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Homework Help: What is this derivative

  1. Jan 17, 2006 #1
    Can someone please tell me the derivative of the following with respect to little 'r' where G is the gravitional constant (6.67x10^-11), Me is mass of earth (5.98X10^24kg), Ms is mass of a spacecraft that has a mass of 1000kg and R which is the distance between the earth and the moon (384400 x 10^3m). Thanks.

    U = (-GMeMs)/(r) - (GMmMs)/(R-r)
  2. jcsd
  3. Jan 17, 2006 #2
    re-write your equation U = (-GMeMs)/(r) - (GMmMs)/(R-r) as:
    U = (-GMeMs)(r^-1) - (GMmMs)[(R-r)^-1]
    Then it is simply using the chain rule. For the first term: Multiply by the exponent, subtract the exponent by 1 to get the new exponent. Everything else is constant so no need to touch it.

    For the 2nd term same thing, except do not forget u also must take the derivative of what is on the inside. Which is -1.
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