# What is meant by "inertial forces", in contrast to the "resultant force"?

For example, in the Wikipedia page on Reynolds number it is said:

The Reynolds number is the ratio of inertial forces to viscous forces within a fluid..

In explanations of the Reynolds number, it is normally stated that the inertial forces equal to the product of the acceleration and mass of a fluid sample. But isn't this just the resultant force by Newton's second law of motion? Why is there a need to have a term "inertial forces" if they basically mean the same as the resultant force on the fluid sample?

No, it is not just a resultant force. The "inertial forces" are defined as $$\rho v^2 L^2$$ where $$\rho$$ is the density, $$v$$ is the flow velocity, and $$L$$ is the length. Note that this is a quantity that has the units of force, but it does not represent an acceleration as in Newton's 2nd law since it is present even when the flow is steady. So this is not an actual force, but a characteristic quantity with units of force that describes how fast the fluid is moving and how much inertia it has.
In contrast the "viscous forces" are defined as $$\mu v L$$ where $$\mu$$ is the viscosity. This also has units of force, but is present during steady flow. So the interpretation is similar. It is a characteristic quantity with units of force that describes how thick the fluid is.