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.
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.
This is how a parachute-free skydive works.