For example:

Why do we use static properties when deriving the speed of sound equation? Why isn't dynamic pressure considered in the process nor in the resulting formula?

$$ \sqrt{k\frac{P}{\rho}} = c$$

Another example are the thermodynamic tables of properties, where you can generally just find the static pressure.

Thank you!


1 Answer 1


The first principles from which the derivation starts include the ideal gas law and Joule's first law. It's static pressure that's proportional to the product of density and temperature in the ideal gas law (and in the 17th- and 18th-century empirical data that the ideal gas law encodes); and it's static pressure that expresses the longitudinal force per area each fluid element exerts on neighbouring fluid elements for the "work done" term in Joule's first law.


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