F.4 Maths...

F=k(M1M2)/d, where k is a constant

If M1 and M2 are both halved and d is doubled,
F'=k(M1/2)(M2/2)/2d=(1/8)[k(M1M2)/d]=(1/8)F

Hence, F'/F=1/8 => F':F=1:8

2010-05-30 17:39:23 補充：
Correct one:

F=k(M1M2)/d², where k is a constant

If M1 and M2 are both halved and d is doubled,
F'=k(M1/2)(M2/2)/(2d)²=(1/16)[k(M1M2)/d²]=(1/16)F

Hence, F'/F=1/16 => F':F=1:16