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Homework Help: Inelastic collision of snadwiches

  1. Jun 11, 2009 #1
    1. The problem statement, all variables and given/known data

    On a greasy, essentially frictionless lunch counter, a submarine sandwich of mass 0.480 kg, moving with speed 3.00 m/s to the left, collides with a grilled cheese sandwich of mass 0.270 kg moving with speed 1.10 m/s to the right.

    1) If the two sandwiches stick together, what is the final velocity? (Take the positive velocities to the right.)

    2) How much mechanical energy dissipates in the collision?

    2. Relevant equations

    Pi = Pf ==> m1v1i + m2v2i = m1v1f + m2v2f

    KEi does not equal KEf

    KE = 0.5 m v^2

    3. The attempt at a solution

    I figured out the first unknown, which came out to be -1.52 m/s.

    I have difficulty finding the answer to the second part "mechanical energy dissipates in the collision". i have no clue on how to approach to this

    in anyhow, I didn't stop working, i had calculated the initial kinetic energy of each particle before they collide which came out to be the following.

    KE for submarine sandwich = 2.16 J
    KE for the other sandwich = 0.16335 J

    and the Final Kinetic energy of the system should equal 0.8664 J

    can anyone please tell me how to figure out the answer for the second part of the question?
    Last edited: Jun 11, 2009
  2. jcsd
  3. Jun 11, 2009 #2


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    You're almost there. How would you interpret the difference in kinetic energies?
  4. Jun 11, 2009 #3
    I really can't think of anything.

    I tried dividing them, once i used Kf/Ki and the other time i put Ki/Kf. Both were wrong answers.

    Would you help me please?
  5. Jun 11, 2009 #4


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    In an inelastic collision kinetic energy is not conserved as you know. But energy as a whole is always conserved so it cannot just disappear. It goes into the deformation of objects,sound, heat etc. How much energy has gone into deforming the sandwiches in your system?
  6. Jun 11, 2009 #5
    Thanks A lot Cyosis, I thought about it a little and i subtracted the final KE from The total Initial energy and got the correct answer. :)
  7. Jun 11, 2009 #6


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    Yep that's correct. The dissipated energy is just the energy that has "disappeared".
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