# Why is pressure always non directional? [duplicate]

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While hammering a nail (before it is in the wall) it is pretty evident that the tip of the nail is going to be applying a force directed along th axis of the nail, then why is it said that pressure is always non directional ?

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## marked as duplicate by Chris White, Brandon Enright, Kyle Kanos, Waffle's Crazy Peanut, John RennieJan 14 '14 at 14:27

Possible duplicate of Define Pressure at A point. Why is it a Scalar? See also Why is pressure in a liquid the same in all directions? for the liquid case. – Chris White Jan 13 '14 at 21:35

I think it's a bit of a wording issue. The Cauchy stress tensor can be decomposed as the sum of a hydrostatic and a deviatoric component as $\sigma=\mathbf{s}+\frac{1}{3}\mbox{Tr}(\sigma)\mathbf{I}=\mathbf{s}+p\mathbf{I}$ where $p$ is the pressure, and since $F=\sigma\cdot\hat{\mathbf{n}}$, the force related to the hydrostatic pressure component simply reduces to $F_{hyd}=p\hat{\mathbf{n}}$, whose magnitude is evidently independent of direction $\hat{\mathbf{n}}$.