A closed-loop $\mathrm{P}\mathrm{Q}\mathrm{R}\mathrm{S}$ carrying a current is placed in a uniform magnetic field. If the magnetic forces on segments $\mathrm{P}\mathrm{S},\mathrm{S}\mathrm{R}$ and $\mathrm{R}\mathrm{Q}$ are $F_1,F_2$ and $F_3$ respectively and are in the plane of the paper and along the directions shown, the force on the segment $\mathrm{Q}\mathrm{P}$ is