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I was watching a pop-sci video about how we never truly touch something, and we just feel the electromagnetic repulsion of atoms.

I knew that but it made me think.

If we cannot truly touch something, then what is friction?

And how -if i hold a pen really tight and move my fingers up and down fast - then both the pen and my fingers will get warm?

If these two never touch, what is generating the heat from the friction?

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  • $\begingroup$ It's actually not so different from macroscopic mechanics: If there is a force between atoms of your hand and those of the pen energy can be exchanged. Because this energy excites many degrees of freedom in the pen and your hand they get warm. $\endgroup$
    – A. P.
    Jan 9, 2021 at 19:57

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Quantum mechanics isn't really needed to explain friction and heat. At smaller scales, friction is basically the collision between protruding edges of rough surfaces. It is because of the electromagnetic repulsion, among other things, that the objects 'collide' and don't pass through each other, even if the collisions are happening without touching. And this is why in solids the linear motion is converted into vibrational or rotational energy in the atoms or molecules through the electromagnetic repulsion, and this kinetic energy is what we perceive as heat.

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Why would you need to touch something for there to be friction or heat?

Imagine moving an air-blower over a pile of balls. You are indirectly pushing the balls around, thus imparting kinetic energy to them. In much the same way, as your finger moves over the pen the electrons in your hand interact with and push around electrons in the material, warming it up.

Now, the actual forces and how they sum-up to give the macroscoping phenomena of friction - that's a rather complicated story. But the cause of the effect is the electric forces bewteen your hand and the pen. You don't need to "touch" in order for there to be such forces, and hence for the surfaces to affect and heat each other.

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