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Homework Help: Gravity weight problem

  1. Nov 24, 2009 #1
    1. The problem statement, all variables and given/known data
    If an apple at the surface of the Earth has a weight of 1 N, its weight three times as far away from the centre of the Earth is

    2. Relevant equations

    f = m1m2/d^2

    3. The attempt at a solution


    so is it 1/3?
    or since it's three times

  2. jcsd
  3. Nov 24, 2009 #2


    User Avatar

    Staff: Mentor

    Re: Gravity

    Welcome to the PF. I'd suggest that you be a little more careful and rigerous in writing the equations.

    On your first equation, you are missing a constant, right? It may not be important in the calculation you are doing, but gravitational force is not equal to what you wrote as m1m2/d^2. It is proportional to that, but not equal.

    And put some variables into the equation to get the ratio, don't just guess. Call the radius of the Earth R, and write the two force equations for the two spots. Then divide them -- what do you get?
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