When a current carrying loop is placed in a magnetic field, the coil experiences a torque given by $\tau = NBiA\sin \theta$. Torque is maximum when $\theta$ = 90^o,i.e., the plane of the coil is parallel to the field ${\tau _{\max }} = NBiA$

Forces ${\overrightarrow F _1}$ and ${\overrightarrow F _2}$ acting on the coil are equal in magnitude and opposite in direction. As the forces ${\overrightarrow F _2}$ and ${\overrightarrow F _2}$ have the same line of action their resultant effect on the coil is zero. The two forces ${\overrightarrow F _3}$ and ${\overrightarrow F _4}$ are equal in magnitude and opposite in direction. As the two forces have different lines of action, they constitute a torque. Thus, if the force on one arc of the loop is $\overrightarrow F$ , the net force on the remaining three arms of the loop is - F.