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Keplers laws and a hypothetical 1/r^3 dependence

  1. Mar 7, 2006 #1
    i've been asked a hypothetical question about Keplers three laws: What if the gravitational force was proprotional to 1/r^3 instead of 1/r^2? And for one of the laws it apparently "isent easy to decide". My thoughts:

    keplers 1. : my first thought was that this law was the "not easy to decide"
    keplers 2. : must still be valid, because the vector product r x F will always be zero, and hence dL/dt = 0 (conservation of momentum for any central force)
    keplers 3 : a planets period would change?

    Is this correct? And can somebody help me clearify matters?
  2. jcsd
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