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Name: Joe
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Question:
How does more surface area make something with air resistance fall slower?



Replies:
Air resistance determines the fastest that something will fall in Earth's atmosphere. This is called terminal velocity.

Terminal velocity depends upon how much air resistance there is versus weight. Air resistance is controlled by the surface area over which the air acts (called a cross section) and speed. A body's shape in the direction of fall is very important - so important that it overcomes all other factors!

For example, the terminal velocity of a 220 lb. skydiver is around 220 miles an hour, give or take a bit. A streamlined 220 pound bomb can approach far higher speeds when falling!

And do not forget: all falling bodies accelerate (speed up) by the same amount until they hit terminal velocity. This acceleration depends upon Earth's gravity and is known to its friends as g, or "little g". Little g is about 32 feet per second every second or 9.80 meters per second each second.

R. W. "Bob" Avakian
Instructor
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology


Joe,

Surface area does not determine how fast something falls. Air resistance is not determined by how much surface area an object has. A way to model air resistance is an object having to push the air molecules out of the way. Shape is just as important, some times more important, than size. Even how an object is rotating can affect air resistance. Probably the two most important things are called cross-sectional area and shape. A wider object has to push more air out of the way for every meter it falls. An object provided with a pointed bottom has to push less hard to do this than the same object with a flat bottom. This is why the front end of a missile is pointed. Also, an object falling faster has to push more air more quickly that the same object falling slowly.

As for falling slower, this also depends on how the air resistance compares to weight. When objects are hardly moving, such as when they are first released, there is no air resistance. As objects speed up, air resistance increases. High resistance objects will eventually reach a speed where air resistance pushing up is just as big as gravity pulling down: the object has reached its greatest speed. It keeps falling without continuing to get faster. Low resistance objects may be able to reach higher speeds, but not always. A big hollow ball that weighs twice as much as a pencil will not fall faster than a pencil because a pencil has to push so little air out of the way to make room for itself. On the other hand, a steel pencil will fall faster than a wood pencil because it takes a higher speed for the air resistance to build up to the weight of the very heavy steel pencil.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Joe,

Air resistance, or drag force, is a complicated phenomenon. The surface area of an object is not a good predictor of drag on the object. The shape of the object plays a strong role, but other factors such as speed and size also play important roles.

Hope this helps,

Burr Zimmerman


Your's is a far more complicated question than I suspect you suspected. Surface area is only one of a number factors that play a part in air resistance (or any other fluid for that matter). For example: It is no accident that golf balls are dimpled. The surface area increases a bit from the indentations, but the range of a golf ball is significantly extended. It is no accident that a tennis player (at least a competitive one where it matters) likes to get the fresh set of tennis balls in a match. It is no accident that a baseball pitcher "rubs up" a new ball thrown in by the plate umpire.

On the other hand as your question implies, 'Why wear a parachute?' The area of physics you are dealing with is "hydrodynamics" and more specifically "aerodynamics". Both are mathematically challenging, complicated areas of engineering and physics.

What Galileo "discovered" or "predicted" was that in the absence of a resisting fluid (e.g. air) that in such a vacuum the feather and the elephant will fall the same distance in the same time interval. The damage to the feather and to the elephant is a different issue because momentum, too, is conserved.

Vince Calder



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