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Conservation of Momentum: A Simple Question

  1. Oct 21, 2008 #1
    I am not sure if I understand the premises under which conservation of momentum can be considered. First of all, I am aware that momentum is conserved if there are no external forces acting on a system. So, if you are looking at two objects colliding, their momentums will always be conserved if there aren't any forces acting on the system itself.

    This brings into question of actually solving for conservation of momentum. Is it always conserved during a collision? What kind of external forces can act during a collision? Is the momentum conserved if you bounce a ball on Earth?

    Another thing I am confused about is the vector relationship to conservation of momentum. Say that before a collision, the object's momentum is conserved since the ground is frictionless. But after the collision, the object's velocity switches direction, yet the magnitude stays the same. In this case, is momentum still conserved? The way I had interpreted is to observe p = mv and if the direction of v changes, then the final vector p is changed. So does momentum change during the collision? If so, does it mean that it's not conserved?

    Sorry for making this topic a little longer than I had hoped for.

    Thank you in advance for any help
  2. jcsd
  3. Oct 21, 2008 #2


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  4. Oct 22, 2008 #3
    Conservation of momentum permits vector directions and magnitudes to change for individual particles/masses...but the total system momentum remains constant in the absence of external forces. Internal forces, being equal and opposite, produce equal and opposite changes in total momentum which cancel.
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