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Homework Help: Mechanical Energy loss due to NC Forces: two railroad cars collide.

  1. Oct 11, 2009 #1
    [SOLVED-Thanks AL]Mechanical Energy loss due to NC Forces: two railroad cars collide.

    SOLVED! - Thanks Al!

    1. The problem statement, all variables and given/known data

    Two railroad cars each of mass 4800 kg and traveling @ 80 km/h collide head on and come to rest. How much thermal energy is produced in this collision?

    2. Relevant equations

    [tex] E_k + E_p = E^{'}_k + E^{'}_p + Q [/tex]

    3. The attempt at a solution

    This problem does not use momentum. Energy considerations only.

    I treat one car as a single entity.

    Energy before = energy after.

    There was only an initial kinetic energy at the beginning and no kinetic energy afterwards. so i assume all the energy was transformed to Q.

    [tex] \frac{1}{2}mv^2 = Q [/tex]

    after finding 80 km/h = 22.2222... m/s

    i solved and got 1.2 x 10^6 J, the answer is double that, 2.4 x 10^6 J, where did i go wrong?
    Last edited: Oct 11, 2009
  2. jcsd
  3. Oct 11, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    There are two moving trains. :wink:
  4. Oct 11, 2009 #3
    OH! I only calculated the amount of thermal energy one train would release, and not the other. So the answer would be Q of both trains (KE of train one + the KE of train two) as it's released simultaneously ?? Ugh... i thought i'd isolate only one train because I wasn't dealing with momentum.. got my head stuck in dynamics :) Thanks a lot Doc!
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