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I've read in books that one can't put one's hand through a table because the table offers a "Normal Reaction" to the hand. And it is also stated that this force is electromagnetic in nature. But what is this force? Can it be explained using classical electromagnetism "in terms of something I'm more familiar with"?

Moreover, if it is this force that stops my hand from going through the table, why is it that certain other solid substances, like sand, don't produce the same result?

Also, is there any limit to this Normal reaction? I mean on pushing too hard my hand might go through the table. Is there any upper limit to this force?

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  • $\begingroup$ Well, to answer your last question, what happens when you push a hammer through the table as hard as you can? $\endgroup$ – Jerry Schirmer Jul 27 '15 at 17:05
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    $\begingroup$ You can't really push your hand through sand - you can just push your hand between the grains. Each grain stays whole, the same as the table. $\endgroup$ – Harry Wilson Jul 27 '15 at 17:24
  • $\begingroup$ Okay, but about the first part of my question? $\endgroup$ – Aritra Das Jul 27 '15 at 17:27
  • $\begingroup$ possible duplicate of What does it mean for two objects to "touch"? $\endgroup$ – pentane Jul 27 '15 at 18:03
  • $\begingroup$ Possible answer: here $\endgroup$ – john Jul 27 '15 at 18:18
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You usually cannot push your hand through the table, because it's a single solid. The atoms are held together by covalent bonds, which are electromagnetic in nature. Sand on the other hand is grainy - the $SiO_2$ grains do not interact with each other and are only held "in place" because of gravity. You can run your hand through sand similar to driving a truck through a classroom full of tables, just by pushing them aside.

There is an upper limit to the normal force excerted by the table. When you push enough, the wood will start to bend slightly and finally crack. The exact amount depends on the kind of wood used, its "age"/environmental history and the geometry of your table.

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    $\begingroup$ Great analogy with the truck and tables. Just hope the class had been dismissed. $\endgroup$ – BMS Jul 28 '15 at 12:52
  • $\begingroup$ The sand on the bottom is interacting with the sand on the top to keep the sand on the top from falling due to gravity. Just like in a solid. But that interaction between grains of sand is obviously different than say, some glass made of silicon and oxygen. I think the key is that when you push on some sand, it can push on the other sand in different directions than what you pushed. Otherwise you are glossing over that sand grains do interact with each other. $\endgroup$ – Timaeus Aug 3 '15 at 18:18
  • $\begingroup$ The difference is that a the particles that make up your glass interact attractively while the different grains of sand only repulse each other.. $\endgroup$ – t0xic Aug 5 '15 at 18:36
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Why is it that two carbon atoms fired at each other will bounce off and not stick together? It is because as the atoms move close together their orbital electrons begin to repel more than their nuclei attract each other's electrons. The result is greater potential energy as they approach and this leads to the tendency to move apart much like compressing a spring. The same is true for the table. Why don't the atoms in the table simply condense into a much tighter packing on their own? Because their normal positions are minima in the potential energy of the system. This also explains why it is hard to pull apart the same material. When you press on the table you are asking millions and millions of atoms to move slightly closer together. The electric potential energy of the system rises quite rapidly and quickly generates a counter force to limit the travel of your hand. In summary, the pushback is purely electrostatic in nature. Mechanics is somewhat of a myth in that nothing ever really touches anything! Even in a collision atoms get close but (as long as they remain with their parent material) never touch!

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