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When I punch a wall, Why doesnt my fist go through instead of staying at rest on the wall? What is cancelling out the force of my punch?

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The molecules that make up the wall exert forces on each other. That is why the wall sticks together in the first place. The molecules are locked into their positions by the forces from the other molecules. If you try to move one molecule out of place, the others exert forces on it that force it back in to place. Generally, the further you try to move the molecule out of place the harder the other molecules push it back.

When your hand comes in and starts pushing on some of the molecules they initially move a small amount in response. But then the force on those molecules from the rest of the wall kicks in and pushes them back, until the response from the other molecules balance out the force from your hand. In hard materials it only takes a tiny displacement to generate a huge response force from the other molecules, which is why it feels like the wall doesn't move at all.

It may help to think of pushing on a trampoline surface. When you push on the surface it moves until the force from the springs pulling it back into place balances the force from you. Every solid surface is pretty much like a trampoline in this way, except that the "springs" are so incredibly strong that it seems like the surface never moves.

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  • $\begingroup$ The forces are acting on two different objects though... so how are they cancelling each other? $\endgroup$ – user243275 Oct 4 at 12:57
  • $\begingroup$ @RandomGuy, Your hand also is made of molecules. Forces between those are what hold your hand together. When the molecules of your hand come close enough to the molecules in the wall, they exert force on each other. $\endgroup$ – Solomon Slow Oct 4 at 13:31
  • $\begingroup$ The forces on the molecules in the wall are 1. the force from your hand and 2. the forces from the other molecules. The forces on your hand are A. the force from your muscles pushing your hand forward and B. the force from the molecules on your hand. Force B on your hand has the same strength as force 1 on the molecules because of Newton's 3rd law. $\endgroup$ – Luke Pritchett Oct 4 at 14:50