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Conservation of momentum (should be easy)

  1. Oct 31, 2006 #1
    An object, with mass 16 kg and speed 19 m/s relative to an observer, explodes into two pieces, one 4 times as massive as the other; the explosion takes place in deep space. The less massive piece stops relative to the observer. How much kinetic energy is added to the system during the explosion, as measured in the observer's reference frame?

    I calculated the original momentum, then i calculated the final momentum. I then solved for vfinal. I got 23.75 m/s for my vfinal. But now I tried calculating delta KE and I'm getting 1624.5 J, but my hw site is telling me it is incorrect. Can someone help me out?
  2. jcsd
  3. Oct 31, 2006 #2
    I believe that your vfinal is correct, but you have probably made some stupid mistake with calculations at the end.
    Initial kinetic energy is 0.5*16*(19)^2 and final KE is 0.5*(16/5)*4*(23.75)^2, and the difference, delta KE is 722J. Check it again!
  4. Oct 31, 2006 #3
    Are you using the correct formula?
  5. Oct 31, 2006 #4
    thanks i was adding both masses for the final momentum, that was my foolish mistake.
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