Page 69 - Aerodynamics for Engineering Students
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Governing equations
of fluid mechanics
2.1 Introduction
The physical laws that govern fluid flow are deceptively simple. Paramount among
them is Newton’s second law of motion which states that:
Mass x Acceleration = Applied force
In fluid mechanics we prefer to use the equivalent form of
Rate of change of momentum = Applied force
Apart from the principles of conservation of mass and, where appropriate, conserva-
tion of energy, the remaining physical laws required relate solely to determining the
forces involved. For a wide range of applications in aerodynamics the only forces
involved are the body forces due to the action of gravity* (which, of course, requires
the use of Newton’s theory of gravity; but only in a very simple way); pressure forces
(these are found by applying Newton’s laws of motion and require no further
physical laws or principles); and viscous forces. To determine the viscous forces we
* Body forces are commonly neglected in aerodynamics.
