the microwelds happen because you apply a finite force over an extremely small area, thus you have extreme pressures, which cause deformation, which causes heat, which welds the materials. At least that is the explanation on a microscopic (but not quantum level)

that doesn''t address why you can''t stick your hand through a desk, or why solids are solid, or what the nature of matter is. This is because the most elegant friction models are by nature large scale approximations of the interactions of matter.

by the same token, quantum mechanics is likely a similar (and similarly incomplete) abstraction of events at even smaller scales. Any explanation of the nature of matter from quantum mechanics will probably end up being a sort of average as well.

This would explain why there is such difficulty marrying all the various theories of matter and its interactions. Each theory is an incomplete approximation, and new theories are the result of trying to explain gaps in previous theories. but none fully describe what is actually happening, because for every observable scale there are an infinite number of smaller ones.

So, if you want to find out how fast a block will slide down a ramp, use newtonian physics. If you want to know how to descibe the interactions of quantum particles, use quantum mechanics. But don''t try to justify newtonian physics in terms of quantum mechanics (or other theories) because they are all incomplete.

On the normal force-

The previous posters are both right, I just thought I would add some detail.

The normal force comes about because a)the object you are pushing on is rigid and b)several rules of physics prevent the atoms in your arm from ''overlapping'' the atoms in the object.

The real questions relating to a and b are a)why are some objects rigid and b)why doesn''t your hand just pass through the wall even if it IS rigid.

For the first part, usually when you push on a wall the normal force is generated by EM interactions. The molecules of the wall are held into the shape of a wall by chemical (read: EM) bonds between the molecules. Trying to deform this structure means you are trying to deform the (EM) molecular bonds. You can try pushing on some jello and see what happens if these bonds are weak. But even highly rigid objects deform somewhat when you push on them. If the walls molecules are held together very tightly they will resist deforming, and how well they do this is called elasticity, malleability, etc. Basically something like a diamond crystal which has rigid covalent bonds between each atom can exert a huge normal force, because this is the force required to deform the crystal structure even a slight amount. This is where the ''response'' of newtons law comes in. When you try to deform this crystal by pushing on it, it pushes back while deforming slightly. This force is EM in nature.

Theres more to the story though. To understand why your arm can''t simply overlap the wall''s molecules, you have to consider several cases. First, when you push against a wall this is a ''small'' force on the molecular level, since you basically spread it out across so many atoms. You''re not going to create a black hole by squeezing anything with your hand, nor will you be able to rip a diamond in half Whats happening here is EM and is because of the way your atoms are made. Positive charge on the inside, and negative charge on the outside. As you bring two atoms close together, they will resist at a certian point. This is electron-electron repulsion, and the charge of the object (neutral,positive,negative) is not whats causing it, its just the fact that you are trying to bring some electrons close together, and they are on the outside of each atom.

Now, heres where the quantum mechanics come in. It is possible to overcome this EM repulsion if you press hard enough (probably not with your hand but hey . If you do this, there is a range of different things that can happen. First you will be bringing the electrons close enough that the ''strong'' nuclear forces will come into play. My understanding is these matter more for protons and neutrons than electron-electron interactions. This, again, is a fundamental force. If you push even harder and overcome these type forces (bringing the electrons very close together) you will start to encounter a ''new'' repulsive force which is entirely from quantum mechanics and isn''t associated with a fundamental force. This new force arises from the pauli exclusion principle, which says no two electrons can occupy the same energy state. You are trying to force the electrons of one atom "through" the electrons of another, but they cannot overlap in this fashion. Remember that electrons act as if they are spread out over the entire atom. Anyway this force really only comes into play in things like black holes, where the EM and strong repulsions are overcome. It turns out even this ''pauli exclusion'' force can be overcome if you press hard enough. Once this happens, there are no more forces to overcome. You ''broke'' the system, so to speak. The electrons and protons of an atom can indeed ''overlap'', and this is a black hole. The entire mass of a black hole seems to be contained in a tiny pointlike space. Or, you could say that every electron, proton and neutron is contained in a single energy state. Although you can''t really go in and test this.

Anyway, the answer for friction is usually just the EM forces I believe, and the same for the normal force. The pauli repulsion force only starts to come into play in things like neutron stars. Its not a fundamental force but someone who knows more relativity and QM than me can tell you all about it