The word aerodynamics comes
from two Greek words: aerios, concerning the air, and dynamis,
meaning powerful. Aerodynamics is the study of forces and the resulting
motion of objects through the air. Judging from the story of Daedalus
and Icarus, humans have been interested in aerodynamics and flying for
thousands of years, although flying in a heavier-than-airmachine has
been possible only in the last hundred years.
Newton's first law states
that every object will remain at rest or in uniform motion in a straight
line unless compelled to change its state by the action of an external
force. This is normally taken as the definition of inertia. The key point
here is that if there is no net force acting on an object (if all the
external forces cancel each other out) then the object will maintain a
constant velocity. If that velocity is zero, then the object remains at
rest. If an external force is applied, the velocity will change because of
the force.
The second law explains
how the velocity will change. The law defines a force to be equal to
change in momentum (mass times velocity) per change in time. Newton also
developed the calculus of mathematics, and the "changes" expressed in the
second law are accurately defined in differential forms. (Calculus can
also be used to determine the velocity and location variations experienced
by an object subjected to an external force.) For an object with a
constant mass, the second law can be more easily expressed as the product
of an object's mass and it's acceleration (F = ma). For an external
applied force, the change in velocity depends on the mass of the object. A
force will cause a change in velocity; and likewise, a change in velocity
will generate a force. The equation works both ways.
The third law states that
for every action (force) in nature there is an equal and opposite
reaction. In other words, if object A exerts a force on object B, then
object B also exerts an equal force on object A. Notice that the forces
are exerted on different objects. The third law can be used to explain the
generation of lift by a wing and the production of thrust by a jet engine.
