1. The problem statement, all variables and given/known data Hi, I've been tasked with showing that the length of vector F_gravity is inversely proportional with radius squared (i.e. |F_vector|=c/r^2) and is central, i.e. consistently directed toward the same point. Apparently, this is the same as (<=>) the acceleration of a particle following an elliptical path being radial (and thus, directed towards the same point as F) *and* it being equal to a_r = - C/r (1) where C is a universal constant. C ends up being 4*π^2*(a^3/T^2), i.e. positive. My questions are: Why is there a minus in (1), and what is the explanation for the <=> being valid? I realize it's Newtons 2nd law, but the constants C and c could conceivably differ by more than m, no? Apologies for the shoddy notions, not used to doing physics in english (or writing formulas outside of Word...) 2. Relevant equations |F_vector|=k*1/r^2 and central <=> a_r = - C*1/r (1) where C is a universal constant. C ends up being 4*π^2*(a^3/T^2), i.e. positive. 3. The attempt at a solution Can't figure it out.