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the internal force [tex]\vec{F_{ij}}[/tex] between two particles is [tex]\vec{F_{ij}}=\nabla_{i} V_{ij}=\nabla_{ij} V_{ij}=-\nabla_{j} V_{ij}[/tex]

where the subscript below [tex]\nabla_{k}[/tex]implies the differentitaion with respect to components of [tex]\vec{r_{k}}[/tex]

i can't get how [tex]\vec{F_{ij}}=\nabla_{i} V_{ij} or \vec{F_{ij}}=-\nabla_{j} V_{ij}[/tex]

the potential energy is dependent on the relative distances between the particles hence there is no meaning to the above two equalities