Newtonian physics are first distinguished from Aristotelian physics and encompass the contributions of Isaac Newton (1642-1717), not only as they pertain to the understanding of the physics of motion but also as they shaped what is now referred to as the scientific method, basically a method which concentrates on a precise, mathematical description of the physical world. With this approach, Newton explained both the motions of heavenly bodies and the motions of objects on or near the surface of Earth by formulating four simple laws: the three laws of motion and the law of universal gravitation. This contribution of Newtonian over pre-Newtonian physics represents one of the most remarkable achievements of mankind's intellectual development. It consisted first of all in radically modifying the two thousand year old prevailing concept of the universe which was based on Aristotle's physics. These foundations were first shaken by Galileo during the Italian Renaissance and by Kepler's laws of planetary motion, but it was Newton who formulated the exact relationship between force and motion and revolutionized our view of the universe by showing that the same physical laws apply to all matter, whether inert or living. Aristotle tried to explain the underlying reasons as to why objects move and he proposed for example that natural motion, such as free-fall, resulted from the objects "wanting" or preferring to lie in their natural state, i.e. in this case, on the ground. He also described another type of motion, "voluntary" motion, such as that shown by a person going from point A to point B because he "wants to." Finally, he believed that a third type of motion could occur, "forced motion," due to an object forcing another to move. Newton formulated the modern concept of force starting with the insight that the only effect which can affect motion are interactions between two objects. Thus, in Newtonian physics, there is only one cause for a change in motion, and it is simply force. Forces may be of a different nature, but they all have the same effect, which is to produce changes in the motion of a body and so, we can conclude that two forces are comparable if they produce the same change when applied in the same context. Aristotle also believed that forces could only act on objects which touched each other. Newtonian physics describe such forces as contact forces but also account for noncontact forces, such as magnetism.
Newtonian physics is also a term understood to refer to the state of physics before the advent of the theory of relativity in the 1920s. Since that time, Newtonian physics have also been referred to as classical physics or pre-relativistic physics, because its laws describe the physical world in a way which can not account for relativistic gravitational effects. In other words, classical Newtonian laws are valid only insofar as gravitational potential energy differences are small compared to mc2 . But as velocity approaches the speed of light, they are no longer valid. Thus, the physics of very massive and very fast objects can only be described by relativity. Newtonian physics also fail to describe the physics of very small objects, such as electrons, atoms and elementary particles, which are accounted for by in quantum theory, also formulated in the 1920's.
The starting point of Newtonian physics are Newton's three laws of motion. The first one states that all objects have inertia; that is, an object will remain at rest or in uniform motion unless acted upon by some outside force. In other words, an object initialty at rest will remain at rest if the total force acting on it is zero, and a moving object will remain in motion with the same velocity in the same direction. And if the total force on an object is not zero, then the object will accelerate.
. Predicting the resulting acceleration is the subject of the second law which states that an object's acceleration is in the same direction as the force exerted on it. The force exerted on that object is equal to the product of its mass and acceleration, or to F = ma, and an object's acceleration can thus be predicted given its mass and the force acting on it.
Newton's third law states that for every action there must be an equal and opposite reaction or that forces occur in equal and opposite pairs: whenever object A exerts a force on object B, object B must also be exerting a force on object A. The two forces are equal in magnitude and opposite in direction.
Physics describe motion and the motion of an object relative to a frame of reference can defined by its position in a Cartesian x, y, z-coordinate system as a function of time t. The first statement implies that frames of reference are identical to each other and that the laws of mechanics apply equally well everywhere. This is because when defining a position, a decision must be made as to where to place x, y and z = 0, and also which respective axis direction will be positive. This involves the selection of the coordinate system and also of a frame of reference in which to place the system. The first is concerned with the actual variable x and the second with the observer's point of view. Any frame of reference in which the first law is valid is termed inertial and Newtonian physics assume that, if not acted on by an external force, a moving object will then continue its uniform motion relative to the frame of reference. Thus, any transformation from one frame of reference whose velocity is different from the other is then linear and the time coordinate is absolute, i.e., it is the same in any frame of reference. While the second revolution in physics brought about by relativity has clearly shown the limitations of Newton's laws, in that they are only true in frames of reference that are not accelerating, i.e., in inertial frames, and that time is a relative concept, it remains that Newtonian physics, unlike Aristotelian physics, does have a range of validity which accurately describes part of our physical world experience.
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