What is the cause of the normal force?

Question

I've been wondering, what causes the normal force to exist? In class the teacher never actually explains it, he just says "It has to be there because something has to counter gravity." While I understand this is true, it never explains <i>why</i>. Whenever I ask anyone else they always respond in a similar way, saying "It has to be there, because the object is not accelerating", and this has become very frustrating. So what is the cause of the normal force? From my reasoning, it has to be one of the four fundamental forces. (Gravity, electromagnetism, the weak force, or the strong force). It would seam to me that electromagnetism would make to most sense (electrons in the outer shells of atoms repelling each other), However, just as I thought this had to be right, I read a thing online about "certain fundamental particles repelling each other when their wave functions overlap". I haven't studied quantum mechanics yet so I'm not really sure what to make of that. If anyone could shed some light on this for me it would be much appreciated.

Answer 1

A normal force can be defined as a contact force exerted by a surface perpendicular to itself. Though the source of such a force is ultimately electron-electron repulsion (van der Waals forces) at an atomic and molecular level, it would be far too impractical to try to deal with it as the sum of an uncountable number of individual interactions, particularly when we can treat it as one collective force applied to an object sitting, leaning, or laying on a surface.

Though normal forces often cancel out gravitational forces and prevent objects from falling, not every force which counteracts the gravitational force is a normal force; only a force caused by sitting, laying, or leaning on a surface,

The same idea of treating many individual molecular interactions as one force applies when we talk about the buoyant force applied to objects placed in a fluid.

Answer 2

You're absolutely correct, it is because of the electromagnetic force. The normal force is essentially the force that keeps you from "falling through the floor". Right now if you stand up, the earth's gravity pulls you down with a force equal to your mass times 9.8 m/s^2; normally you would start accelerating downwards, but instead you stay still because as you are pulled downward, you are pulled into the floor. The floor is made up of atoms and molecules that are firmly in place and under the force of your feet will push back against you. If you could see the particles, you would see the electrons in your feet and the electrons in the floor repelling each other with their electric fields. This is the source of the normal force; this explanation is often tedious and professors prefer to just rely on your intuition to understand that clearly we aren't all falling though the floor therefore there is this normal force holding us up. This concept can be tricky for many students that are new to physics.