what is it exactly? and in my book they had an equation F = (4)(pi)^2(m)(r)/T^2 then they said by using keplers law... they arrived to a new equation that relates the gravitational force exerted by the sun which is... F = m/r^2 If Keplers law says T = r^3/2 how in the heck did they go from F = (4)(pi)^2(m)(r)/T^2 to F = m/r^2 by substituting T for r????? Where did (4)(pi)^2(r) go ????????