An object that is falling through a vacuum is subjected to only
one external force, the gravitational
force, expressed as the weight of the object. The weight
equation defines the weight (W) to be
equal to the mass of the object (m) times the gravitational acceleration
(g), which is 9.8 meters per square second on the surface of the
earth. The gravitational acceleration decreases with the square of
the distance from the center of the earth. For many practical
problems, we can assume this factor to be a constant.
An object that is moving because of the action of gravity alone is
said to be **free falling**.
If the object
were falling through the atmosphere, there
would be an additional drag force acting on
the object. And the physics involved with
describing the motion of the object would be more complex.

The motion of a free falling object can be described by Newton's
second law of motion, force (F) =
mass (m) times acceleration (a). We can do a little
algebra and solve for the acceleration of the object in terms of the
net external force and the mass of the object ( a = F / m).
The **net external
force** is just the weight of the object (F = W). Substituting into
the second law equation gives: a = W / m = m * g / m = g .
The **acceleration** (change of velocity) of the object then
becomes the gravitational acceleration. The mass, size, and shape of
the object are not a factor in describing the motion of the object.
So all objects, regardless of size or shape or mass (or weight) will
free fall at the same rate; a beach ball will fall at the same rate as
an airliner.
Knowing the acceleration, we can
predict the velocity and location of a free
falling object at any time.

The remarkable observation that all free falling objects fall at
the same rate was first proposed by **Galileo,** nearly 400 years
ago. Galileo conducted experiments using a ball on an inclined plane
to determine the relationship between the time and distance traveled.
He found that the distance depended on the square of the time and
that the velocity increased as the ball moved down the incline. The
relationship was the same regardless of the mass of the ball used in
the experiment. (The story that Galileo demonstrated his findings by
dropping two cannon balls off the Leaning Tower of Pisa is just a
legend.).

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- Beginner's Guide Home Page

*byTom
Benson
Please send suggestions/corrections to: benson@grc.nasa.gov *