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# What Is a Drag Equation?

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The drag equation is a formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid. The drag equation is D = Cd * A * .5 * r * V^2. The drag equation states that drag (D) is equal to the drag coefficient (Cd) times the density (r) times half of the velocity (V) squared times the reference area (A).

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In order for the drag equation to be accurate, the objects must have a blunt form factor and the fluid must have a large enough Reynolds number to produce turbulence behind the object. The drag equation was created by Lord Rayleigh, who originally used (L) squared instead of (A ). (L) stood for linear dimension. The reference area (A) is normally defined as the area of the orthographic projection of the object on a plane perpendicular to the direction of motion. It's the same as the cross sectional area for objects that aren't hollow with a simple shape, such as a sphere. (A) may be significantly larger than the area of any cross section along any plane perpendicular to the direction of motion for other objects, such as a rolling tube or the body of a cyclist.

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