
    My original idea was the use the equation I mentioned above and then use the quadratic equation to get the answer, but that didn't work out so well.
  5. Jan 14, 2007 #4


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    Please dont double, triple post the same question.
  6. Jan 14, 2007 #5


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    firstly, let's remember that there are many ways to do the same problem. My way is to do it in two steps. By the way the equations I listed are correct, the second one is just: distance travelled = av. speed x total time.

    Logic: how long does it take to stop from initial speed assuming the full [tex]-4.5m/s^2[/tex], then in that time how far the vehicle will travel, then what's left for reaction time? etc....
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