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How does the conservation of momentum explain this?

  1. May 30, 2010 #1
    Picture the earth without any atmosphere or air molecules. If I were to throw a rock with 0.5 kg straight up from the surface of the earth it will reach a velocity of zero at its peak? At this moment the momentum of the rock becomes zero. Where does the momentum go if its always conserved?
     
  2. jcsd
  3. May 30, 2010 #2

    Doc Al

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    Staff: Mentor

    The momentum of the rock by itself is not conserved. But the momentum of the rock + earth system is. The vector sum of the momenta of each remains constant throughout the motion.
     
  4. May 30, 2010 #3
    The earth moves
     
  5. May 30, 2010 #4
    The rock has gravitation of its own. As it accelerates toward Earth, reducing its upward speed, Earth also accelerates upward toward it. The velocity change the Earth experiences is far smaller than that experienced by the 0.5 kg mass, but the Earth masses much, much more, so their changes in momentum are equal and opposite.
     
  6. May 30, 2010 #5

    jtbell

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    Staff: Mentor

    As the rock goes up and down, the earth goes down and up, by a much smaller distance of course.
     
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