F _ ( 12 ) = − γ m s ( 1 ) m s ( 2 ) r _ ( 1 ) − r _ ( 2 ) | r _ ( 1 ) − r _ ( 2 ) | 3 {\displaystyle {\underline {F}}^{(12)}=-\gamma m_{s}^{(1)}m_{s}^{(2)}{\frac {{\underline {r}}^{(1)}-{\underline {r}}^{(2)}}{\left|{\underline {r}}^{(1)}-{\underline {r}}^{(2)}\right|^{3}}}}
