1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Potential energy in 2 gravity fields

  1. Sep 13, 2015 #1


    User Avatar

    Hi everyone,

    First, sorry if the question is stupid, I do not have any background
    in Physics. Just very curious.

    Potential energy due to gravity is:
    m * g * h Joules

    Now, suppose I lift a weight on Moon at a given height. I will have:
    1.6 * m * h Joules

    I maintain it at that height and go to Earth. There, I will have:
    9.8 * m * h Joules

    Where does the added energy come from ? I cannot see why it would
    come from the Moon Earth travel itself, as only the mass matters during
    the motion ...

    Thanks for your time and answers !


  2. jcsd
  3. Sep 13, 2015 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    First of all, the mgh formula is only a valid approximation as long as the gravitational field may be considered homogeneous. At scales as large as the distance from the gravitational center, this is no longer true.

    You cannot do this. In order to transport it to Earth, it needs to change its height from both the lunar surface and the Earth's surface. This involves a change under which the gravitational field most certainly is not homogeneous. In addition, the zero-point of the different potentials are chosen differently. You are comparing apples and carrots.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook