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Conservation of energy, momentum

  1. Mar 7, 2010 #1
    i don´t quite understand:

    if you have a system made up of three pool balls. one of them is moving with a speed of 1 meters per second, the two other balls are standing still. if the moving ball hits the two other balls, in a way that the moving ball is standing still afterwards, then the two other balls should have a speed of 0.5 meters per second each, so that momentum is conserved in this system, right?
    if that is the case than there is a loss of kinetic energy, because 1 ball moving at a certain speed has double the kinetic energy, than two balls moving at half the speed?
    but energy is supposed to be conserved in a system two.
    can someone please help get rid of that twist in my head, what am i getting wrong here? is momentum not being conserved in a system?
  2. jcsd
  3. Mar 7, 2010 #2
    Yes momentum is conserved. And yes there is a loss of kinetic energy to the environment.

    However a rolling ball also has rotational energy so you need to add all types of energy to do an energy balance.

    You always have to be careful accounting for all the energy changes when using conservation of energy conservation of momentum can be a safer bet (although less intuitive) this is will come home if you ever study elementary particle physics.

    There is an equivalent situation in electricity.

    Consider two identical capacitors, one charged, one not charged.

    Now connect the discharged one across the charged one.

    What is the voltage across the combination and what is the energy state of the system?
  4. Mar 7, 2010 #3
    Your conservation of momentum argument is wrong, because the balls can move in different directions, so their speed can be larger, but their momentum can still be equal to momentum of the first ball.
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