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Name:  Jeffrey F.
Status:  student
Age:  20s

Location: N/A
Country: N/A
Date: 2000-2001
Question:
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?

Replies:
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.

Kenneth Mellendorf


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