m2 = 20 (slider from 1 to 20, step 0.1)
m1 = 16.2 (slider from 1 to 20, step 0.1)
d = 15 (slider from 1 to 15, step 0.1)
k: m1 * m2 / d^(2.0)
M1 = (0, 0)
M2 = Point(Circle[M1, d])
b = Segment(M1, M2)
A: (k, 0.0)
u = Vector(M1, A)
C: (b - k, 0.0)
a = Vector(M2, C)