A closed loop PQRS carrying a current is placed in a uniform magnetic field. If the magnetic forces on segments PS, SR and RQ 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 QP is :

Question

$\begin {array} {1 1} (1)\;F_3-F_1+F_2 & \quad (2)\;F_3-F_1-F_2 \\ (3)\;\sqrt{(F_3-F_1)^2+F_2^2} & \quad (4)\;\sqrt{(F_3-F_1)^2-F_2^2} \end {array}$