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I think I understand how pressure works with gases. More molecules bouncing around -> more random impacts -> stronger force.

But I realized to my embarrassment that I don't understand how solid things press on each other, on a molecular level. Say I put a block of iron on my head. If I put another one on top of it, I feel twice the weight. The two blocks together can tear through thin paper where one block can't. But the contact between my head (or paper) and the blocks is just a very thin layer of atoms of the lower block's structure. If the lower block doesn't move when I put the upper one on it, what causes this thin layer to "press" on my head (or paper) more? When the two blocks together tear through thin paper, where does the force come from that acts on the paper molecules - it can't be gravity from the upper block, right? And how come that whatever this source of pressure is only depends on the weight of the upper block, and not on what it's made out of, iron or wood?

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If you like this question you may also enjoy reading this and this Phys.SE posts. –  Qmechanic Jul 21 at 8:32
    
Each iron block is absolutely on different from (for example) "a hand pressing on you". Imagine Arnie Schwarzenegger pressing one hand down on your head or shoulder. You're feeling a force - created by his muscles. If he uses two hands, you feel two of the forces. It's utterly the same with the example bricks. The only difference is the force, is caused by gravity, not "muscles." So, it's that simple. –  Joe Blow Jul 21 at 10:51

2 Answers 2

You have a chain of action and reaction. There is twice the weight on our head because the forces felt by the lower block are its weight, plus the action of the top block (minus the reaction force of your head).

Then how is it on a molecular scale? Well, just the same: if you imagine a crystalline solid with horizontal layers, one layer feels the action of those over it plus its own weight, and this sum needs to be balanced by the layer below. Molecularly, the force between layers is transmitted by the short range repulsion forces (a bit like a very tough spring that would be between your head [lower layer] and the block [upper layer]).

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You're saying that essentially every solid is a spring, right? And when a top block is lying on the bottom block, the bottom block's atoms are necessarily a little closer to each other in its lattice? Is it measurable/detectable by how much they are closer? –  Avva Jul 21 at 10:18
    
@Avva: yes, the compression of the block is related to the applied force by the bulk modulus. –  John Rennie Jul 21 at 10:20

You're mixing up different things. Two blocks of iron press your head more than one block because the Earth's gravity pulls two blocks stronger than one. This is why two blocks can tear through the paper where one can not.

The molecular bit comes into consideration if you ask why the top block doesn't pass through the bottom one. This is because the electron clouds of the surface atoms on the top and bottom block repel each other, so the two blocks never really touch at the molecular level.

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I don't see how I'm mixing different things, although perhaps I'm not expressing myself well enough. Think of the paper the moment you add a top block and it tears. What is the added force on the paper molecules that overcomes their binding? It can't be 'gravity from the top block' - gravity actually pulls top block and paper together (insignificantly). Gravity acts on top block where it sits, far from paper. What's the added force that acts on paper, and how is it "transmitted" from top block to paper, if there's a solid bottom block between them that doesn't change? –  Avva Jul 21 at 9:58
    
It's not "gravity from the top block". Instead, it's the gravity from the earth on the blocks that creates the extra weight. Remember that weight = mass X acceleration. Also remember that Newton said F=ma. Hence, doubling the mass also doubles the force on the paper. –  hdhondt Jul 21 at 10:30

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