Why is normal force included in the calculation of either static or kinetic friction?

Question

Please explain why is normal force always included in calculating the static or kinetic friction

Answer 1

It basically comes down to the microscopic view of the two surfaces. They are never 100% smooth and have peaks and valleys. The larger the Normal Force the more the peaks and valleys compress into each other making it harder for the two surfaces to move relative to each other. In the case of static friction, we get microscopic high pressure points of contact which have a tendency to "weld" to each other. That is one of the main reasons static friction is larger than kinetic.

Answer 2

Friction occurs when two non-ideal "rough" objects are rubbing against each other. For this to happen, you need to have some force holding the two objects together. With no attractive force, there would be no friction - the objects would just float away or bounce off of each other if you tried to move them. The normal component of the force is used to calculate friction because the amount of friction is different depending on the angle of incidence of the applied force. Think about how much easier it is to move a heavy couch across the floor if you push horizontally (normal force=gravity only) than if you push downward (normal force=gravity plus your added downward force).