The New Science of Strong Materials (1968) | Hacker News Reader

The New Science of Strong Materials (1968)

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27 pointstosh11 comments

Something I wondered about a lot early in my physics education is how the normal force "knows" to be just enough to oppose your weight and keep you from falling through the floor or flying up. It's just the right amount!

I feel like I got some satisfying enough answers that I stopped being curious about it, but not enough that I could explain it to someone else.

It doesn’t know, it’s just a coincidence. Everything that could fall through has fallen and everything that could go up has already gone up.

Are we talking neutrinos here?

Op said how does the floor know with what force to push back.

Well guess what Op, it doesn’t. It’s a coincidence.

Suppose you had a vertical series of random floors that exert random levels of force. If you step on one and fall through, then you just fall onto the next floor. And if that one doesn’t exert enough force, you fall through that one as well. This could continue, until you finally hit a floor that exerts sufficient force back to you, and that is the floor you will rest on, perhaps forever, if these falls killed you in the process.

It is by sheer coincidence you found a floor capable of holding you. In essence this is what must have happened at some point with everything in the universe, giving us the world we have today, a world built entirely by coincidence.

I think there's something to this. It tickles my intuition.

That’s a kind of anthropic principle argument, but the other part of it is that the floor was designed to not collapse. If it was designed for you to fall through then it’d do that.

Think of everything in the world as objects with springiness , nothing is "absolutely rigid" , rigid objects in Newtonian mechanics are really really tight springs that deform very little to push back with a huge force. So a "rigid" block on a "rigid" table is really two springy objects that deform to equilibrium i.e when the normal force exactly equals the weight of the block

It's fairly easily explained by just thinking of the surface as being covered in tiny springs, or trampolines maybe. Just these are tiny springs made of electromagnetic forces.

Anything sitting still on the surface is compressing the springs exactly enough until they push back enough to resist further compression, and not enough that they push more. This is a stable equilibrium.

The normal force from a surface can exceed the force needed to just push back against the weight of an object, if you can compress the springs enough. That's why things can bounce.

It’s definitely not just the right amount for my floors. They creak and bounce every time I walk across them!

I was expecting a discussion on the Pauli exclusion principle.

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