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Homework Help: Gravitational Field Question

  1. Jun 3, 2012 #1
    1. The problem statement, all variables and given/known data
    A 12 kg meteor experiences an acceleration of 7.2 m/s^2, when falling towards the earth.

    a.) How high above the earth's surface is the meteor?

    b.) What force will a 30 kg meteor experience at the same altitude?

    2. Relevant equations

    Fg = ma , Fg =Gm1m2 / r^2

    3. The attempt at a solution

    a = Fg / m, thus:

    Fg / m = Gm1m2 / r^2


    r = √ (Fg / Gm1m2(m))

    I got this far, somewhat understanding what I was doing the equation manupulation. Please help me! I searched this question up, but that thread was not helpful.

    I don't know how to solve the question. The altitude is the radius from the earth? and since a = Fg / m, :S I equated this with the Fg = Gm1m2 / r^2, :|.

    Please help!
  2. jcsd
  3. Jun 3, 2012 #2


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    Do you understand what "Gm1m2/r^2" means? What is G? What are m1 and m2?
    A really major error is setting "a= Fg/m= Gm1m2/r^2". "Gm1m2/r^2" is the force due to gravity, not the acceleration. What you want is Fg= ma= Gm1m2/r^2.

    Now, again, what are m1 and m2?

    That wolyld be correct but you didn't! You equated it to a, not Fg.

  4. Jun 3, 2012 #3

    Yes I do understand what the equation: Fg = Gm1m2 / r^2 this is what my notes say (taking an online physics course, they provide notes)

    G = gravitational field constant, m1 and m2 are the two masses, Fg = the force of gravity acting on each object,

    r = the distance between the centres of the two masses.

    this is the newtons law of universal gravitation in the above equation form.

    So ma = Gm1m2 / r^2 ??

    which means r = √ (ma / Gm1m2)?

    m1 would be the earths mass and m2 would be the meteor mass? but the mass, m2, would cancel out right?
  5. Jun 3, 2012 #4


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    It looks as if you have three different masses, m1, m2 and m. There are only two objects, so there should only be two different masses.
  6. Jun 3, 2012 #5
    The top equation is right, but you solved for r wrong.
    And you are right about m2, it does cancel.
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