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How does weight add up to press on things?

  1. Jul 20, 2014 #1
    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, microscopically. 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?
  2. jcsd
  3. Jul 20, 2014 #2


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    To bounce they need some kind of repulsion, right?

    The atoms at the boundary also repel each-other.

    Not directly, but transmitted through the lower block

    Because that's how pressure is defined: force per area. And the force here depends on weight.
  4. Jul 20, 2014 #3
    >Not directly, but transmitted through the lower block

    What exactly does this mean, "transmitted"? Microscopically?
  5. Jul 20, 2014 #4


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    Electromagnetic forces.
  6. Jul 20, 2014 #5


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    Atoms in a solid (or gas or liquid) have electrons on the outside. Like charges repel. Not really any difference between a solid, liquid or gas in that respect. It's why gas molecules bounce off the walls of a vessel rather than disappear into them.
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