Keplers laws and a hypothetical 1/r^3 dependence (1 Viewer)

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Hello,
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?
 

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