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Principles of conservation of energey

  1. Jun 6, 2012 #1
    1. The problem statement, all variables and given/known data

    A train of mass 50 onnes traveling at 6o m/s collides head on into a stationary train which has a mass of 30 tonnes. If the two vehicles lock together after impact, calculate using the principles of conservation of momentum, what will be

    a) their velocity after impact?
    b) the loss of kenitec energy

    i have had a attempt at A but i do no no where to start with B or what formula to use. this is my attempt at A)

    2. Relevant equations

    M1V1+M2V2=MfVf
    M1= 50000kg
    M2= 30000kg
    V1 = 60m/s
    V2 = 0 m/s

    3. The attempt at a solution

    MfVf = (50000*60)+(30000*0) = 3000000
    3000000/ (50000+30000) = 37.5m/s

    Could some one please help with Part b and help me with what formula to use
     
  2. jcsd
  3. Jun 6, 2012 #2

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Although you titled this "conservation of energy" I assume you realize that there is NO conservation of energy here. There is, however, conservation of momentum and that is what you used, correctly, to find the speed after the collision.

    The kinetic energy before the collision is (1/2)mv^2= (1/2)(50)(60)^2. The kinetic energy after the collision is (1/2)(50+ 30)(37.5)^2= (1/2)(80)(37.5)^2. Subtract the energy after the collision from the kinetic energy after the collision to find the loss of kinetic energy.
     
  4. Jun 6, 2012 #3
    hi hallsofivy thank you for the very prompt reply.

    sorry about the title, that was a typo.

    ive manged to work out before the collision as 90 MJ and after as 56.2 5MJ so the answer is 33.75 MJ?
     
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