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Homework Help: A quick dirivative Question

  1. Apr 28, 2006 #1
    Alright, I'm doing a dynamical magnetism problem for my physics class. I've got it almost done except I need to take the derivative of 1/x^3 with respect to time. X in this case is a variable for meters, I want to turn this into an expression with velocity (Velocity is given) but I'm not sure what that is.

    d(1/x^3)/dt = ? I'm pretty sure its not just 1/v^3 but that would be nice ^_^. Any help would be appreciated.

    Would this just be -4x^-3*v?
    Last edited: Apr 28, 2006
  2. jcsd
  3. Apr 28, 2006 #2


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    No, you differentiated wrongly. What is [tex]\frac{d}{dx}(x^{-3})[/tex] ?

    Then [tex]\frac{d}{dt}(x^{-3}) = \frac{d}{dx}(x^{-3})(\frac{dx}{dt}) = v\frac{d}{dx}(x^{-3})[/tex] as you correctly surmised.
  4. Apr 28, 2006 #3
    k, I guess I have a bigger problem then originally thought since X is not given :/.
  5. Apr 28, 2006 #4


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    What ? :confused:

    x is a variable denoting displacement right ? And don't mix up the cases - stick to small x.

    All I'm saying is you differentiated wrong.

    What is [tex]\frac{d}{dx}(x^n)[/tex] ? This should be in the textbook. Now plug in n = -3.
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