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Homework Help: Potential energy.

  1. May 11, 2010 #1
    1. When a body of mass 'm' is taken from the surface of the earth to a height equal to the radius of the earth (R) then the change in its Potential energy is______

    got the solution as follows:
    1) when on the surface of the earth the P.E is mgR (considered it as initial P.E)
    when the body is taken to height 'h=R' frm the surface of earth the P.E is mg(R+R) = 2mgR (considered it as final P.E)
    .. . change = Final P.E - Initial P.E = 2mgR-mgR= mgR

    another approach:
    2) on the surface of earth P.E = 0
    when raised to "h=R" its P.E = mgR
    . . change in P.E= mgR

    out of these two approaches which is considered to be the best.
  2. jcsd
  3. May 11, 2010 #2
    U = mgh is only a valid approximation when near Earth's surface. You must use the gravitation potential energy
  4. May 11, 2010 #3
    can u make it more clear..
  5. May 11, 2010 #4
    The gravitational potential energy is [tex] U = \frac{-GMm}{r} [/tex]
    where r is the distance from the center of mass of the Earth. You can use u = mgh as an approximation when close to Earth's surface.
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