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Conservation of momentum and energy question

  1. May 26, 2010 #1
    Hello everyone,

    Let's say there was gun and a bullet with a mass of 1kg at rest. The bullet and guns speeds were 2 m/s in opposite directions after firing if I right the equations.

    Kinetic energy

    0.5m1v12 + 0.5m2v12 = 0.5m1v22 + 0.5m2v22


    Why doesn't this equation work when objects are at rest. Mathematically I can understand but why physcially. Can anyone explain what is happening to kinetic energy in this scenario. Why does conservation of momentum equation hold in in inelastic collisions but not this one. I understand some energy is lost but why is it not affecting momentum.
     
  2. jcsd
  3. May 26, 2010 #2

    Vanadium 50

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    Where does the energy to propel the bullet come from?
     
  4. May 26, 2010 #3
    Ok let's say it came from gun powder. Then energy from gun powder would be given to the bullet, making it fly. Now the gun hasn't lost any energy or gained any but still it recoils. Why does it recoil?
     
  5. May 26, 2010 #4

    Doc Al

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    The point is that mechanical energy is not conserved, so your equation doesn't apply. (But you can write an equation for conservation of momentum.) The explosion of the gun powder provides energy to both the bullet and the gun.
     
  6. May 26, 2010 #5
    Thanks for the reply. Why does gun powder give a forward force to the bullet and backward force to the gun. Also why is this question not about gun powder and bullet. If gun powder provides the force, the reaction force should be towards gun powder, not the gun.

    EDIT: Let me think this again. If force is rate of change of momentum. Since gun powder is part of the gun, the gun changed the momentum of the bullet. Oh so you can't change the momentum of something else without that momentum coming from you, meaning losing it. Oh so the gun retaliates. Oh so basically if you want to displace something, the person who is trying to displace the object is going to get displaced as well atleast a tiny bit. May be that is what momentum is saying. Is this r
     
    Last edited: May 26, 2010
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