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Dissipation of mechàanical energy in collision of sandwiches

  1. Jun 16, 2008 #1
    On a greasy, essentially frictionless lunch counter, a submarine sandwich of mass 0.500 kg, moving with speed 3.50 m/s to the left, collides with a grilled cheese sandwich of mass 0.200 kg moving with speed 2.00 m/s to the right.

    How much mechanical energy dissipates in the collision?

    I calculated the final velocity (-1.93 m/s) but I don not know where to go after that
     
  2. jcsd
  3. Jun 16, 2008 #2
    In this collision, think about what's being conserved and what isn't.

    Momentum is always conserved. That leaves just one thing that isn't.
     
  4. Jun 16, 2008 #3

    Doc Al

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    Staff: Mentor

    Compare the initial total KE of both sandwiches to the final KE.
     
  5. Jun 16, 2008 #4
    i think that the kinetic energy is what is not being conserved because it is being transfered to puck b.

    the initial ke of a is
    a: ke= .5(.5)(-3.5^2)= 3.0625
    b: ke= .5 (.2)(2^2)= 1

    and the final ke is ke= .5(.5+.2)(-1.93^2)= 1.304

    I had all these and subtract them from eacj other and got -2.76
     
  6. Jun 16, 2008 #5
    Pretty sure that's right, although the negative sign is unnecessary. As long as you understand that your answer is the change in kinetic energy, that is, the kinetic energy of your system went down by that much, you're good.
     
  7. Jun 16, 2008 #6
    I just tried that one and my homework says that -2.76 and 2.76 are not right. Anything else i can do?
     
  8. Jun 16, 2008 #7

    Doc Al

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    Staff: Mentor

    Redo that second calculation.
     
  9. Jun 16, 2008 #8
    thanks. i went back through my math and found an error, the answer is -2.16 J
     
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