Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Universal Gravitation and Local G

  1. Jan 21, 2010 #1
    K, so I dont quite understand how I find a persons weight on a different planet using the said person's weight, the mass of the other planet, and the radius of the planet?

    So, for example:

    Max weighs 1000 N on Earth. What would his weight be on a planet with twice the mass of Earth, but with the same radius?

    How would I go about answering this and similar questions? Thanks for your help!!
  2. jcsd
  3. Jan 22, 2010 #2


    User Avatar
    Gold Member

    Weight is a force, F:

    [tex] F = G \frac{m_1 m_2}{r^2} [/tex]

    If you have a constant, known m1 and a constant r (like Earth) the equation can be simplified down to:

    [tex] F = m_2 g [/tex]


    [tex] g = \frac{G m_1}{r^2} [/tex] (basically all the other values in the equation besides m2. m1 was the mass of Earth, so m2 is the so-called "test mass" (you or me, or a ball, for instance).

    So look at the first equation. What happens to F when we double m?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook