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Homework Help: Physics Forces Question

  1. Dec 16, 2008 #1
    1. The problem statement, all variables and given/known data

    A freight train has a mass of 2.8 × 10^7 kg.
    If the locomotive can exert a constant pull
    of 8.1 × 10^5 N, how long would it take to
    increase the speed of the train from rest to
    77.6 km/h? (Disregard friction.) Answer in
    units of s.

    2. Relevant equations


    3. The attempt at a solution

    I've tried everything, but I really have no clue on how to do this problem, if anyone can point me in the right direction or tell some formulas that would help it would be much appreciated.
    Last edited: Dec 16, 2008
  2. jcsd
  3. Dec 16, 2008 #2


    User Avatar
    Homework Helper

    You know the force.

    You know the mass.

    So surely you have the acceleration don't you?

    And if you have acceleration what it that? Isn't that the rate of change of velocity? And if you know how fast velocity is changing can't you figure how long it will take to get to that speed?
  4. Dec 16, 2008 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Do it in two steps:
    (1) Find the acceleration.
    (2) Use kinematics to find the time. (Convert that speed to standard units: m/s.)
  5. Dec 16, 2008 #4
    Thanks guys I found my mistake

    It was I kept finding the acceleration in m/s and never changed the final velocity from k/h to m/s

    The answer was 745.13 seconds
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