Where did "μN" come from? Friction on the molecular level is basically the attractive forces between the molecules, but while dealing with macroscopic objects we use the value "μN" as the magnitude of friction, where N is the magnitude of the normal reaction force, and it acts in the direction opposite to relative motion. Where did "μN" come from? It does seem that its tougher to move an object the harder it is pushing down, but did this result only come experimentally? Is there a mathematical proof for it?