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Homework Help: Energy required to move an object to an altitude twice the radius of Earth

  1. Nov 23, 2011 #1
    1. The problem statement, all variables and given/known data
    How much energy is required to move a 1 000-kg object
    from the Earth’s surface to an altitude twice the Earth’s

    2. Relevant equations
    U = - G Me m / r

    3. The attempt at a solution

    I'm just using the above equation with 2r (twice the radius) and get 3.1 x 10e10 J, but the book says different...

    Any help?? Thanks!
  2. jcsd
  3. Nov 23, 2011 #2


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    Gold Member

    Plugging in a certain radius just tells you the potential energy an object has at that distance from the center. The question is asking you how much energy it takes to move from 1 radius (the surface) to another radius. So you need to find a difference in potential energy.

    In this case, you need to find the potential energy difference between the mass at the surface and at that 2r altitude. This must be the energy required to bring it from the surface to the high up distance.
  4. Nov 24, 2011 #3
    actually, I think I figured it out. The problem is somewhat bad worded and I believe it can confuse people. It says an ALTITUDE twice the radius of Earth, which means two times that distance ONCE the object has left the Earth, so actually it would be 3r.... right?

    I guess it's right, I got the correct answer ;) thanks for the help!!
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