Suppose a closed loop with constant current around the loop. The loop is in a uniform constant B field. The force is [tex]\vec F=I\left (\oint_c d\vec l\right ) \times \vec B.[/tex] Being a closed loop [itex]\oint_c d\vec l=0[/itex]. So there is no force acting on the loop. BUT at the same time, we know there is torque, so there is force!!!! How is that possible?