Name: Jeffrey F.
My question concerns the force equation for air
resistances, specifically, the quadratic drag equation: F=.5DrCV^2
I developed a derivation for this equation by using the concepts
presented in the development of the ideal gas laws relating pressure to
the motion of the gas molecules bouncing off the container walls.
However, it seemed to me that it was necessary to ignore the
translational motion of the air molecules when calculating the impulse
experienced by an object due to the change in momentum of the air
molecules. Only when ignoring the translation motion of the air molecules
does my equation intergrate into the above equation.
Is the translational motion associated with the air molecules ignored
in that equation? If so, how can it even be valid?
It is important to realize that many equations used in science are
approximations. The drag force is useful when the object moving through the
air is moving much faster than the air itself. When in a fluid with a large
current, a scientist must view the motion relative to the fluid. True drag
is more complicated, depending on factors such as aerodynamics and
temperature. The drag equation is a very good approximation if the fluid is
not moving too fast or is not too thick. The equation can not be used with
honey as the fluid.
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Update: June 2012