Contact forces are forces that come into play only when two objects come in contact with each other. In this wikipedia article, it is written that

Molecular and quantum physics show that the electromagnetic force is the fundamental interaction responsible for contact forces. The interaction between macroscopic objects can be roughly described as resulting from the electromagnetic interactions between protons and electrons of the atomic constituents of these objects.

If I agree with this then I’m unable to understand everyday phenomena, let’s say I push on a couch, if I use my hand to apply a force on one end of couch, the electrons of my hand will repel the the electrons on that end of the couch and will move a little inwards, this moving causes a repulsion between electrons within the couch and the electrons which have moved, this process continues till the motion of the other end of the electrons is caused. But when the last electron (of the other end) gets repelled then how would this cause the translation of my couch? If those electrons (at the other end) gets repelled a moves a little farther then they will replel the electrons in air molecules and the process gonna continue.

I want to know how does the microscopic explanation of the contact forces explain the observable translation or rotation of macroscopic objects?

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    $\begingroup$ the electrons will not move as a gas, the structure of solids is complicated, a simple model valid for many materials is that the atoms behave as it attached by springs. so you basically are pushing a spring and these pushed the rest but they will not move around the other side of the object. their relative positions will remain more or less fixed so it is as if you were pushing a large charged particle whose charges do not move around (the analogy is vey loose that is why I dont post it as an answer) $\endgroup$
    – user65081
    Jan 22, 2020 at 17:01
  • $\begingroup$ @Wolphramjonny I like your analogy but how pushing a spring gonna cause a translation, after all this analogy comes down to contact forces only. $\endgroup$
    – user240696
    Jan 22, 2020 at 17:10
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    $\begingroup$ Re, "If those electrons (at the other end) gets repelled a moves a little farther then they will replel the electrons in air molecules..." Well, yes, actually. If you push hard enough to slide the couch across the floor, Then the air will move out of the couch's way. $\endgroup$ Jan 22, 2020 at 17:16
  • $\begingroup$ @SolomonSlow So, how the translation of the couch gonna happen? $\endgroup$
    – user240696
    Jan 22, 2020 at 17:18
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    $\begingroup$ @Knight, Air doesn't push back very hard*. When you move a couch, or your own body, or anything else in a room full of air, the air swirls around to make way for you/it. [* The difficulty of pushing air out of your way increases as you try to move faster. Stick your hand out the window of a car that's cruising at highway speed, and you will feel significant push-back from the air.] $\endgroup$ Jan 22, 2020 at 17:20

1 Answer 1


Contact forces between solid objects arises from electromagnetic interactions.

But when we think about what happens when some force is applied at a region of the object, it is important not to forget its capacity to resist to shear stresses.

If instead of pushing the couch with the hand we use a point like object as a big nail, that capacity is being tested at its limit.

The atoms or molecules of the solid not only have average distances according to QM potentials x distance, but that potentials are not isotropic, and they can't easily rotate (as liquid molecules) to deform according to shear stresses.

  • $\begingroup$ Thank you for replying to me. Means the shear stress (i.e. our push) causes no shearing strain but the whole motion as @Wolphramjonny said they move in synchrony ? $\endgroup$
    – user240696
    Jan 24, 2020 at 8:16
  • $\begingroup$ Yes, after a very short time, when an elastic wave transmits the contact force to the whole object. That is why the contact produces a sound. $\endgroup$ Jan 24, 2020 at 15:11

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