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B Law of conservation of momentum in a non isolated system.

  1. Nov 29, 2016 #1
    Is the law of conservation of momentum valid in non isolated system . What is an isolated system? My teacher confuses me that the law is valid in an non isolated system . They even say that when we say about the isolated system (meaning they are present ) we do not consider the air resistance or frictional forces is that true? They give me the example of a gun recoiling , I am confused .
     
  2. jcsd
  3. Nov 29, 2016 #2

    BvU

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    Isolated in this case means no external forces and no mass crossing the boundary. A tennis ball in the air is not an isolated system: it will fall to the ground. A gun also, but if you look in the direction of where it's aiming then the momentum in that direction is conserved.
     
  4. Nov 29, 2016 #3
    @BvU what are you talking about the direction?
     
  5. Nov 29, 2016 #4

    BvU

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    You need a constant force to keep the gun from dropping to the floor. But to calculate the kickback you can use conservation of momentum in the direction the bullet flies.
     
  6. Nov 29, 2016 #5

    jbriggs444

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    [and make sure that you place the boundaries around the "system" so that both gun and bullet remain inside]
     
  7. Dec 3, 2016 #6
    What will be the conclusion , will the law of conservation of momentum hold in an non isolated system .
     
  8. Dec 3, 2016 #7

    BvU

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    No. See #2.
     
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