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Solid Motion in Fluid: The Drag force

By fluid we mean non-solid, essentially liquid or gas. One definition of a fluid could be something that a solid can move with a vanishing driving force in the limit of vanishing speed. Thus by its very definition a fluid does not produce a drag force for v=0. On the other hand when v increases we know from experience that a force must be applied to maintain the motion and hence the drag force appears with increasing speed. As for kinetic friction the drag force is always directly opposed to the velocity vector. Experiments show that to a reasonable approximation the drag force follows the expression
equation38
tex2html_wrap_inline120 is the density of the fluid . For air at the surface of the earth we have tex2html_wrap_inline122. A is the area of the solid as seen in projection along the direction of motion, and tex2html_wrap_inline126 is a dimensionless constant which varies from 0.1-1 and depends on the fluid as well as the solid. v is the speed of the solid in the fluid.

Thus we get large drag force when an object with large cross sectional area moves at high velocities in a dense fluid. The constant tex2html_wrap_inline126 must be measured: it is a phenomenological parameter which is difficult to calculate from first principles.





Collin Broholm
Mon Sep 29 10:16:07 EDT 1997