Newton's Laws of Motion

Newton's Laws of Motion are three physical laws that, together, laid the foundation of classical mechanics. They describe the relationship between a body and the forces acting upon it, and its motion in response to those forces. More precisely, the first law defines the force qualitatively, the second law offers a quantitative measure of the force, and the third asserts that a single isolated force does not exist. These three laws have been expressed in several ways, over nearly three centuries, and can be summarized as follows:

First Law

In an inertial frame of reference, an object either remain at rest or continues to move at a constant velocity, unless acted upon by an outside force.

Second Law

In an inertial frame of reference, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F=ma. (It is assumed here that the mass m is constant).

Third Law 

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

The three laws of motion was first compiled by Sir Isaac Newton in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687. Newton used them to explain and investigate the motion of many physical objects and systems. For example, in the third volume of the text, Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler's laws of planetary motion.

Some also explain a fourth law which states that forces add up like vectors, that is, that forces obey the principle of superposition.

x-----x

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