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Homework Help: The Gravitational Force and the distance from the earth

  1. Sep 11, 2008 #1
    1. The problem statement, all variables and given/known data
    Find the altitudes above the Earth's surface where Earth's gravitational field strength would be (a) two-thirds and (b) one-third of its value at the surface. [Hint: First find the radius for each situation; then recall that the altitude is the distance from the surface to a point above the surface.]

    2. Relevant equations

    I do believe i use F=Gm1m2/r^2

    3. The attempt at a solution

    Well, actually im at a little loss for this question..
  2. jcsd
  3. Sep 11, 2008 #2


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    The hint tells it all.

    Set 1 equation = F, another to 2/3 F another to 1/3 F.

    Solve by dividing one equation by another.
  4. Sep 11, 2008 #3


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    That's the right equation.

    Let's say at the surface we have

    Fo=G m1 m2/ro^2

    and we want to find the distance r1 where the force F1 is

    \frac{F1}{Fo} = \mbox{???} = 2/3

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  5. Sep 11, 2008 #4
    problem is that, it doenst give me the distance for the altitude...
  6. Sep 11, 2008 #5


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    Yes, that distance is what we are trying to figure out.

    As you said, F=Gm1m2/r^2 . Can you use that equation to express the ratio of two forces,

    F1 / F0

    where F1 is the force at distance r1, and F0 is the force at distance r0?
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