A rectangular loop with sides of 20 cm and 10 cm carries a current...

The net magnetic force on any closed current-carrying loop placed in a uniform magnetic field is always zero.

The net force is given by $\vec{F}_{net} = \oint I(d\vec{l} \times \vec{B})$.

Since the magnetic field $\vec{B}$ is uniform, it can be taken out of the integral:

$\vec{F}_{net} = I(\oint d\vec{l}) \times \vec{B}$

For any closed loop, the vector sum of all length elements is zero, i.e., $\oint d\vec{l} = 0$.

Therefore, $\vec{F}_{net} = 0$.

Alternatively, the forces on the two longer sides are zero as they are parallel to the magnetic field. The forces on the two shorter sides are equal in magnitude but opposite in direction, cancelling each other out. Thus, the net force is zero.