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In classical mechanics, Newton's laws of motion are three laws that describe the relationship between the motion of an object and the forces acting on it. The first law states that an object either remains at rest or continues to move at a constant velocity, unless it is acted upon by an external force. The second law states that the rate of change of momentum of an object is directly proportional to the force applied, or, for an object with constant mass, that the net force on an object is equal to the mass of that object multiplied by the acceleration. The third law states that when one object exerts a force on a second object, that second object exerts a force that is equal in magnitude and opposite in direction on the first object. The three laws of motion were first compiled by Isaac Newton in his ''Philosophiæ Naturalis Principia Mathematica'' (''Mathematical Principles of Natural Philosophy''), first published in 1687.See the ''Principia'' on line a
Andrew Motte Translation
/ref> Newton used them to explain and investigate the motion of many physical objects and systems, which laid the foundation for Newtonian mechanics.

Laws

Newton's first law

The first law states that an object at rest will stay at rest, and an object in motion will stay in motion unless acted on by a net external force. Mathematically, this is equivalent to saying that if the net force on an object is zero, then the velocity of the object is constant. :$\sum \mathbf = 0\; \Leftrightarrow\; \frac = 0.$ where $\mathbf$ is the force being applied ($\sum$ is notation for summation), $\mathbf$ is the velocity, and $\frac$ is the derivative of $\mathbf$ with respect to time $t$. Newton's first law is often referred to as the ''principle of inertia''. Newton's first (and second) laws are valid only in an inertial reference frame.

Newton's second law

The second law states that the rate of change of momentum of a body over time is directly proportional to the force applied, and occurs in the same direction as the applied force. :$\mathbf = \frac$ where $\mathbf$ is the momentum of the body.

Constant Mass

For objects and systems with constant mass, "We may conclude emphasizing that Newton's second law is valid for constant mass only. When the mass varies due to accretion or ablation, n alternate equation explicitly accounting for the changing massshould be used." mphasis as in the original/ref> the second law can be re-stated in terms of an object's acceleration. :$\mathbf = \frac = m\,\frac = m\mathbf,$ where is the net force applied, is the mass of the body, and is the body's acceleration. Thus, the net force applied to a body produces a proportional acceleration.

Variable-mass systems

Variable-mass systems, like a rocket burning fuel and ejecting spent gases, are not closed and cannot be directly treated by making mass a function of time in the second law; The equation of motion for a body whose mass varies with time by either ejecting or accreting mass is obtained by applying the second law to the entire, constant-mass system consisting of the body and its ejected or accreted mass; the result is :$\mathbf F + \mathbf \frac = m$ where is the exhaust velocity of the escaping or incoming mass relative to the body. From this equation one can derive the equation of motion for a varying mass system, for example, the Tsiolkovsky rocket equation. Under some conventions, the quantity $\mathbf \frac$ on the left-hand side, which represents the advection of momentum, is defined as a force (the force exerted on the body by the changing mass, such as rocket exhaust) and is included in the quantity . Then, by substituting the definition of acceleration, the equation becomes  = .

Newton's third law

The third law states that all forces between two objects exist in equal magnitude and opposite direction: if one object ''A'' exerts a force F''A'' on a second object ''B'', then ''B'' simultaneously exerts a force F''B'' on ''A'', and the two forces are equal in magnitude and opposite in direction: F''A'' = −F''B''. The third law means that all forces are ''interactions'' between different bodies, or different regions within one body, and thus that there is no such thing as a force that is not accompanied by an equal and opposite force. In some situations, the magnitude and direction of the forces are determined entirely by one of the two bodies, say Body ''A''; the force exerted by Body ''A'' on Body ''B'' is called the "action", and the force exerted by Body ''B'' on Body ''A'' is called the "reaction". This law is sometimes referred to as the ''action-reaction law'', with F''A'' called the "action" and F''B'' the "reaction". In other situations the magnitude and directions of the forces are determined jointly by both bodies and it isn't necessary to identify one force as the "action" and the other as the "reaction". The action and the reaction are simultaneous, and it does not matter which is called the ''action'' and which is called ''reaction''; both forces are part of a single interaction, and neither force exists without the other. The two forces in Newton's third law are of the same type (e.g., if the road exerts a forward frictional force on an accelerating car's tires, then it is also a frictional force that Newton's third law predicts for the tires pushing backward on the road). From a conceptual standpoint, Newton's third law is seen when a person walks: they push against the floor, and the floor pushes against the person. Similarly, the tires of a car push against the road while the road pushes back on the tires—the tires and road simultaneously push against each other. In swimming, a person interacts with the water, pushing the water backward, while the water simultaneously pushes the person forward—both the person and the water push against each other. The reaction forces account for the motion in these examples. These forces depend on friction; a person or car on ice, for example, may be unable to exert the action force to produce the needed reaction force. Newton used the third law to derive the law of conservation of momentum; from a deeper perspective, however, conservation of momentum is the more fundamental idea (derived via Noether's theorem from Galilean invariance), and holds in cases where Newton's third law appears to fail, for instance when force fields as well as particles carry momentum, and in quantum mechanics.

History

The ancient Greek philosopher Aristotle had the view that all objects have a natural place in the universe: that heavy objects (such as rocks) wanted to be at rest on the Earth and that light objects like smoke wanted to be at rest in the sky and the stars wanted to remain in the heavens. He thought that a body was in its natural state when it was at rest, and for the body to move in a straight line at a constant speed an external agent was needed continually to propel it, otherwise it would stop moving. Galileo Galilei, however, realised that a force is necessary to change the velocity of a body, i.e., acceleration, but no force is needed to maintain its velocity. In other words, Galileo stated that, in the ''absence'' of a force, a moving object will continue moving. (The tendency of objects to resist changes in motion was what Johannes Kepler had called ''inertia''.) This insight was refined by Newton, who made it into his first law, also known as the "law of inertia"—no force means no acceleration, and hence the body will maintain its velocity. As Newton's first law is a restatement of the law of inertia which Galileo had already described, Newton appropriately gave credit to Galileo.

Importance and range of validity

Newton's laws were verified by experiment and observation for over 200 years, and they are excellent approximations at the scales and speeds of everyday life. Newton's laws of motion, together with his law of universal gravitation and the mathematical techniques of calculus, provided for the first time a unified quantitative explanation for a wide range of physical phenomena. For example, in the third volume of the ''Principia'', Newton showed that his laws of motion, combined with the law of universal gravitation, explained Kepler's laws of planetary motion. Newton's laws are applied to objects which are idealised as single point masses, in the sense that the size and shape of the object's body are neglected to focus on its motion more easily. This can be done when the object is small compared to the distances involved in its analysis, or the deformation and rotation of the body are of no importance. In this way, even a planet can be idealised as a particle for analysis of its orbital motion around a star. In their original form, Newton's laws of motion are not adequate to characterise the motion of rigid bodies and deformable bodies. Leonhard Euler in 1750 introduced a generalisation of Newton's laws of motion for rigid bodies called Euler's laws of motion, later applied as well for deformable bodies assumed as a continuum. If a body is represented as an assemblage of discrete particles, each governed by Newton's laws of motion, then Euler's laws can be derived from Newton's laws. Euler's laws can, however, be taken as axioms describing the laws of motion for extended bodies, independently of any particle structure. Newton's laws hold only with respect to a certain set of frames of reference called Newtonian or inertial reference frames. Some authors interpret the first law as defining what an inertial reference frame is; from this point of view, the second law holds only when the observation is made from an inertial reference frame, and therefore the first law cannot be proved as a special case of the second. Other authors do treat the first law as a corollary of the second. The explicit concept of an inertial frame of reference was not developed until long after Newton's death. These three laws hold to a good approximation for macroscopic objects under everyday conditions. However, Newton's laws (combined with universal gravitation and classical electrodynamics) are inappropriate for use in certain circumstances, most notably at very small scales, at very high speeds, or in very strong gravitational fields. Therefore, the laws cannot be used to explain phenomena such as conduction of electricity in a semiconductor, optical properties of substances, errors in non-relativistically corrected GPS systems and superconductivity. Explanation of these phenomena requires more sophisticated physical theories, including general relativity and quantum field theory. In special relativity, the second law holds in the original form F = dp/d''t'', where F and p are four-vectors. Special relativity reduces to Newtonian mechanics when the speeds involved are much less than the speed of light. Some also describe a fourth law that is assumed but was never stated by Newton, which states that forces add up like vectors, that is, that forces obey the principle of superposition.

* Euler's laws of motion * Hamiltonian mechanics * Lagrangian mechanics * List of scientific laws named after people * Mercury, orbit of * Modified Newtonian dynamics * Newton's law of universal gravitation * Principle of least action * Principle of relativity * Reaction (physics)

References

Bibliography

* * * * * * ;Historical For explanations of Newton's laws of motion by Newton in the early 18th century and by the physicist William Thomson (Lord Kelvin) in the mid-19th century, see the following: * * *