A $4\mathrm{.}0-kg$ and $a6\mathrm{.}0-kg$ box rest side$-$by$-$side on a perfectly smooth, level floor. A horizontal force of $40$ N is applied to the $4\mathrm{.}0-kg$ box, pushing it against the $6\mathrm{.}0-kg$ box, and, as a result, both boxes slide along the floor. How hard do the two boxes push against each other?