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Homework Help: Linear Momentum/Energy Loss

  1. Aug 16, 2012 #1
    1. The problem statement, all variables and given/known data

    A 1110 kg car traveling initially with a speed of 25.0 m/s in an easterly direction crashes into the rear end of a 9900 kg truck moving in the same direction at 20.0 m/s (Fig. P9.21). The velocity of the car right after the collision is 18.0 m/s to the east.

    image: http://www.webassign.net/sb5/p9-21.gif

    (a) What is the velocity of the truck right after the collision?

    (b) How much mechanical energy is lost in the collision? (Use input values with an adequate number of significant figures to calculate this answer.)

    (c) Account for this loss in energy. (Select all that apply.)
    Internal energy
    Elastic Potential
    Gravitational Potential

    2. Relevant equations

    m1v1i + m2v2i = m1v1f + m2v2f

    3. The attempt at a solution

    So I figured out parts a and b, but I cannot get part c.
    Honestly, I do not remember anything like this question from class.
    So far I these were my guesses:
    (1) just kinetic
    (2) kinetic, internal, elastic
    (3) kinetic, elastic

    But the truth is I don't really know what this means, and I want to stop guessing and actually understand it…
    I think kinetic would be one of them because both vehicles slow down.
    I don't know what else though

    Thanks for your help
  2. jcsd
  3. Aug 17, 2012 #2


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    Homework Helper

    welcome to pf!

    hi lo31415926535! welcome to pf! :smile:
    i think they mean account for mechanical-energy-before minus mechanical-energy-after

    if all the energy was included (and not merely the mechanical energy), then the difference would be zero

    so, essentially, they're asking you to list the energy that isn't mechanical energy :wink:
  4. Aug 17, 2012 #3


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    Science Advisor

    You should be able to drop "Gravitational Potential Energy" from that list- the vehicles don't go up at all! And the same for "Elastic Potential Energy". In fact just seeing "potential" in those should tell you they cannot be responsible for loss of mechanical energy in a collision.
  5. Aug 17, 2012 #4
    thank you both! i got it :)
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