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Quite Easy Question PLEASE HELP

  1. Nov 28, 2007 #1
    Im a father trying to help my daughter with a Math problem.. And Im feeeling a little dumbfounded with this one.
    This is a summary of the info she has
    g=9.8m/s2

    Point A to B is 12km
    uniform acceleration 3/4 of Total distance
    deceleration 1/4 of Total distance
    Total Time 20mins

    Whats the max velocity of the train?

    I would apreciate if some could help me to solve this and teach me how to do it, Ill look like a genius to my daughter.

    Kind Regards

    Kai
     
  2. jcsd
  3. Nov 28, 2007 #2
    You haven't given all the information. For example, if the deceleration isn't uniform then the velocity can go arbitrarily high.

    g seems irrelevant.

    have ds=ds1(9km)+ds2(3km), t1=20min-t2, assume v@0km=v@12km=0 & const. a1,a2.
    d(v^2)=v@9^2-0^2=2.a1.ds1=2.a2.ds2 => a2=3a1
    v@9-0=at=a1(20min-t2)=3a1.t2 => t2=5 => v@9=..

    Not sure whether you'll actually be able to figure that out.. anyway, this belongs in the homework forum.
     
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