Weight is the force
generated by the gravitational attraction of the earth on any object.
Weight is fundamentally
different from the aerodynamic forces,
lift and drag.
Aerodynamic forces are mechanical forces and the object has to be
in physical contact with the air which generates the force. The
gravitational force is a field force; the source of the force does
not have to be in physical contact with the object to generate a force.
The nature of the gravitational force has been studied by
scientists for many years and is still being investigated by
theoretical physicists. For an object the size of an airplane flying
near the earth, the descriptions given three hundred years ago by Sir
Isaac Newton work quite well. Newton published his theory of
gravitation with his laws of motion in
1686. The gravitational force, F, between two particles equals a
universal constant, G, times the product of the mass of the particles, m1 and m2,
divided by the square of the distance, d, between the particles.
F = G * m1 * m2 / d^2
If you have a lot of particles acting on a single particle, you
have to add up the contribution of all the individual particles. For
objects near the earth, the sum of the mass of all the particles is
simply the mass of the earth and the distance is then measured from
the center of the earth. On the surface of the earth the distance is
about 4000 miles. Scientists have combined the universal
gravitational constant, the mass of the earth, and the square of the
radius of the earth to form the gravitational acceleration, g
. On the surface of the earth, its value is 9.8 meters per
square second or 32.2 feet per square second.
g = G * m earth / (d earth)^2
The weight W, or
gravitational force, is then just the mass of an object times the
W = m * g
Since the gravitational constant (g) depends on the square of the
distance from the center of the earth, we would expect that the
weight of an object would decrease with altitude. Let's do a
test problem to see how much the weight changes. If an airplane is
flying at 35000 feet (about 7 miles) the distance to the center of
the earth is about 4007 miles. We can calculate the change in the
gravitational constant as the square of (4000/4007) which equals
.9965. If the airplane weighs 10000 pounds on the surface of the
earth, it weighs 9965 pounds at 35000 feet; it has lost 35 pounds, a
very small amount compared to 10000 pounds.
You can view a short
of "Orville and Wilbur Wright" discussing the weight force
and how it affected the flight of their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page