Translations:Adventist Youth Honors Answer Book/Health and Science/Physics/31/en
From Pathfinder Wiki
At the atomic level you can think of bumpy surface like sand paper rubbing against another surface. When the two surfaces are at rest, the high spots of one surface fit into the valleys of the other surface and it takes quite a bit of force to move one over the other. Once they are moving, the two surfaces bounce from peak to peak like a skier only hitting the tops of the moguls. We observe that it takes more energy to start pushing an object than to keep it moving once moving.
