Newtonian physics encompasses the contributions of Isaac Newton (1643-1717) to the understanding of the physics of motion. Newton's contribution to science also includes his refinement of the scientific method, which concentrates on a precise, mathematical description of the physical world. Using 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: his three laws of motion and the universal law of gravitation. This contribution of Newtonian physics represents one of the most remarkable achievements of humanity's intellectual development.
Newtonian physics radically modified the two-thousand year-old prevailing concept of the universe which was based on Aristotle's physics. These foundations were first shaken by physicist Galileo Galilei (1564-1642) 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. Newton revolutionized the view of the universe by showing that the same physical laws apply to all matter, whether inert or living. Aristotle (384-322 BCE) tried to explain the underlying reasons as to why objects move and he proposed that "natural motion" (such as freefall) resulted from the objects "wanting" or preferring to lie in their natural state--in this case, on the ground. Aristotle 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, "forced motion," could occur due to an object forcing another to move.
Newton formulated the modern concept of force, starting with the insight that only interactions between objects can affect motion. 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 when they are unbalanced, which is to produce changes in the motion of a body. Aristotle believed that forces could only act on objects that touched each other; Newtonian physics describes such forces as contact forces but also accounts for noncontact forces (like gravitational forces).
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 initially at rest will remain at rest if the total force acting on it is zero, and a moving object will remain at a constant velocity (constant speed and direction). If the total force on an object is not zero, then the object must accelerate.
Predicting the resulting acceleration is the subject of the second law, which states that an object's acceleration is directly proportional to the total (net) force acting on the object and is in the same direction as that force. This acceleration is also inversely proportional to its mass, meaning that more massive objects are less apt to accelerate for a given net force. The most common form of Newton's second law is F = ma, or force is equal to the product of an object's mass and its acceleration.
Newton's third law defines force as the interaction between two objects, and states that for every action there must be an equal and opposite reaction. Thus forces must 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.
Newtonian physics was firmly entrenched in scientific belief until the advent of German-Amerian physicist Albert Einstein's (1879-1955) special and general theory of relativity in the early twentieth century. Since that time, Newtonian physics has also been referred to as "classical physics" or pre-relativistic physics, because its laws describe the physical world in a way that cannot account for relativistic gravitational effects. In other words, Newtonian laws are valid only when gravitational potential energy differences are small compared to mc2, where m is the mass of an object and c is the speed of light. The laws are also invalid in the analysis of objects with speeds that approach the speed of light. Thus, the physics of very massive and very fast objects can only be described by relativity. Newtonian physics also fails to fully describe the physics of very small objects (such as electrons, atoms, and elementary particles), which are accounted for in quantum theory.
Newton's laws are valid in frames of reference moving at constant velocities well below the speed of light. Frames of reference moving at constant velocity relative to one another are called inertial frames of reference. In order to apply Newton's laws in accelerated frames of reference, fictitious (imaginary) forces must be introduced to explain the motion of objects. For example, when a car suddenly stops, an unrestrained passenger will keep moving forward through the windshield. Relative to the surface of the Earth, Newton's first law of inertia clearly explains this observation. Relative to the car (a non-inertial reference frame), it appears as if something pushes the passenger forward. This "something" is not a true force according to Newton's third law, and thus is referred to as a fictitious force. While the revolution in physics brought about by relativity has clearly shown these limitations of Newton's laws, Newtonian physics maintains a range of validity for many applications in the physical world.