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Homework Help: Not sure why I can't use v=v0 + at on this problem.

  1. Sep 14, 2014 #1
    1. The problem statement, all variables and given/known data

    Freight trains can produce only relatively small acceleration and decelerations.

    (a) What is the final velocity of a freight train that accelerates at a rate of 0.0400 m/s^2 for 8.00 min, starting with an initial velocity of 4.00 m/s?

    2. Relevant equations

    v=v0 + at
    x=x0 +v0t +1/2at^2
    v^2=v0^2 +2a(x-x0)

    3. The attempt at a solution

    Why can't i use the first equation I listed to solve this problem? Using that equation, you end up with v = 76.8 m/s

    If you first use the second equation, and then the third equation, you end up with v = 23.2 m/s

    Thanks in advance!
  2. jcsd
  3. Sep 14, 2014 #2


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    Science Advisor

    The first eqn gives the same as the 2nd and third.

    v = 76.8 m/s is wrong, 23.2 m/s is right you may have pressed a wrong button on your calculator.

    v=v0 + at
    v = 4 m/s + (0.04 m/s^2)*(8 min * 60s/min) = 23.2 m/s
  4. Sep 14, 2014 #3


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    Science Advisor

    The first equation is exactly what you're supposed to use. Using the second and third seems roundabout.

    But how are you getting 76.8 m/s with the first equation? Can you show your work?
  5. Sep 14, 2014 #4
    Oh goodness, it's been a long weekend haha. I accidentally set it up as 4 m/s TIMES (0.04m/s^2)(480s).

    Thanks guys
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