So I have been told that friction is caused by microscopic bumpiness in the surfaces of objects. Based on Friction= (coefficient of friction)x(normal force), if normal force is 0, then there is no friction. However, where is the line drawn between microscopic bumpiness causing friction and other things. Why cant the force of a nail holding up a picture not be considered friction? Or can it? Based on the above formula, the friction would be 0 though since the is no normal force perpendicular to the direction of friction. Is the formula a simplified version that is only accurate to a certain level of bumpiness and afterwards, its no longer considered friction? Also why is dynamic friction the same for all speed? Or is it? From my understanding, dynamic friction breaks bumps easier (due to more momentum) and/or does not all the to surfaces to come all the way into each other. Thanks!