Density and fluid pressure
Density is mass per volume; pressure is normal force per area.
Key knowledge
Average mass density is m/V. Fluid pressure parameterizes isotropic normal stress, with positive p denoting compression: uniform signed pressure p on a plane area A produces normal force magnitude |p|A. With nonuniform pressure, force must be integrated over the surface. Pressure and density describe different quantities and neither is a total force.
Values and equations
ρ_average=m/V; dF_pressure=−p n dA
- ρ_average
- Average mass density kg m⁻³
- m
- Mass contained in V kg
- V
- Volume m³
- p
- Fluid pressure Pa
- F_pressure
- Pressure force on a surface element of the chosen body N
- n
- Outward unit normal of that body 1
- A
- Surface area m²
Conditions and scope
- V>0; distinguish average density from local density in a nonuniform material.
- The scalar pressure force concerns static or isotropic pressure stress; viscous shear and general solid stress require additional tensor components.
Sources · 1
References
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Evidence and review
Read §14.1 Density and Pressure definitions and force/area treatment. The signed differential normal force and absolute-value force magnitude are disclosed surface-element/sign-convention expressions of the same pressure definition; no general tensor stress is reduced to a scalar.
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Entry details
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- foundation-physics-density-pressure
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- foundations-refresh.1
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