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Q & A: Air Resistance and Landing Time

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Most recent answer: 10/22/2007
Q:
How does air resistance affect an object’s landing time, and what variables does air resistance depend on?
- Andee (age 16)
Missouri City, TX USA
A:
When an object is dropped from rest (and we ignore air resistance) the time it takes to reach the ground depends only on the initial height and the acceleration of the object. This acceleration is due to the force of gravity, and is the same for all objects.

As any object falls it usually encounters some degree of air resistance, which is caused by the object's surface colliding with air molecules. The two most common factors that determine the air resistance are the speed of the object and its cross-sectional area. Greater speed causes greater air resistance, and increased area increases air resistance as well. It also depends on an object's shape. Parachutes have lots of air resistance because of their big cross-sectional area and also their cupped shapes. Cars, airplanes and boats are designed with smooth shapes to reduce air (and water) resistance. In general, this air resistance will mean that an object dropped from a certain height will take longer to reach the ground.

It's not too clear what is meant by "landing time" in this question, however. For an airplane, it could mean the length of time between the time at which it starts descending from its cruising altitude (usually 30,000 feet) until the time it pulls into the gate at the airport, or the time it takes between the first touch of the wheels on the runway and the time it comes to rest. In both of these instances, air resistance is a big factor (the free-fall gravity example above results in a very hard crash-landing -- the "landing time" is very short, but it is not a landing anyone would want to make).

In order to get down from cruising altitude to the ground without hitting too hard, air resistance plays a big role. When a plane drops from a high altitude to a lower one, it naturally speeds up just like anything that is dropped. It has to lose this energy somehow, and so the pilot will slow the engines down, but usually this is not enough. The pilot will also extend the landing gear and the flaps on the wings, both of which increase the air resistance and slow the plane down. The more air resistance there is, the faster the plane can slow down to make a safe landing. On the runway, the pilot will even put the engines in reverse (this is done with air diverters on the sides of the engines), blowing air in front of the airplane, to slow the plane down even faster. If there is too much air resistance, then the airplane will flutter like a feather in the wind, or like a piece of paper. The reverse-engine trick works on the ground because the wheels help keep the plane rolling safely and stably while there is so much resistance to its motion.

You may notice birds doing the same kind of thing when they land. They swoop down, and then just as they come in for a landing, they start flapping their wings backwards, pushing air in front of them so they come to a graceful stop and can grab onto a small branch. Air resistance helps them slow down quickly and come in for a good landing.

(published on 10/22/2007)

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