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Sir Isaac Newton Sir Isaac Newton, born in 1643, was an Englishman, renowned for both his science and mathematical knowledge. He invented calculus, pioneered the field of optics, and investigated philosophy. However, he may be most well known for his groundbreaking work, Philosophiae Naturalis Principia Mathematica, or as it is more commonly known, simply Principia.
Gravity Many people have heard the story of Sir Isaac Newton sitting under an apple tree; as he sat, an apple suddenly fell from the tree, hitting him on the head. And from this knock, it is said that he suddenly came up with the notion of gravity. Now, no one really knows if this story is accurate, but we do know that Newton acknowledged that the concept of a "universal force" came to him in the country. Newton said that he suddenly realized that it is the exact same force pulling an object to earth, as that which keeps the moon in earth's orbit. Newton realized that the force of universal gravitation makes every pair of bodies attract. The magnitude of this force is directly proportional to the masses of the bodies, and inversely proportional to the square of the distances: Principia In August of 1864, Newton was visited by Edmund Halley, an English astronomer, who found that Newton had completed an extensive investigation into gravity and motion. Halley was so enthralled by what he saw that he convinced Newton to publish his findings. Finally, in 1867, Newton's Philosophiae Naturalis Principia Mathematica was published. In it, were explorations into gravity and motion - to be specific, Newton's three fundamental laws of motion, aptly named the First, Second, and Third Laws, as they are referred to today in physics. Each law stated a basic concept of physics:
Newton's Three Laws In many sections of AeroNet, you probably noticed the connections to the Third Law, especially in reference to thrust. However, connections can be made to all three laws within AeroNet. Newton's First Law states that something stationary will remain stationary, and something moving will continue to move, unless a force acts on it. The most obvious example of an aerodynamic connection is with drag. A plane would never need thrust once at cruising speed if there were no drag - according to Newton's First Law, the plane would never slow down! In fact, it would require reverse thrust to slow it down. However, the force of drag, of the wind against the plane, is constantly there. In order for the plane to stay at cruising speed, the force from thrust must counteract the force of the drag. In the Second Law, Newton states that the acceleration of a body, or its change in speed, is dependent upon the force on that body. Here, one can observe the dependence on this law by lift and weight. As a plane is flying through the air, its altitude should remain relatively constant. So if its vertical speed is approximately 0, it should not be changing, except for small corrections. In order for the vertical speed to remain approximately 0, the vertical acceleration should stay nearly zero. By the Second Law, this requires that the vertical force on the plane remain small. Since the total force on the plane is equal to the sum of the forces, the weight must be canceled out by the lift, so that the sum is small. In order for a plane to propel itself, it must use an aspect of Newton's Third Law: that every force has an opposite force directed at the source. A plane propels itself by forcing small particles, both air and fuel, backwards at high speed. The force the plane exerts on the particles is reciprocated, and the air and fuel mixture pushes back against the plane, propelling it forwards. 
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