| MadSci Network: Physics |
Hello Roxane,
I want to give you a little technical background on the subject of falling objects that will help you to better understand my answer to your question:
When an object is dropped from a great height, there are essentially two
forces that come into effect:
1) Acceleration due to gravity
2) Drag force due to air resistance.
Gravity causes a body (or YOUR body in this case) to accelerate downward
to the ground at a rate of 9.8 meters per second, per second. This means
that if you were free falling for just 20 seconds (and there was NO air
resistance) your speed at that time would be
9.8 m/sec.sec x 20 sec = 196 m/sec (equal to 438 miles per hour)
This is near the speed of jet airliner!
Fortunately, there IS drag force from the air to slow you down. The drag force acts in the opposite direction of gravity, and is proportional to the speed 'squared'of an object moving through the air. (As an example, if an object is moving through the air at 100 m/sec and has a drag force of "5", then at 200 m/sec (double), the drag force will be "20" (2x2, or 4 times as large).
You may have heard the term "terminal velocity". That is the maximum speed that an object will fall through the air. What is happening at terminal velocity? The Drag force is exactly equal to the Acceleration of gravity against the mass of the object (or the WEIGHT of the object). This means that when a 100 pound person free falls, they will accelerate until their speed through the air causes the drag force to also equal 100 pounds. At this point, they will accelerate no more, but will continue to fall at a this constant speed.
One more complication (and this is finally getting to your question): The Drag Force is related to the shape of the object moving through the air. The bigger the profile hitting the air, the bigger the drag. Think of a skydiver- when they are free falling (before opening the 'chute) they will 'lay' out flat, with their arms and legs stretched out fully. This is the largest profile through the moving air. Their drag force will be greatest in this position, and their terminal velocity will be the slowest. If they rotate their bodies straight up and down with their arms and legs held in tightly, their profile and their drag force will be the smallest, and their terminal velocity will be the highest.
The answer to your question is in the last paragraph- To fall the slowest, you don't want to swing your arms, but position your body to present the largest profile to the moving air.
HOWEVER, before you hurl yourself off of a tall building, please be aware that a sky diver, even in the maximum profile position will still fall at about 120 mph, more than enough to flatten you into a pancake on impact.
So, the real answer to your question is that you need a parachute to get you down in one piece!
One more note: The way a parachute works is simply to create a VERY large drag profile out of your weight. The terminal velocity is then reduced to a speed that will not injure you when you hit the ground.
Happy (and Safe) Landings!
Jay Shapiro
[note added by MadSci Admin: Look at the following previous answers for some additional background on terminal velocity:
In addition, there are many more previous answers in our archives on the subject which you can find by using our search engine.]
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