Which of Newton's Laws Apply When
a Parachute Falls
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.