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The stress induced in a pipe by internal pressure is $$P_r \over t$$ where $P$ is the applied pressure, $r$ the pipe radius and $t$ the pipe wall thickness. The strain induced in the pipe wall is $$\Delta r \over r$$ where $\Delta r$ is the change in radius. The basic stress strain relationship is $$\sigma = E \epsilon$$ where $\sigma$ is the stress, $... 0 Is mass just a special case of it always being zero, or are there cases that it's not? This question is equal to question below: Does exist any system that its mass isn't constant? You have already answered your question: By definition mass cannot be transferred to or from a system. 0 A gas can be approximated as an ideal gas. You then assume that the particles don't feel each other and that they are infinitely small. The potential is zero. The particles can only have kinetic energy. If you would make a simulation of such a system the particles can literally move through each other. If you make this ideal gas approximation, it is ... 0 You can model the gas a a collection of hard spheres of some radius$r$, and do the correction relative to the limit as$r\to\infty$perturbatively. What you find is that for a fixed pressure and temperature amd number of molecules the first order correction to the law replaces$V$with$V-\frac{4n\pi r^3}{3}\$. This tells you that the volume to use is the ...