Why should you care about friction? To be honest, you probably should not. It is better you continue working on your stuff. Honestly close this page now and start doing something useful. Fine, if you decide to stay here, you were warned.

I was probably in 7th grade of school, I was reading a physics book and it mentioned that when we are walking, it is friction that prevents us from sliding. As obvious as it looks now, at that point I was literally hitting harder asphalt to test this mysterious force.

To be completely honest, it is still mysterious to me. When we imagine a force, the first thing that comes to mind is an arrow pointing at a surface. Not along a surface, but normal to it. How can a force be directed along a surface. What would stop it from moving. A friction? But what it really is.

Let's look at the case when the friction is as minimal as possible. Imagine yourself running on ice. Or better go do it instead of reading this 7th grade physics. If you stayed here 2nd time, you must be really interested in the topic. So, you have already figured out by now that roughness is the key to this mysterious sleeping force (because it is parallel to the ground, like laying in a bed, get it? Anyway ...).

Roughness is micro bumps in a surface, which also have some roughness (micro bumps), which also have bumps, which ... Most probably it rings a bell about fractals. So a surface is a collection of plane irregularities at different scales in the perpendicular direction. That was a complicated sentence. So, after we lost the last uninterested reader here goes something more exciting.

I strongly believe that friction is the same normal force that comes to mind when we thing about a force. If we zoom into the extrusions, the parallel sliding force is resisted by them. This shear force is hitting the extrusions. And how it is hitting them? Perpendicularly, like Newton's 3rd law of motion. Look it up if you forgot, but I believe you still remember it.

At every small scale, this shear force is at the micro level the same old normal force resisted by the extrusions (roughness). Obviously the rougher the surface, the stronger is shear resistance and the higher is friction. But how do we quantify that?

That is probably for another day, because I am going to sleep. Good night.