The motion of an aircraft through the air can be explained and described by
physical principals discovered over 300 years ago by Sir Isaac Newton. Newton worked
in many areas of mathematics and physics. He developed the theories of gravitation
in 1666, when he was only 23 years old. Some twenty years later, in 1686, he
presented his three laws of motion in the "Principia Mathematica Philosophiae
Naturalis." The laws are shown above, and the application of these laws
to aerodynamics are given on separate slides.
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 of an
object changes when it is subjected to an external force.
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 most 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 m, the second law states that the
force F is the product
of an object's mass and its acceleration a:
F = m * a
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.
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