motion, the change of position of one body with respect to another. The rate of change is the speed speed, change in distance with respect to time. Speed is a scalar rather than a vector quantity; i.e., the speed of a body tells one how fast the body is moving but not the direction of the motion.
..... Click the link for more information. of the body. If the direction of motion is also given, then the velocity velocity, change in displacement with respect to time. Displacement is the vector counterpart of distance, having both magnitude and direction. Velocity is therefore also a vector quantity. The magnitude of velocity is known as the speed of a body.
..... Click the link for more information. of the body is determined; velocity is a vector U [−3,1] and V [5,2], one can add their corresponding components to find the resultant vector R [2,3], or one can graph U and V on a set of coordinate axes and complete the parallelogram formed with U and V
..... Click the link for more information. quantity, having both magnitude and direction, while speed is a scalar quantity, having only magnitude.

Types of Motion

Uniform motion is motion at a constant speed in a straight line. Uniform motion can be described by a few simple equations. The distance s covered by a body moving with velocity v during a time t is given by s=vt. If the velocity is changing, either in direction or magnitude, it is called accelerated motion (see acceleration acceleration, change in the velocity of a body with respect to time. Since velocity is a vector quantity, involving both magnitude and direction, acceleration is also a vector. In order to produce an acceleration, a force must be applied to the body.
..... Click the link for more information. ). Uniformly accelerated motion is motion during which the acceleration remains constant. The average velocity during this time is one half the sum of the initial and final velocities. If a is the acceleration, v_othe original velocity, and v_fthe final velocity, then the final velocity is given by v_f=v_o + at. The distance covered during this time is s=v_ot + 1-2 at². In the simplest circular motion the speed is constant but the direction of motion is changing continuously. 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=v²/r, where v is the speed and r is the radius of the circle.

The Laws of Motion and Relativity

The relationship between force force, commonly, a "push" or "pull," more properly defined in physics as a quantity that changes the motion, size, or shape of a body. Force is a vector quantity, having both magnitude and direction.
..... Click the link for more information. and motion was expressed by Sir Isaac Newton Newton, Sir Isaac, 1642–1727, English mathematician and natural philosopher (physicist), who is considered by many the greatest scientist that ever lived.
..... Click the link for more information. 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 third law implies that the total momentum of a system of bodies not acted on by an external force remains constant (see conservation laws conservation laws, in physics, basic laws that together determine which processes can or cannot occur in nature; each law maintains that the total value of the quantity governed by that law, e.g., mass or energy, remains unchanged during physical processes.
..... Click the link for more information. , in physics). Newton's laws of motion, together with his law of gravitation gravitation, the attractive force existing between any two particles of matter .

The Law of Universal Gravitation

Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it
..... Click the link for more information. , provide a satisfactory basis for the explanation of motion of everyday macroscopic objects under everyday conditions. However, when applied to extremely high speeds or extremely small objects, Newton's laws break down.

Motion at speeds approaching the speed of light light, visible electromagnetic radiation . Of the entire electromagnetic spectrum , the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm.
..... Click the link for more information. must be described by the theory of relativity relativity, physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference.
..... Click the link for more information. . 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. When the motions of extremely small objects (atoms and elementary particles elementary particles, the most basic physical constituents of the universe.

Basic Constituents of Matter

Molecules are built up from the atom , which is the basic unit of any chemical element .
..... Click the link for more information. ) are described, the wavelike properties of matter must be taken into account (see quantum theory quantum theory, modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
..... Click the link for more information. ). The theory of relativity also resolves the question of absolute motion. When one speaks of an object as being in motion, such motion is usually in reference to another object which is considered at 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. All these motions are relative.

It was once thought that there existed a light-carrying medium, known as the luminiferous ether ether or aether, in physics and astronomy, a hypothetical medium for transmitting light and heat (radiation), filling all unoccupied space; it is also called luminiferous ether. In Newtonian physics all waves are propagated through a medium, e.g.
..... Click the link for more information. , which was in a state of absolute rest. Any object in motion with respect to this hypothetical frame of reference would be in absolute motion. The theory of relativity showed, however, that no such medium was necessary and that all motion could be treated as relative.

Bibliography

See J. C. Maxwell, Matter and Motion (1877, repr. 1952).

motion

Change in position of a body relative to another body or with respect to a frame of reference or coordinate system. Motion occurs along a definite path, the nature of which determines the character of the motion. Translational motion occurs if all points in a body have similar paths relative to another body. Rotational motion occurs when any line on a body changes its orientation relative to a line on another body. Motion relative to a moving body, such as motion on a moving train, is called relative motion. Indeed, all motions are relative, but motions relative to the Earth or to any body fixed to the Earth are often assumed to be absolute, as the effects of the Earth's motion are usually negligible. See also Brownian motion; periodic motion; simple harmonic motion; simple motion; uniform circular motion.

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Types of Motion

The Laws of Motion and Relativity

The Law of Universal Gravitation

Basic Constituents of Matter

Bibliography

motion

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