![]() ![]() Three laws that form the foundation of classical mechanics. See Dynamics, Force, Kinetics (classical mechanics) The newtonian laws have proved valid for all mechanical problems not involving speeds comparable with the speed of light and not involving atomic or subatomic particles. Third law: If two particles interact, the force exerted by the first particle on the second particle (called the action force) is equal in magnitude and opposite in direction to the force exerted by the second particle on the first particle (called the reaction force). Second law: The acceleration of a particle is directly proportional to the resultant external force acting on the particle and is inversely proportional to the mass of the particle. They are stated as follows:įirst law: A particle not subjected to external forces remains at rest or moves with constant speed in a straight line. Three fundamental principles which form the basis of classical, or newtonian, mechanics. The Columbia Electronic Encyclopedia™ Copyright © 2022, Columbia University Press. The theory of relativity showed, however, that no such medium was necessary and that all motion could be treated as relative. Any object in motion with respect to this hypothetical frame of reference would be in absolute motion. It was once thought that there existed a light-carrying medium, known as the luminiferous ether, which was in a state of absolute rest. Although a person sitting in a car is at rest with respect to the car, both in motion with respect to the earth, and the earth is in motion with respect to the sun and the center of the galaxy. When one speaks of an object as being in motion, such motion is usually in reference to another object which is considered at rest. The theory of relativity also resolves the question of absolute motion. When the motions of extremely small objects (atoms and elementary particles) are described, the wavelike properties of matter must be taken into account (see quantum theory). The equations derived from the theory of relativity reduce to Newton's when the speed of the object being described is very small compared to that of light. Motion at speeds approaching the speed of light must be described by the theory of relativity. ![]() However, when applied to extremely high speeds or extremely small objects, Newton's laws break down. Newton's laws of motion, together with his law of gravitation, provide a satisfactory basis for the explanation of motion of everyday macroscopic objects under everyday conditions. The third law implies that the total momentum of a system of bodies not acted on by an external force remains constant (see conservation laws, in physics). The relationship between force and motion was expressed by Sir Isaac Newton in his three laws of motion: (1) a body at rest tends to remain at rest or a body in motion tends to remain in motion at a constant speed in a straight line unless acted on by an outside force, i.e., if the net unbalanced force is zero, then the acceleration is zero (2) the acceleration a of a mass m by an unbalanced force F is directly proportional to the force and inversely proportional to the mass, or a = F/ m (3) for every action there is an equal and opposite reaction. The acceleration causing this change, known as centripetal acceleration because it is always directed toward the center of the circular path, is given by a= v 2/ r, where v is the speed and r is the radius of the circle. In the simplest circular motion the speed is constant but the direction of motion is changing continuously. The distance covered during this time is s= v o t + 1-2 at 2. If a is the acceleration, v o the original velocity, and v f the final velocity, then the final velocity is given by v f= v o + at. The average velocity during this time is one half the sum of the initial and final velocities. Uniformly accelerated motion is motion during which the acceleration remains constant. ![]() If the velocity is changing, either in direction or magnitude, it is called accelerated motion (see acceleration). The distance s covered by a body moving with velocity v during a time t is given by s= vt. Uniform motion can be described by a few simple equations. Uniform motion is motion at a constant speed in a straight line.
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