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If a mass became pulled up from Earth, its U increases and its K

  1. Jun 7, 2008 #1
    If a mass became pulled up from Earth, its U increases and its K decreases, but if it became pulled into space, its U becames 0. So, could we say that Gravity is not a conservative field in large scale, but a negative Energy?
  2. jcsd
  3. Jun 7, 2008 #2

    Doc Al

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    Staff: Mentor

    Realize, since we take U = 0 at infinity as the reference point, that the U of the system starts out negative and increases to zero.
    Not sure why you'd conclude that gravity is not conservative at large scale. Realize that the sign of potential energy depends on your reference point.
  4. Jun 7, 2008 #3


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    Science Advisor

    Well, no. If it is pulled into space, then there is a force acting on it which would increase its total energy. It might well keep the same K (Kinetic energy) while increasing its potential energy. If it already has a large kinetic energy on earth and uses only that energy to move away from the earth, with no external force, then the kinetic energy decreases while the potential energy increases so the total energy remains the same.

    Again, no. Its potential energy does NOT become 0 "in space". For gravitational problems, potential energy (which, as Doc Al said, is always relative to some reference point) is taken to be 0 at infinity, not just in space. If you take an object to have a very large negative potential energy on earth (and considering only the earth's gravity) the at several light years from the earth it would have a very small, but still non-zero negative, potential energy.
  5. Jun 7, 2008 #4
    Ok, great, you are very good.
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