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Conservation of momentum

  1. Sep 3, 2015 #1
    1. The problem statement, all variables and given/known data

    apparently momentum is only sure to be conserved in a closed system - no external forces. So that means if a toy car collides with a wall then the total momentum efter wont be the same as before. That is because the wall would
    be joined to something else and ultimately to the Earth.

    What exactly is a closed system with no external forces? And isn't everything on the earth connected to earth?

    2. Relevant equations
    3. The attempt at a solution
     
  2. jcsd
  3. Sep 3, 2015 #2

    DEvens

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    A closed system is exactly what you said. No external forces.

    The idea of "system" is fairly artificial. You draw a "bag" around the things you are interested in. That's your system. For purposes of conservation of momentum, you have to take notice if any forces go in or out through the "bag."

    So if your system is just the toy car, then it is not closed. Forces involving the wall penetrate in through the "bag."

    But suppose instead of a wall it was a big block of concrete. And both the car and the stone were floating in space with nothing contacting them. And you include both masses as part of your "system." Then for your system there are no forces penetrating in or out. And momentum is conserved. It may be very hard to accurately measure the change in velocity of the big block of concrete when a small toy car hits it. If the toy car masses 100 grams, and the concrete masses 1E8 grams, for example, the change in velocity of the concrete will be correspondingly small.
     
  4. Sep 3, 2015 #3
    So you mean that there are things that can affect the wall other than the car?
     
  5. Sep 3, 2015 #4

    DEvens

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    ^_o
     
  6. Sep 3, 2015 #5
    Sorry im gonna sound stupid but this is hard
     
  7. Sep 8, 2015 #6

    HallsofIvy

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    The wall is attached to other walls that make up a building, the building is attached to the earth. Yes, there are a lot of things that 'affect the wall other than the car'.
     
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