Newton's Laws describe the behavior of a particle which may be subject to forces by its environment.
Newton's First Law: An isolated particle at rest or in motion stays at rest or in motion with the same velocity.
Newton's Second Law: A particle subject to a force accelerates in the direction of the force, in proportion to the force, and inversely proportional to its mass. In mathematical form we write: F=ma, where F is the force exerted on the particle, m is the mass of the particle, and a is the acceleration of the particle.
(note: this describes the effect of an action by the environment on the object. Since it includes the possibility of no action as a special case, it can be understood to generalize the first law. This law introduces the concept of the intertial mass of an particle which is defined to be a property of the particle.)
Newton's Third Law: Every action (force) has an equal and opposite reaction.
(note: this describes the reciprocal action of two particles upon each other, thus the action and reaction affect different particles).
Note: Newton's Laws describe respectively the largest scale behavior of an isolated system, a system in interaction with its environment, and two systems interacting with each other.
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