== You're pushing the load straight along the horizontal surface at a constant speed.
== The speed and direction of the load are not changing, so there is no acceleration.
== From Newton's 2nd law of motion, we know that F = m A . Since A=0, F must also be zero. That means NO NET FORCE is acting on the load.
== But how can that be ? You're pushing it with 90 N !
== The NET force is zero. So something else must be pushing the box, also with 90 N of force, exactly opposite to YOUR force, and they add up to zero.
== THERE's your force of friction !
