Reynolds numbers and "inertial force" I am an undergrad physics major taking an engineering course that just introduced the concept of reynolds numbers. When I try to get an idea of how the Reynolds number is physically derived, I keep running into the definition that it is the "ratio of inertial forces to viscous forces acting on a fluid." Where the "inertial force" is defined as F=mass*acceleration. As I understand newtons 2nd law, it means that for an object with mass M to accelerate with acceleration A, the object must be acted on by a net force ∑F=M*A. But F = M*A is not an actual force, but rather a description that says that "acceleration is directly proportional to force and inversely proportional to mass." The only other time I have seen this concept of "inertial force" is with respect to non-inertial reference frames where objects experience a pseudo-force due to the acceleration of the frame, but that doesn't seem to apply here. Can anybody explain to me what is meant by "inertial force," or otherwise give me a intuitive or straightforward description of what the Reynolds number describes??