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Pressure is a scalar quantity, and I think I understand this in the context of pressures exerted by gases and liquids. However, I struggle to understand this in the context of solids. If I use a hammer to hit a nail into a wall, how is the pressure acting evenly in all directions? Intuitively, it seems like the pressure is directed in a specific direction, unlike what happens in a gas that explodes or an object submerged in water. Wouldn't there be a difference in the pressure experienced by the atoms in the wall if I hit them with the nail at a different angle, and surely there would be a difference in pressure for different atoms in the wall, as well as the air molecules beside the wall where the nail hits. Am I getting this confused with force or stress, and if so, what even is pressure (other than force divided by area)?

All explanations are welcome, but I would prefer intuitive explanations where possible that could be understood by a high school student, unless more complicated math is needed to help explain what is going on,

Juan
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In the case of solids, when you apply a force on the surface you have what is called a traction vector. This vector is defined locally as the force density over the surface.

Inside the solid you have stress. This is a quantity that is of a tensor nature. The traction on the surface is the projection of the tensor in the direction of the unit normal. That means that you can have both, shear and normal forces on surfaces of a solid.

nicoguaro
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