For my birthday, my wife got me a great gift - memories. She wanted to give me the chance to do something I had never done before. That something turned out to be indoor skydiving. Sadly I do not have pictures from the event, but I've been thinking about it and how it works.
I remember in high school physics doing all of our assignments in a friction-less environment. This was always a little frustrating to me because physics is applied mathematics that demonstrate how the world works and the world does not exist in a friction-less state. I understand the reason for this - high school students are probably not prepared for the calculations necessary to compute those numbers and high school teachers may what to stay as far away from fluid dynamics as possible.
Anyway, I bring that up because as I was doing research on this, I realized how important it is. First - let me lay out a misconception that I've been operating under, that terminal velocity (TV) is a constant, given value. I figured that the whole idea of terminal velocity - the point when a falling object no longer accelerates and thus maintains a constant velocity - was that it was an absolute. I didn't know why people stopped accelerating I just figured it was one of those points attached to the law of gravity. That doesn't make any sense, but there it is.
As it turns out TV is a function of several factors, many you could probably suspect: the objects mass, acceleration due to gravity (9.8 m/s/s), density of the fluid (in this case air), the drag coefficient based on the fluid in question, and the projected area of the object. That last factor is the most variable when it comes to skydiving. This is because terminal velocity is the point where the resistance of the air is pushing against you (i.e. up) at the same rate that gravity is accelerating you (i.e. down). So the very force that causes reentering space craft, meteorites, and steaks to heat up is also the one that limits how fast you can fall.
According the the terminal velocity Wikipedia article, the common TV for skydivers is about 122 mph or 54
meters per second. This is with the belly down, arms and legs out, commonly seen pose. Head down, skydivers may go as fast as 200 mph or "almost the terminal velocity of the Peregrine Falcon diving down on its prey." The world record is 843.6 mph held by Felix Baumgartner when he bailed out of a helium balloon on the edge of space last year. Up that high he was able to increase his speed because the air density was much less then closer to the surface. All in all, quite impressive.
I looked up all this because the certificate they gave us after our "jump" said that we had reached terminal velocity. I thought about how that works if we weren't really falling. But again, TV can be manipulated if you can control the different factors. In a vertical wind tunnel you can control the amount of air resistance. Simply pump enough upward force to counter gravity and your downward velocity is neutralized. When you enter the camber, they have you put your hands on your chest and fall into the wind. Even if you fell for a full two seconds, you would only be going about 19.6 meters/second (~1.1 kilometers/hour) so from what I can see, you would indeed reach terminal velocity it would just be a very slow one. Because of the controlled environment they can really whip the air resistance up to where your body does not need to be in the standard pose to remain afloat.
So, once again I feel vindicated in my believe that the more you know about something the more you can exploit it. By learning more about the relationship between gravity and air resistance (which resulted in a discussion of terminal velocity) we can see how they can be controlled. I've always said that until the force that holds things down was understood we could never fly. Now, I can actually say that I have flown. And it was pretty cool.
I remember in high school physics doing all of our assignments in a friction-less environment. This was always a little frustrating to me because physics is applied mathematics that demonstrate how the world works and the world does not exist in a friction-less state. I understand the reason for this - high school students are probably not prepared for the calculations necessary to compute those numbers and high school teachers may what to stay as far away from fluid dynamics as possible.
Anyway, I bring that up because as I was doing research on this, I realized how important it is. First - let me lay out a misconception that I've been operating under, that terminal velocity (TV) is a constant, given value. I figured that the whole idea of terminal velocity - the point when a falling object no longer accelerates and thus maintains a constant velocity - was that it was an absolute. I didn't know why people stopped accelerating I just figured it was one of those points attached to the law of gravity. That doesn't make any sense, but there it is.
As it turns out TV is a function of several factors, many you could probably suspect: the objects mass, acceleration due to gravity (9.8 m/s/s), density of the fluid (in this case air), the drag coefficient based on the fluid in question, and the projected area of the object. That last factor is the most variable when it comes to skydiving. This is because terminal velocity is the point where the resistance of the air is pushing against you (i.e. up) at the same rate that gravity is accelerating you (i.e. down). So the very force that causes reentering space craft, meteorites, and steaks to heat up is also the one that limits how fast you can fall.
According the the terminal velocity Wikipedia article, the common TV for skydivers is about 122 mph or 54
meters per second. This is with the belly down, arms and legs out, commonly seen pose. Head down, skydivers may go as fast as 200 mph or "almost the terminal velocity of the Peregrine Falcon diving down on its prey." The world record is 843.6 mph held by Felix Baumgartner when he bailed out of a helium balloon on the edge of space last year. Up that high he was able to increase his speed because the air density was much less then closer to the surface. All in all, quite impressive.I looked up all this because the certificate they gave us after our "jump" said that we had reached terminal velocity. I thought about how that works if we weren't really falling. But again, TV can be manipulated if you can control the different factors. In a vertical wind tunnel you can control the amount of air resistance. Simply pump enough upward force to counter gravity and your downward velocity is neutralized. When you enter the camber, they have you put your hands on your chest and fall into the wind. Even if you fell for a full two seconds, you would only be going about 19.6 meters/second (~1.1 kilometers/hour) so from what I can see, you would indeed reach terminal velocity it would just be a very slow one. Because of the controlled environment they can really whip the air resistance up to where your body does not need to be in the standard pose to remain afloat.
So, once again I feel vindicated in my believe that the more you know about something the more you can exploit it. By learning more about the relationship between gravity and air resistance (which resulted in a discussion of terminal velocity) we can see how they can be controlled. I've always said that until the force that holds things down was understood we could never fly. Now, I can actually say that I have flown. And it was pretty cool.
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