Question

Difference between stress and pressure

Answer 1

Pressure and stress have the same units: pounds per square inch (psi) or newtons per square meter (N/m^2) also known as pascal (Pa). Pressure is a force per unit area typically used inside a fluid (liquid or gas) and is the same in all directions at a point inside the fluid. Stress is also force per unit area but is typically used inside solids. The difference is that the stress can be different in different directions inside the solid. Also that stress can be a shear stress or a normal stress (tension or compression). In the solid, the stress state is describe using a tensor (a matrix of values) at each point indicating the stress in different directions,whereas the pressure is scalar, a single value. It is possible to have stresses in a fluid too.Inside a solid, the state of stress can beconstant (unvarying with time) in response to static forces applied to the solid. A fluid will flowin response to forces,so the stress would vary with time. In a fluid, a stress can come about due to flowing of the fluid. For instance, when a fluid is flowing past asolid, there is a shear stress inside the fluidnext to the solid surface due to viscosity of the fluid. There is also a pressure there and a normal force pressingagainst the wall.

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Answer 2

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