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Homework Help: Unit for Gravitational Attraction Between Objects?

  1. Feb 18, 2007 #1
    1. The problem statement, all variables and given/known data
    1. The problem statement, all variables and given/known data
    Two objects, one with a mass of 200g, and the other with a mass of 1 kg are placed 10 cm apart. What is the gravitational attraction between the objects?

    Background: I'm performing an experiment in which a horizontal beam is suspended just above the ground with weights at either end. The larger weights are placed away from the smaller ones, so that the beam would have to turn for the distance between te large and small weights to decrease. I need to find the gravitational attraction between the objects, expressed in... well first I need to know which unit it would be expressed in.

    I think it would be expressed in Newtons. Is this correct?
  2. jcsd
  3. Feb 18, 2007 #2


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    Yes, you are correct.
  4. Feb 18, 2007 #3
    Ok, so the formula F=G(mm)/d^2 would give the force in newtons.

    The gravitational force being 6.6742 x 10^-11*m^3*s^2*kg^2

    So F= 6.6742 x 10^-11*m^3*s^2*kg^2 (200g*1000g)/ 0.01 ?

    Which would give a force of... how do I calculate using G? Everything else I can do, but what do I need to do with G?

    An example question would help, and no, this is not homework, seeing as the Ontario Grade 8 curriculum most certainly does not involve such mathematics.
  5. Feb 18, 2007 #4


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    G is not a force, it is called the universal gravitational constant. It is the same everywhere. For your calculation, you must convert all your units into kg, m, s, so everything is consistent. For example in your equation you have (200g*1000g). You need to change the mass from grams to kilograms, to be consistent with the units you see in G. The same goes for distance, it must be in meters.

    It's not too difficult, just multiplication and division.
  6. Feb 18, 2007 #5


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    This is a good problem to get you used to unit manipulation. You did everything right, except as hage pointed out, you can't use grams because the number you are using for G only works with kilograms. I've cleaned up the math a bit so you can see step-by-step where different units combine and cancel

    Last edited: Feb 18, 2007
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