i am examining a proof of kepplers first law for my conics project and im a bit rusty on the physics, it begins withe the following line
To begin with, we will start off by applying Newton's law of motion and Newton's law of universal gravitation together to find that
m a = (-G m M/r^2) r
i think there is an extra r in this equation? if no where does it come from??
here is the link if it helps
The Attempt at a Solution
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