In the 17th century, a philosopher and mathematician, Sir Isaac Newton, propounded three basic laws of motion. It is certain that he did not have the airplane in mind when he did so, but almost everything we know about motion goes back to his three simple laws. These laws, named after Newton, are as follows:

   Newton's first law states, in part, that:

      A body at rest tends to remain at rest, and a body in motion tends to remain moving at the same speed and in the same direction.

   This simply means that, in nature, nothing starts or stops moving until some outside force causes it to do so. An airplane at rest on the ramp will remain at rest unless a force strong enough to overcome its inertia is applied. Once it is moving, however, its inertia keeps it moving, subject to the various other forces acting on it. These forces may add to its motion, slow it down, or change its direction.

   Newton's second law implies that:

      When a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and is directly proportional to the applied force.

   What we are dealing with here are the factors involved in overcoming Newton's First Law of Inertia. It covers both changes in direction and speed, including starting up from rest (positive acceleration) and coming to a stop (negative acceleration, or deceleration).
 

Newton's third law states that:       Whenever one body exerts a force on another, the second body always exerts on the first, a force which is equal in magnitude but opposite in direction.    The firing of a shotgun or large caliber rifle provides a "bang" in more ways than one. It may not be the most pleasant way to get your "kicks," but the recoil of a gun as it is fired is a graphic example of Newton's Third Law (Fig. 17-2). The champion swimmer who pushes against the side of the pool during the turn around, or the infant learning to walk - both would fail but for the phenomena expressed in this law. The charge leaving the gun, or any push against some surface which "pushes back," illustrates this mutual action/reaction. In an airplane, the propeller moves and pushes back the air; consequently, the air pushes the propeller (and thus the airplane) in the opposite direction - forward. In a jet airplane, the engine pushes a blast of hot gases backward; the force of equal and opposite reaction pushes against the engine and forces the airplane forward. The movement of all vehicles is a graphic illustration of Newton's Third Law.

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