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Homework Help: Law of universal Gravitation

  1. Nov 18, 2013 #1
    1. The problem statement, all variables and given/known data

    The radius of the earth is 6400km. A 7200n space craft travels away from the earth. what would be the weight of the space craft at the following distances from the Earths surface: 6400km, 12800km

    2. Relevant equations


    3. The attempt at a solution
    Just want to confirm I'm doing this right
    So first I want to find the mass of the space craft so I use the fg=mg formula. I know that the mass of the earth is 5.98*10^24kg.

    Gravitational constant is 6.67*10^-11

    I use the universal gravitation formula

    The r will just be the radius of the earth PLUS the distance from the earth's surface. (Converted to m)

    Then just plug into the equation. Am I doing this right?
  2. jcsd
  3. Nov 18, 2013 #2
    It is not wrong but too complicted. They give these nice numbers for a reason.
    Look at the numbers. By what factor increases the distance? What does this tell about the change in force?
  4. Nov 19, 2013 #3
    The factor increases by 2 (6400+6400) then 3 (6400+12800)

    So u mean I would just multiply 7200 by 1/4 for the distance 12800 from th earths center
    And 1/9 for the distance 19200 ?
  5. Nov 19, 2013 #4
    You have what is known as 'an inverse square law'


    Force due to gravity decreases with 1/r^2
  6. Nov 19, 2013 #5
    Yes. This will do it.
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