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Homework Help: Help needed with accelleration problem !

  1. Jan 20, 2005 #1
    help needed with accelleration problem...urgent!

    I am having trouble wiht one of the questions on my first physics HW asignment.
    I've tried solving it different ways but cant seem to get the correct answer.

    the question reads:
    An engineer in a locomotive sees a car stuck on the track at a railroad crossing in front of the train. When the engineer first sees the car, the locomotive is 280m from the crossing, and its speed is 24m/s.
    if the engineer's reaction time is .55s, what should be the magnitude of the minimum deceleration to avoid an accident?

    I have been using the formula:
    Vfinal^2 - Vinitial^2 = 2a(xfinal - xinitial)

    V being velocity, a being accelleration and x being position.

    I solve this equation for a and begin substituding in.
    I know that my Vinitial is 280 - 34(.55) because you have to take out ground covered in the engineers reaction time.

    but when I substitute in everything else I alwasy get -1.079
    because of the wording of the question I have tried it with and without the negative sign, but it is still wrong.

    any help would be greatly appreciated
  2. jcsd
  3. Jan 20, 2005 #2


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    Science Advisor
    Homework Helper

    The sign should be minus,because it's a DECELERATION.The modulus is much,much smaller than what u've gotten.Since the logics and the formula u used are good (24*0.55,not 34*0.55,okay??),i advide you to check the arithmetics...

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