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Homework Help: Change in kinetic energy

  1. Dec 6, 2008 #1
    1. The problem statement, all variables and given/known data
    In the 1950s, an experimental train that had a mass of 2.50 x 10^4 kg was powered across the level track by a jet engine that produced a thrust of 5.00 x 10^5N for a distance of 509m.

    a)Find the work done on the train
    b)Find the change in kinetic energy
    c)Find the final kinetic energy of the train if it started from rest
    d)Find the final speed of the train if there were no friction.

    2. Relevant equations

    3. The attempt at a solution

    W= (5.00 x10^5N)(509m) = 2.545 x 10^8

    I'd typically find the change in KE when I have differences in velocity b/c of the equation KE=(1/2)mv^2, but I don't have v in this equation and I don't know how to find it when I don't have time (t).

    I know KE changes when there is change in height, but the train runs on a level track, does that mean there is none?

    Then in my review book it says W= KE initial - KE final = change in KE
    Does that mean the work I found done on the train is the same as the change in KE involved? I don't know...

    I used v(f)^2 = v(i)^2 = 2ad
    v(f)^2= 2(9.8)(509)

    I really don't know how to approach this one...
  2. jcsd
  3. Dec 6, 2008 #2


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    Homework Helper

    Answer is the same for a,b and c.
    For (d), find acceleration a = thrust / mass
    then use the formula v(f)^2 = v(i)^2 + 2ad to find v(f)
  4. Dec 6, 2008 #3


    User Avatar
    Homework Helper

    Welcome to PF.

    Yes the work done will result in an increase in kinetic energy.

    The problem generally wants you to ignore frictional forces.
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