Luke Aikins went skydiving without a parachute. This is crazy—not because it's impossible, but because really bad things happen if you make a mistake. If you haven't seen the video, it shows Aikins stepping out of a plane at 25,000 feet and falling into a net—without using a parachute.

Clearly there are some physics questions here. Let's get started.

Does it matter that he jumped from 25,000 feet?

The short answer? No. It doesn't really matter if you jump from 25,000 feet or 15,000 feet. In both cases, you'd have a final speed of about 150 mph. Although it's true that a ball dropped from 4 meters will hit the ground at twice the speed as a ball dropped from 1 meter, that only works if we can ignore the air resistance force.

What is the air resistance force? This is a force that an object feels as it moves through air. You can feel this yourself when you put your hand out the window of a moving car. This force depends on:

The speed of the object's motion in the air.

The size of the object.

The shape of the object.

The density of the air.

Since the force depends on the speed, an object dropped from rest will initially have zero air resistance force. The gravitational force will pull the object down and it will accelerate as it falls. When the acceleration is in the same direction as the velocity, this means that the object speeds up. Now that the object is moving downward, there is an air resistance force. This force increases with speed. Eventually, the air resistance force will be equal in magnitude to the gravitational force and the object will fall at a constant velocity. This is called terminal velocity.

Terminal velocity depends on the shape and size of the object as well as its mass. For a human skydiver, terminal velocity usually is between 120 and 150 mph. Here's a simple experiment: Take one coffee filter and a stack of two coffee filters. Drop them.

Since the coffee filters have the same shape and size, the only significant difference is the mass. The double stack will have twice the gravitational force as it falls so it will achieve higher terminal velocity.

OK, so a skydiver jumping from 25,000 feet should get to the ground with the same speed as a skydiver jumping from 15,000 feet. But are there any other differences? My guess is that it would be better to jump from a higher starting point to allow more time for making corrections to land in the net. Also, if you are gonna go out, go out with style.

There is one other big difference between 25,000 and 15,000 feet—the air density. At 25,000 feet the partial pressure of oxygen is so low that your brain won't function correctly. Skydivers and mountaineers address this with oxygen masks. I won't say anything more beyond suggesting you check out this awesome video by Destin (Smarter Every Day) on the effects of high altitude on the body.

How does the net stop a jumper?

Stopping a human is all about acceleration. The acceleration depends on the change in velocity and the time it takes to make this change. No matter what, this skydiver will slow from 150 mph to 0 mph—the question is how long it will take. If the acceleration of a human is too high, bad things can happen, including injury and even death. NASA has a nice chart on the human tolerance to acceleration. From this you can see that humans can survive up to 30 G's for very short periods.

A traditional skydiver will have an acceleration when the parachute is opened. This can take a few seconds to slow the jumper and give an acceleration of acceptable levels. If you want to stop in a net instead of a parachute, you need to think about time. How do you stop in a long enough time interval? The only answer is to make a net that stretches a large distance. This will give the skydiver a longer stopping time with a reasonable acceleration. If you want to stop with an acceleration of 10 G's, you would need a stretch distance of at least 2.3 meters (7.5 feet). Looking at the video, it appears that Luke stretched the net quite a bit more than 7 feet.

Why does he flip on his back?

At the end of Luke's fall, he rolls over from facing down to facing up. I'm not a professional net jumper, so I am going to speculate on his reasons. First, humans have a higher tolerance to accelerations that are "eyeballs in"—that's actually what they call it. This is the acceleration that would be in the direction that your nose points if you are flat on your back. So, by flipping over he is "eyeballs in." Second, being on your back is better for your arms and legs. The net will bend body parts in toward your torso as the net stretches. If you are facing down, this could push your arms and legs in a way they weren't designed to bend. It could hurt.

Of course there is a downside to flipping onto your back—you can't see where you are going. But maybe it's best to not know exactly what's going to happen at the very end.