1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    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.

    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


    User Avatar
    Homework Helper

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


    User Avatar
    Homework Helper

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


    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


    User Avatar
    Homework Helper

    [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 ?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook