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 remains 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 maintains a constant velocity. Prior to Newton's investigation, people believed that one needed to constantly apply a force to maintain a constant velocity. Newton's law states quite the opposite; no force is needed to maintain a constant velocity. Forces produce a change in velocity not the velocity itself. If all external forces are balanced and the velocity is zero, then the object remains at rest. If an external force is applied, the velocity changes because of the force.

The second law explains how the velocity changes. The law defines a force to be equal to change in momentum (mass times velocity) per change in time. 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.

Acceleration is the change in velocity with change in time. For a constant external applied force, the acceleration is inversely proportional to the mass. For the same force, a lighter object has a higher acceleration than a heavy object. A force causes a change in velocity; and likewise, a change in velocity generates a force. The equation works both ways. Newton developed the calculus of mathematics. 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.

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 propeller.

You can view a short movie of "Orville and Wilbur Wright" explaining how Newton's Laws of Motion described the flight of their aircraft. The movie file can be saved to your computer and viewed as a Podcast on your podcast player.

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