Drag Force
The first of two Chute
Types, the drag parachute, works by lessening, and
then at a certain speed of descent, canceling, the force
of gravity with an opposing force. The same kind of
drag force can be felt when you are
riding a bicycle, and you can make it vary by adjusting
how tall you sit or by using different shapes of helmet
or clothing. This drag is actually made up of two kinds
of force -- one based on the shape, called form
drag, and also one called skin friction,
which is much like the friction when two object rub
against each other. In air, at the sizes and speeds
typical of parachutes -- even model ones -- the first
of these is dominant and we will focus on that. (At
the scale of dust particles, on the other hand, it is
the second force that predominates.)
So what factors contribute to this “form drag”?
There are three factors that model parachute designers
need to care about:
(1) The velocity of the chute is critical,
as drag is related to the velocity squared.
A doubling of the speed increases the drag force fourfold.
(2) The amount of frontal area facing
the airflow. Keeping this factor large can be accomplished
by designing a parachute that gets inflated quickly
and stays inflated during descent.
(3) The shape, and the material out of which the chute
is made, which determine respectively how the air flows
and the amount of skin friction, are bundled up into
a dimensionless number called the drag coefficient.
The higher this number, the greater the drag force.
A fourth factor does not concern parachute drops in
a classroom -- density of air does
not change detectably in these conditions so it can
be treated as a constant -- either separately or bundled
in as part of a combined factor with the drag coefficient
(which will then no longer be dimensionless).
A World Without Drag
What would falling be like without a drag force caused
by air? This is a subject of classical mechanics and
Newton's Laws. Without this factor, which increases
with speed and frontal area and the shape of the object,
a light body would fall as fast as a heavier one.
This happens in places like outer space, and as well
on our Moon. During one of the lunar walks in the 1970’s,
astronauts conducted a classic freefall Challenge --
they dropped a feather and a hammer at the same time.
You can see a movie clip of this Challenge to the right.
As Newton would have predicted, the heavier and lighter
bodies accelerate at the same rate until they hit the
ground at the same time. Why? The greater force of gravity
on the more massive hammer is countered by its equally
great inertia.
So why does a heavier object with the same shape and
profile fall faster than a similar lighter object in
Earth's atmosphere? Find out by reading the next page
on Terminal Speed.
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