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History : Chute Types : Newton's Laws : Drag Force : Terminal Speed : Real/Model Chutes               
Which of Newton's Laws Apply When a Parachute Falls
(and When)?

A simple explanation of how a parachute works says that after being fully deployed, the chute reaches a constant rate of descent or terminal speed. When this happens, the force of gravity acting on the parachute and its rider is balanced by an equal and opposite drag force, resulting in a zero net force on the parachute. Newton's First Law of Motion says that when a system has no net force acting on it, that system will not change in speed or direction of motion. "A body at rest will remain at rest, and a body in motion will remain in motion." The second part of the First Law thus applies when a parachute is fully deployed and has reached terminal speed -- when the gravitational and drag forces cancel one another out, the speed of descent in this condition is maintained.

Newton's Second Law of Motion applies both when the chute is first deployed, and when it lands. Before the chute fully opens, drag force is smaller than gravity's force, so there is net force downward. The second Law says that when there is a net force, a body will accelerate in the force's direction. (A net force can either change a body's speed or direction of motion or both.) As the chute system increases in speed, more and more drag gets created. The downward net force gets smaller, and so the body continues to speed up, but less quickly. Once drag friction equals gravity's force, there is no net force on the object. It then descends at a constant speed (First Law) .

When the parachutist finally reaches the ground, the force of the ground upward momentarily overwhelms the downward gravitational force, resulting in a net force up, and causing the body to decelerate rapidly. This is the second time when, during a parachute's journey, Newton's second Law applies. When the parachute comes to rest, the force from the ground equals that of gravitational force, and the chute is without motion (First Law).

A more complicated model explaining how parachutes works is beyond the scope of this project. However, such a model would describe how a chute deploys, what happens when the canopy folds in gusty air, or begins swaying during descent. It would tell how changes in direction or speed occur, why certain canopies flip when released, and the effects of wind or turbulence as the chute travels downward.

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