If friction cancels force why is work done when kinetic energy doesn't change

[Fusilli_Jerry89]

Can someone please explain to me how work is the change in energy, yet if you are applying a force of 100N to a box, and 100N of friction is making the net force zero, how is work being done when the box is already in motion, yet there is no change in kinetic of gravitational potential energy?

 

[denverdoc]

Work is often a difficult concept to wrap your mind around, but extremely useful in most cases. From Newton's first law we could determine that without external forces, the box once given a shove would remain at that velocity for all eternity. That it requires a constant force to maintain velocity, implies that there is a force operating against its motion. The energy ends up as heat.