# Air Resistance

by on September 26, 2010

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Skydiving

Air resistance, also called drag, refers to forces that oppose the relative motion of an object through a fluid, liquid, or gas. These forces act in a direction opposite to the oncoming flow velocity. Air resistance depends on velocity. When considering air resistance in astrodynamics, it is an inefficiency that requires additional thrust during lift-off, but is an added efficiency when returning to Earth. Apparently, what goes up, comes down a little more easily.

Size and shape are the two factors that affect air resistance. Air resistance works with surface area, so the more surface area, the more air resistance. Try this: drop two pieces of paper(one crumpled, one flat). The crumpled one will fall faster because it meets less air resistance over its smaller exposed surface.

When an object is falling air resistance acts to push it back up. This is only true for objects falling straight down. If the object was falling left or right, then air resistance would be opposite. Air resistance is the opposite of gravity for an object falling down. It pushes up while gravity pushes down.

When an object is falling, it will reach a point where it can no longer accelerate. That is its terminal velocity. Gravity is stronger than air resistance on all falling objects, but there is a small timeframe when air resistance overpowers gravity. A good example is a skydiver. The skydiver falls until reaching terminal velocity(gravity is stronger). After the rip cord is pulled, air resistance is stronger for a short time. Eventually, gravity exerts itself again and the diver falls to the ground.

Air resistance increases as velocity increases. The main reason that the space shuttle needs fireproof shielding is that the friction created by air resistance heats the surrounding air enough to cause an intense fire.

Air resistance is a major factor in calculating the amount of fuel that is required to lift a space craft into low Earth orbit. It comes into play for every plane trip that leaves an airport. Actually, there is no way to avoid air resistance in every aspect of your daily life.

We have written many articles about air resistance for Universe Today. Here’s an article about free fall, and here’s another article about air resistance.

If you’d like more info on Air Resistance, check out the NASA’s Falling Object with Air Resistance. And here’s a link to an article, The Way Things Fall.

We’ve also recorded an episode of Astronomy Cast all about Gravity, including 0 gravity. Listen here, Episode 102: Gravity.

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