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How long does it take it increase speed?

  1. Feb 5, 2008 #1
    1. The problem statement, all variables and given/known data
    A freight train has a mass of 1.2e+7 kg. If the locomotive can exert a constant pull of 8.4e+5 N, how long does it take to increase the speed of the train from rest to 82 km/h?


    2. Relevant equations
    (mass*velocity)/time=force


    3. The attempt at a solution
    (1.2e+7 * 82)/h = 8.4e+5

    i solved for h
    h = (8.4e+5)/(9.84e+8)
    h = 8.54e-4

    then i had to convert from hours to min.
    8.54e-4*60

    so i got
    5.1e-2 min

    this is my first time posting on this forum so i hope i followed the right format.
     
  2. jcsd
  3. Feb 5, 2008 #2

    rock.freak667

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    82km/h = [itex]\frac{82*1000m}{3600s}[/itex]
     
  4. Feb 5, 2008 #3
    can you explain the above?
     
  5. Feb 5, 2008 #4

    rock.freak667

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    I just converted the speed from km/h to m/s which is usually what you should do.

    82km/h

    in 1h train travels 82km
    => in 1h train travels 82*1000m
    in 3600s train travels 82000m
    in 1s train travels 82000/3600 m

    therefore the velocity in m/s is 820/36
     
  6. Feb 5, 2008 #5
    that doesnt answer my question, i think im messing up on the formula. I have one submission left so i dont want to risk it
     
  7. Feb 5, 2008 #6

    rock.freak667

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    [tex]F=\frac{mv}{t} \Rightarrow t=\frac{mv}{F}[/tex]

    [tex]t=\frac{(1.2*10^7 kg)*((\frac{82000}{3600} ms^-1)}{8.4*10^5 N}[/tex]



    Can you calculate now?


    But your answer was [itex]5.1*10^{-2}[/itex]minutes. Doesn't that seem like a small amount of time to go from 0 to 82km/h ?
     
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