Dot Physics

Stratos Jump for Mere Mortals

Crazy, but I was on CNN Saturday night. They contacted me at the last minute to talk about the Red Bull Stratos Jump. Here is a screen shot to show that I am not making this up (or that I have awesome photoshop skillz).

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Looking back, maybe I looked like an idiot. Really though, it wasn’t my fault. I thought we were going to talk about physics. The first two questions threw me for a loop. Here are the two questions and my response (roughly paraphrased):

Will Felix survive the jump?

Answer: I guess so.

Is there a scientific reason for this jump?

Answer: I thought we were going to talk about physics. So….maybe?

Maybe it wasn’t actually that bad. However, thinking about this, I want to give it another shot. So, what are the key take-home points I would like the general public to know about the Red Bull Stratos Jump? (in no particular order)

Forces and terminal velocity

There are really only two forces acting on the skydiver as he or she falls. There is the gravitational force – in this case it is essentially (but not exactly) constant. Then there is the air resistance force. This is a force from the skydiver colliding with the air. A couple of key things about the air resistance force:

  • It depends on the density of air. This is important in this case since the density of air changes with altitude.
  • It also depends on the surface area of the object as well as the shape. How about we just assume these don’t change.
  • It depends on the square of the velocity in the air.
  • The air resistance force is always in the opposite direction as the motion in the air (so, for this case, that will always be up).

There are really only three ways these forces can combine resulting in three different types of motions.

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The key thing about forces is that they CHANGE the velocity of an object. If the total force is zero (like case C) then the velocity will not change. For a skydiver, this is called terminal velocity. Normally, a skydiver starts the jump at around 10,000 feet. Sure the air is thinner up there than at the ground, but not THAT much thinner. This means that the skydiver quickly reaches a point where the air resistance is equal (but in the opposite direction) as the gravitational force and travels at a constant velocity the rest of the fall.

The key difference for the Stratos Jump is that Felix will start at an altitude where the density of air is really really small making a small air resistance force. This also makes for a very large terminal velocity (you would have to go super fast for the air resistance to be as large as gravity). During the time period A the total force is in the same direction as the way the jumper is moving, so that makes the jumper speed up.

As the jumper gets into higher density air, the air resistance force gets really large really quick. This makes the air resistance force much larger than the weight. Now (during time period B above) the net force is in the opposite direction as the motion of the skydiver. For this case, the skydiver will be slowing down.

Eventually, the speed will slow down making the air resistance force smaller so that they are equal (time period C). If there are equal forces in opposite directions on the skydiver, this is the same as no forces. If there are no forces on an object (or no net force) the velocity will be constant. This is terminal velocity.

How fast will he go?

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The common answer to this question is that Felix will not go that fast because the fastest a skydiver can fall is around 200 mph. This true for normal skydivers where they change their body position to have a smaller cross sectional area. This means that for the air resistance to be equal to the gravitational force, they have to go faster.

For Felix, jumping from 120,000 feet, this doesn’t hold true. The key difference is the very low density of air that will allow him to go super fast. He could reach speeds near 700 mph.

Faster than sound

This is a tricky question. The key thing here is “what is the speed of sound?” In the most basic model of gases, the speed of sound only depends on the temperature. So, as you go higher and the temperature goes down, so does the speed of sound. Felix will not have to go 740 mph (the speed of sound at sea level) to break the sound barrier.

Will the forces be too great?

For this particular jump, there will be a larger than normal air resistance force (see above) as the skydiver transitions from going very fast in low density to higher density air. If you start at 120,000 feet, this air resistance force will produce an acceleration less than 2 times normal gravitational feelings (2 g’s).

What about the science?

This is the question I failed at in the interview. But I am ready now. Is there any scientific reason to do this? The best answer may be that science can be found everywhere. Think of all the things man has done that produced some cool scientific idea. These are not always planned experiments. The key is to just keep your eyes open and observe. You will never know what you will find.

From an engineering view, this jump will test some useful stuff. How do you get a man so high in the atmosphere? How about the spacesuit? How about the parachute? Also, maybe he can collect some atmospheric data. What about human performance at such a high altitude?

Finally, from a learning viewpoint, I think this is a great problem for introductory physics courses. Oh – Red Bull, please collect and share acceleration data during the fall. Please?

More details

Is this not enough? Do you want more details (in terms of the physics?) Here are a few posts you might like:

Comments

  1. #1 Jason Buell
    May 24, 2010

    I’m on to your games now Rhett! From Makezine. If they had been true blog readers though they would have interviewed you after the jump and then had you use Tracker Video to break things down.

  2. #2 Rhett Allain
    May 24, 2010

    @Jason,

    Wow – that link is awesome. I am going to have to try that.

    Next up – Rhett on the Today Show!

  3. #3 D. C. Sessions
    May 24, 2010

    Based on absolutely nothing but WAG, I’d guess that terminal velocity for a human body (padded by altitude suit) would be well short of Mach 1.0 — and the air temperature would make that pretty slow.

    There’s still enough air at that altitude to keep a subsonic aircraft (think U-2, although they don’t go quite that high) up, for instance. The density is down to the point where rolloff is pretty slow, too.

  4. #4 Art
    May 24, 2010

    They contacted you and you assumed it was as a physicist. Non-physics questions gave you a little pause. In future you might do better to realize that their audience is a bunch of beer drinking dopes whose knowledge of physics is limited to some vague recollections of people dropping cannonballs off of towers and that physics had something to do with making the bomb.

    In other words you, being a physicists, has little to do with it. Your a stand in as the all-purpose ‘science guy’. And talking to an audience of poorly motivated folks operating at middle school level.

    You really did a good job of it. But you would have done a better job if you had chugged a couple of beers beforehand to take the hard science edge off and tune you in to the beer addled, couch potato frequency that TV operates on. I say chugged because a loud belch early in the interview would endear you to the TV audience. As long as you make only a cursory effort to apologize and carry on as if nothing had happened.

    You have to realize that the audience is amotivational 16 to 25 year-old and the main uses for Red Bull is to keep the drunks awake so they can drink more, and to get hungover people moving again.

  5. #5 Dan Meyer
    May 24, 2010

    Transcript. Hey-o!

    ——————-

    LEMON: Why? I have to ask him that. I wish I could ask him that question. You know, this seems like the craziest science experiment ever, right? So we asked a scientist to help us understand what Baumgartner is trying to do. So Rhett Allain is an associate professor of physics at Southeastern Louisiana University. And he joins us from Hammond, Louisiana, tonight.

    Good to see you, sir.

    Is Felix, Fearless Felix going to survive this?

    PROF. RHETT ALLAIN, SOUTHEASTERN LOUISIANA UNIVERSITY: He probably can. I mean, Joe did it from 102,000 feet. 120,000 feet will be a lot faster, but he’ll experience a lot of the same things. There won’t be any forces outside of the realm of what a normal person should be able to take.

    LEMON: I would imagine there is some — a scientific reason to do this. Maybe it helps us understand you know what it’s like to do something. Maybe it helps NASA. I don’t know. I don’t understand why someone would want to do this.

    Is there a scientific reason to do this?

    ALLAIN: Oh, I don’t know about the science of this. I mean, I approach this from the science of how can we apply basic physics that a college freshman would look at in terms of this case. Atmospheric science. Maybe there is something there. That is not really something that I’m too much of an expert in.

    LEMON: Yes. What I’m trying to get to is there a scientific reason to do it or is this just for the rush or the adrenaline to see what it feels like. By the way, he is being sponsored by Red Bull. We should say that.

    So what is it like to fall so far, so fast?

    ALLAIN: Well, I mean, the thing that is different here is at that high altitude, the density of air is very low. So you have these two competing forces, gravity and air resistance. And with the very, very low density, the air resistance isn’t that much so you can really go real, real fast until you get to lower atmosphere and then you start slowing down. So he can go significantly fast.

    LEMON: All right.

    Hey, listen, it is good to see you. Thank you so much. Appreciate you spending a Saturday night with us. And we wish Felix, Fearless Felix Baumgartner the best of luck. God speed as I say. We hope he is safe.

    Thank you very much, sir.

    ALLAIN: Thank you.

    ——————-

  6. #6 Rhett Allain
    May 24, 2010

    @Dan,

    Where did you find that? Or can you just type really really fast?

  7. #7 Patrick
    May 28, 2010

    Dan’s rendition of the interview is definitely a bit off in some important details but maybe I’m just a perfectionist. Anyway, I like Art’s post. I’d like to add that I was watching the interview with my sister (not a physicist btw) and she initially replied that you didn’t answer the question after the interviewer’s 2nd question. I immediately paused the video and replied, in your defense: “Did you hear that question (or series of questions)? Did you hear how broad and vague it was? I wouldn’t know how to begin to answer it either?” Apparently all my sister heard was: “Blah blah…. question about scientific reasoning… blah blah…. How will it feel for Felix to fall from such a height?” This is the question (based on my sister’s non-scientist point of view) that American’s heard when they watched this interview. So I guess Art is right. A good way to interest the average unmotivated American would be to start with saying a joke like “It would feel awesome!” Hind sight is 20:20, I know. But then you could go into the details of forces acting on Felix and show how much academic value you have in this context.

  8. #8 Rhett Allain
    May 28, 2010

    @Patrick,

    I think Dan’s rendition is the official CNN transcript – http://www.cnnstudentnews.cnn.com/TRANSCRIPTS/1005/22/cnr.09.html.

    And yes, this could have been so much more – a great learning opportunity. But instead it was a great PR opportunity, and I thank CNN for that.

  9. #9 Skydivers
    September 9, 2010

    Thank you for the tip. Looks like I need to listen more when these guys are trying to teach me skydive speak…

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