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Mechanics - work done conservation of energy *Help needed*

  1. Feb 26, 2012 #1
    A train with a mass of 250 tonnes starts from rest and accelerates up an incline of 1 in 100 attaining a speed of 45 Kph after traveling 200m. If the frictional resistance to motion is constant at 30KN calculate the work done by the engine using the principle of conservation of energy

    2. Relevant equations
    Possible relevant equations:

    Work done= Force*Distance
    Kinetic energy = 1/2*Mass*Velocity^2
    Potential energy= Mass*Gravity (9.81)* Height

    3. The attempt at a solution

    Force= 250*103 Kg*12.5= 3 125 000 kg/m/s
    Work done= 3 125 000*200= 625 000 000(J)
    Not sure whether this is along the right lines or not. I am also unsure where the frictional resistance and gravity is used.

    Thank you for your time.
  2. jcsd
  3. Feb 26, 2012 #2


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    net force is mass times acceleration. You are not using the net force or the acceleration.
    Your incorrect values are calculating the net total work done. You are looking for the work done by the engine only.
    the problem is asking you to use conservation of energy, not force and kinematic equations. What is the conservation of energy equation that relates work and energy?
  4. Feb 27, 2012 #3
    Sorry to be a pain, but I do not know how to go about this, please can you explain?
    Thank you
  5. Feb 27, 2012 #4


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    Say, Eddy, if you are asked to solve the problem using energy methods, you should know about the possible energy equations to use, for example, you should know that the work done by non conservative forces (like the engine force and friction force in this example) must equal the change in PE plus the change in KE of the system. Give it a try.

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