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Homework Help: Derivative of Newton's Law of Gravitation

  1. Mar 21, 2010 #1
    1. The problem statement, all variables and given/known data
    Newton's Law of Gravitation says that the magnitude (F) of the force exerted by a body of mass (m) on a body of mass (M) is

    F = (GmM)/(r^2)

    where G is the gravitational constant and r is the distance between the bodies.

    Find dF/dr and explain it's meaning. What does the minus sign indicate?

    2. Relevant equations
    none

    3. The attempt at a solution

    using implicit differentiation:
    dF/dr = d/dr [(GmM)/r^2]
    = [(r^2)d/dr(GmM) - (GmM)d/dr(r^2)] / (r^4)
    = [(r^2)G(d/dr(mM)) - (2rGmM)] / (r^4)
    = [(r^2)G(m * dM/dr + M * dm/dr) - 2rGmM] / (r^4)

    Ok, I know this can be simplified, but that seemed even more convoluted when I did that. I'm really not sure what to do from here. I expected the derivative to be something a little more intuitive and to result in a negative expression based on the question. I would expect dF/dr to equal a negative expression and the significance of the minus sign to be indicative of a force of attraction since the derivative is with respect to distance and the distance is decreasing. Please help me with the derivative, because If I can get that right, then I just need to express what's going on in words.
     
  2. jcsd
  3. Mar 21, 2010 #2

    tiny-tim

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    Hi Bear_B! :wink:

    (try using the X2 tag just above the Reply box :wink:)
    ooh, so complicated! :cry: no, m and M are constants.

    Try again. :smile:
     
  4. Mar 21, 2010 #3
    Ok, I had seen that result worked out elsewhere when I couldn't get on here. Can you please tell me why m and M are constants? I thought the equation to determine the gravitational force was for an unknown mass m and an unknown mass M at a unknown distance r from each other...knowing that m and M are constants makes the differentiation simple, I just want to understand why I am treating m and M as constants. Thanks
     
  5. Mar 21, 2010 #4
    F = GmM/r2
    dF/dr = GmM [ d/dr (r-2)]
    = GmM (-2r-3)
    = GmM (-2/r3)
    dF/dr = -2GmM / r3

    ok, so here is the answer, but why are m and M constants?
     
  6. Mar 21, 2010 #5

    D H

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    Because they have nothing to do with r. They are masses.
     
  7. Mar 22, 2010 #6

    tiny-tim

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    Hi Bear_B! :smile:

    (just got up :zzz: …)

    Yes, m and M are variables just like r,

    but they are independent variables (they can be varied separately, without affecting the others).

    So when the question asks "Find dF/dr", it means differentiate with respect to r, keeping all other variables constant. :smile:
     
  8. Mar 22, 2010 #7
    Thanks. Now I understand the concept that the Force (F) is being taken with respect to distance (r) determining the rate of change of F as r changes for two objects with mass m and M. Since dF/dr is asking how F changes when r changes it has to assume that the m and M are constants unless otherwise indicated. I think I get it now. I just wanted a conceptual understanding of the problem beyond how to do the problem mechanically (because that is the easy part).

    ^^ This is my (basic) understanding of why the derivative assumes m and M as constants when differentiating with respect to r.
     
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