Complete the following statement. The first condition of equilibrium…

Sum of all the forces on the body : $F_{net}$

We know that, at equilibrium, the potential energy is extremum.

Also, $F_{net} = \frac{dU_{net}}{dr} $

As : $\frac{dU_{net}}{dr} = 0 \text{ for extreme U}$

$\Rightarrow F_{net} = 0$