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Name: Roxy
Status: student
Grade: 9-12
Location: CO 
Country: N/A
Date: 1/4/2006


Question:
I wish to find the air resistance on a flat plate (which is perpendicular to the direction of air flow) using the formula of R= KpSV^2, where p represents the density of air, S the frontal area of the body, and V the velocity. I am told that K is a coefficient depending on the shape of the body and found by experiment. I do not fully understand what it is and have no idea how to find it in an experiment. Also, I do not know how to get the other terms.


Replies:
The study of the drag of objects in air is a subset of the branch of engineering known as "fluid mechanics."

For ordinary speeds in air, the following equation seems to work. Drag force = Cd * A * p * V^2 /2. Where A is the frontal area, V is the speed, p (rho) is the density of the air and Cd is the drag coefficient. This is similar to your equation except for the factor of two. The density of air is about 1.225 kg/m3.

Cd is small or large depending on whether the object is shaped to allow to pass easily through the air or not. For example, a rough sphere has Cd of 0.4. A flat plate 1.17. A cube 1.05 (flat face facing wind). An airplane about 0.012. A car about 0.3. A truck about 0.9. An upright person about 1.2. A sports motorcycle 0.6. A parachute 1.42. A "streamlined body" is shaped to have as low a drag coefficient as possible and is about 0.04.

That equation is only approximate. Other factors that can influence drag are speed and roughness. Different equations are used for drag in water, drag at supersonic speeds, or drag at very low speeds.

Bob Erck



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