solve for rF= k (m1m2/r^2)
\(F= k \frac{ m _{1}m _{2}}{r^2} \\ \\ r eq 0\\ \\ r^2F =k\cdot m _{1}m _{2} / :F \\ \\r^2= \frac{k\cdot m _{1}m _{2} }{F}\\ \\r = \sqrt{\frac{k\cdot m _{1}m _{2} }{F}}\)
