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Name: Jess
Status: student
Grade: 9-12
Location: NY
Country: N/A
Date: 1/17/2005


Question:
Why does air friction depend on the surface area?


Replies:
Air friction is due to the object moving through the air having to move molecules of air aside. The larger the object the more air has to be moved aside and so the larger the air friction. Even surfaces parallel to the direction of motion of the object generate friction since molecules of air collide with the surface and are so pushed in the forward direction by the collision. By Newton's Second Law (Action and Reaction), they push back on the object, thereby generating more air friction.

Best, Dick Plano


The simple way of looking at it is that small things can be pushed through the air easier than big things because less air needs to be pushed aside or around.

Also, even with equal surface area, some SHAPES move easier through the air than others. A standard example is the "teardrop" shape which is easier to push through air (or water) than a flat plate.

Engineers have found out that there is a lot of drag if the shape disturbs the air a lot. For example, if you push a flat plate through the air, it leaves lots of whirls of turbulent air behind it. It requires energy to create that turbulence. On the other hand, a smooth "aerodynamic" shape leaves hardly any turbulence behind it after it goes by.

If you stand near a highway when a large semi-truck goes by you will feel blast of disturbed air that follows along behind it. The truck has a large surface are and it is not aerodynamic either. A small smooth car has much lower friction and drag and disturbs the air much less.

Bob Erck


I would not exactly call it "surface area". Most air drag depends on the "frontal area", also called the frontal cross-section area. After all, the moving object must push aside an amount of air proportional to that frontal area. That air has mass, and once it is moving, it often zings off in some direction as a little gust of wind. That is lost energy. It takes work to push that air up to speed. This is a picture of the "dynamic drag" typical of "turbulent flow".

At extremely low speeds in air, or in thick viscous liquids like honey, the drag is different. The liquid is too syrupy to swirl or have gusts of wind; it only moves as much as required by your immediate pushing and then softly but promptly stops. The viscous behavior dominates the mass-like behavior. This "viscous drag" _does_ depend on the total surface area.

A long, thin rod-shape, moving end-first, will have twice the viscous drag of a shorter rod of same diameter and half the length. The two would have nearly the same dynamic drag.

Viscous drag is proportional to velocity. Dynamic drag is proportional to velocity-squared. So dynamic drag always gets bigger than viscous drag at some high speed. Honey will swirl if you swish it briskly enough. Air will act viscous if you move slowly enough. This drag will be very weak, but it can be measured.

Jim Swenson


Take your index finger, lay it flat on a table, and slide it along. Feel the speed and drag on your finger. Now, turn your finger 90 degrees , so it touches on its side. Now, using the same force and speed, slide your finger. You should feel more speed. There also was the feeling of less drag. Why? If everything was the same, the force, and speed of your hand, the only difference was the area. The bottom of your finger has less surface area, or contact, with the table, compared to the side of your finger. The forces you were feeling were frictional forces. It is no different with the force of air, or the friction caused by air, it is dependant on area. The issue may be you do not see air moving, unless it carries particles, like smoke. That is why wind tunnels add smoke to see the flow. Cars and planes are aerodynamically designed to minimize the friction, or drag from the wind. Bullets do not have flat faces do they?

James Przewoznik



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