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Homework Help: Momentum. Kinetic energy lost in the collision.

  1. Apr 8, 2012 #1
    A railroad car of mass M moving at a speed v1 collides and couples with two coupled railroad cars, each of the same mass M and moving in the same direction at a speed v2.

    (a) What is the speed vf of the three coupled cars after the collision in terms of v1 and v2? (Use M, v_1 for v1, and v_2 for v2 as appropriate.)

    a) (M*v_1+M*v_2+M*v_2)/(3M)

    (b) How much kinetic energy is lost in the collision? Answer in terms of M, v1, and v2. (Use M, v_1 for v1, and v_2 for v2 as appropriate.)

    b) I don't get this I had this for the answer. 1/2*M*[-2/3*v_1^2 +4/3*v_1*v_2-2/3v_2^2]
    but it is wrong...

  2. jcsd
  3. Apr 8, 2012 #2


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    Staff: Mentor

    While the expression is correct, you can simplify it some more.
    Can you show how you arrived at your result?
  4. Apr 8, 2012 #3


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    Homework Helper

    Note the sign: It is the change of KE, not the lost KE.
    And simplify the expression.

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