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The Physics of the Red Bull Stratos Jump

•, By Rhett Allain

In short, yes.

As Felix falls from a very high altitude, there are essentially two forces acting on him. There is the gravitational force pulling down and an air resistance pushing up. During the entire jump (even up to 120,000 feet) the gravitational force will be pretty constant. The air resistance force is not constant. It depends on his speed as he moves through the air. The faster he goes, the greater the air resistance force.

For a normal sky diver jumping out of a plane, the jumper will fall and increase speed. However, before too long the speed will increase to a value such that the air resistance force has the same magnitude as the gravitational force. At this point, the net force on the jumper will be zero and he or she will fall at a constant speed. This speed is call the terminal velocity and has a typical value around 120 mph (around 50 m/s).

There is one important difference between Felix's jump and a normal sky diver - the density of air. At 120,000 feet, the density of air is much lower than it is near the surface of the Earth. This means that the air resistance force will be much smaller at this high altitude (for the same speed) than it would be when he is lower. The result is that he will keep increasing in speed to a value much higher than 120 mph.

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