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Gravity is a force that likes to pull everything down to Earth at 9.8 meters per second per second, which means that if you were falling out of an airplane, you'd end up going 9.8 meters per second faster every second that you're in the air.

This would be your speed in that direction, or your velocity, after the first second, and the second second, and the third, the fourth, the fifth, the sixth... it's not ideal, basically. Fortunately for us, however, this speeding-up process doesn't continue on indefinitely, and that's because of a lovely thing called terminal velocity.

When an object moves through the atmosphere, that object is literally slamming into particles of air. This is something you can feel if you move your hand around really fast, or if you stick it out of the window of a moving car. The air molecules can't get out of the way fast enough, so they build up, creating high pressure. And on the other side of the moving object, molecules can't fill in fast enough, which creates a small vacuum, or low pressure. The difference in pressure is drag.

Every falling object has a speed at which the force of drag equals the force of gravity on that object. And at the moment it reaches that speed, the object will stop accelerating, and maintain a constant, or terminal, velocity. As you continue to fall through the air, you'd stop accelerating at about 54 meters per second, and so that's your terminal velocity. That's about 121 miles per hour.

Before you hit the ground, though, it would be nice to increase your drag some more. That's what the parachute is for - after it opens, you begin to decelerate, and your velocity decreases, until your velocity equals the new force of the drag. This is your new terminal velocity - about 5 meters per second, or about 12 miles per hour - if you want a safe landing.