The Physics Behind Parachutes Para The speed of the skydiver As a skydiver falls, he accelerates downwards, gaining speed with each second. The increase in speed is accompanied by an increase in air resistance . This force of air resistance counters the force of gravity. As the skydiver falls faster and faster, the amount of air resistance increases more and more until it approaches the magnitude of the force of gravity. Once the force of air resistance is as large as the force of gravity, a balance of forces is attained and the skydiver no longer accelerates. The skydiver is said to have reached a terminal velocity. The cross-sectional area of the skydiver A skydiver in the spread eagle position encounters more air resistance than a skydiver who assumes the tuck position or who falls feet (or head) first. The greater cross-sectional area of a skydiver in the spread eagle position leads to a greater air resistance and a tendency to reach a slower terminal velocity. The importance of cross-sectional area to skydiving is also demonstrated by the use of a parachute. An open parachute increases the cross-sectional area of the falling skydiver and thus increases the amount of air resistance which he encounters. Once the parachute is opened, the air resistance overwhelms the downward force of gravity. The net force and the acceleration on the falling skydiver is upward. An upward net force on a downward falling object would cause that object to slow down. The skydiver thus slows down. As the speed decreases, the amount of air resistance also decreases until once more the skydiver reaches a terminal velocity. The forces gravity and air resistance play the major role in a parachute and they can be shown mathematically.  F=ma, where m=mass of the object and parachute, and a=their acceleration. http://www.physicsclassroom.com/mmedia/newtlaws/sd.cfm When the skydiver is in the aircraft there are two forces counter acting. The force of gravity is pulling down, and there is a normal force pushing back from the floor of the aircraft on the skydiver. Once the skydiver leaves the aircraft, he is feeling the full acceleration downward due to gravity. The speed starts to increase until Terminal Velocity is reached. Once Terminal Velocity is reached the skydiver is no longer under acceleration, not to say he isn't heading towards earth at deadly speeds, it's just that the forces of gravity and air resistance are countering each other. http://www.northallertoncoll.org.uk/physics/module%202/terminal%20velocity/terminal%20velocity.htm This can be shown as F in y direction => AR = mg Air resistance = the mass x gravity. Once the parachute is pulled the skydiver under goes acceleration very quickly! This time however the acceleration is in the opposite direction. As the skydiver descends to earth his speed is reduced further and further, until eventually traveling at a relatively safe speed downward. At this point air resistance and gravity equal each other and there is no more acceleration. If you have ever been skydiving, you know the feeling you get when the chute is pulled? Its a felling of weightlessness almost completely floating! Then there's the Landing! Upon landing the skydiver feels a very fast deceleration due to his feet meeting the ground. Ahh its a sweet feeling! Then of course we have our landing! Sky diving is a thrill, but everyone knows its only a good day when you touch back down safely. Here is a video you must see! http://youtu.be/ur40O6nQHsw TH