Three particles $\mathit{P}$, $\mathit{Q}$ and $\mathit{R}$ are placed on a straight line as shown in the figure. The masses of $\mathit{P}$, $\mathit{Q}$ and $\mathit{R}$ are $\sqrt{3}m$, $\sqrt{3}m$ and $\mathit{m}$ respectively. The gravitational force on a fourth particle $\mathit{S}$ of mass $\mathit{m}$ is equal to