Skydiving – A Little (Simple) Skydiving Physics

Skydiving looks both exciting and scary. But to make it more of one and less of the other is pretty simple. It will help to get a little bit of information about how such an amazing activity is even possible. It just takes an ultra simple physics lesson. No math required.

When a skydiver leaves the plane, they can jump out, up or down. For simplicity, we’ll just assume that any downward push they give can be ignored. So, immediately, there are two forces acting on the skydiver’s body. One is the force of gravity, which is everpresent and pretty close to constant over the length of the fall. The other main force is air resistance.

Gravity acts (ignoring small effects) toward the center of the Earth. Your body pulls on the Earth an extremely small amount, but the difference in mass between it and you makes it no contest. You accelerate toward the Earth much, much more than it does toward you. So, the net effect – obviously – is you speed up and move down.

But there’s a force acting in the opposite direction – air resistance. And it is not constant. As you accelerate, your speed increases. That’s what acceleration means. You go faster and faster. But the faster you go, the higher the air resistance becomes up to a point.

After a few seconds, and a few hundred or a thousand feet, the two forces are equal. When that happens, your acceleration becomes zero. You don’t slow down and you don’t speed up. But you already have a speed as a result of the earlier acceleration. That speed is approximately 120 miles per hour (200 kilometers per hour). That’s called ‘terminal velocity’.

‘Approximately’ because the exact number depends critically on the shape and area of the falling body. That’s why a skydiver will achieve a higher speed of free fall if they are pointed nose down with their arms at their sides and legs together versus the common position of arms out, face and belly toward the Earth.

Other small effects, like the air density, humidity and local air pressure, can be ignored here. At much higher elevations, where the air is much less dense, they can’t be ignored.

What happens next is pretty important. A drogue (a small chute) is released that has lines attached to the main canopy. The small chute fills with air and pulls the main out, which fills with air. Now the body starts to decelerate because of the sudden (but not too sudden) increase in air resistance. The large area and shape of the canopy traps a lot of air, providing high resistance. The body slows down again.

Now the process is slightly reversed. As the body slows down, the air resistance decreases. Remember, before, the body was moving faster and faster and the air resistance increased accordingly. Now the same principle applies in reverse. As the body slows, the air resistance decreases.

In short order (a few seconds), the forces balance again and you achieve a new terminal velocity, one that is much lower. You float down at about 10 miles per hour. But there’s one more small step. You hit the ground.

When you touch down, the force of gravity is balanced now by a force from the solid Earth. Air resistance is negligible at this point. That balance is achieved very quickly but not instantly. There is another (rapid) deceleration, until you finally come to a complete stop.

Physics can be such a rush.

 

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