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Homework Help: How exactly does friction/external forces affect the momentum?

  1. Nov 10, 2009 #1
    How exactly do friction forces and other external forces change (mostly decrease) the momentum of an object?

    We know that the conservation of momentum theory only applies to closed systems with no external forces, yet in real life, there always is external forces that must surely affect the momentum. How do exactly do they affect it?

    (I know that friction does work on the object, and therefore decreases the kinetic energy. H/o, shouldn't this not change the momentum? For ex., in inelastic collisions, kinetic energy is lost yet momentum is still conserved...)
  2. jcsd
  3. Nov 10, 2009 #2


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    External forces just push things around and make them go faster or slower, which changes their momentum. But I think this isn't what you were asking. Can you elaborate?
  4. Nov 10, 2009 #3
    I can see that friction forces affect objects' velocity, which I suppose, should affect there momentum. However, I'm confused as to why in an inelastic collisions, the objects lose kinetic energy (and therefore velocity, right?) and yet still conserve momentum?

    To elaborate further, our physics teacher had us do a lab where we collided two balls together and "measured" the momentum to test out the conservation of momentum theory. The initial momentum was close to the final momentum, of course, however, there was still some slight errors. Besides human error, what else could have caused this slight loss of momentum?
  5. Nov 10, 2009 #4


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    The conservation of momentum is a direct consequence of Newton's third law. Here's a simplified proof:

    m_a applies a force on m_b and m_b applies a force on m_a. m_a accelerates at F/m_a; m_b accelerates at F/m_b in the other direction. After a while, v_a=F/m_a*t and v_b=F/m_b*t. m_a*v_a=F*t and m_b*v_b=F*t, so m_a*v_a = m_b*v_b. That's the conservation of momentum.

    Kinetic energy is not conserved because the energy is turned into things like heat and sound.

    External forces: gravity (if the table wasn't perfectly level), air resistance, rolling friction, etc. If you just measured mv and said that was the momentum, you would have neglected the balls' rotational motion, which is another source of error.
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