r r = i r r * * i r 0 0 i p e× F = ± k * i i ( )r i * 2 p e ± k < F 0 * : F 1 * : F 2 * : F 3 * :... > 1 4 9 16 ± m m× * F = ± g p e i i * 2 ( ) r i g m p m e r * i k r 0