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).

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.

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?

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: