# Motion

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## motion,

the change of position of one body with respect to another. The rate of change is the speedspeed,
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.
of the body. If the direction of motion is also given, then the velocityvelocity,
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.
of the body is determined; velocity is a vectorvector,
quantity having both magnitude and direction; it may be represented by a directed line segment. Many physical quantities are vectors, e.g., force, velocity, and momentum.
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 accelerationacceleration,
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.
). 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, vo the original velocity, and vf the final velocity, then the final velocity is given by vf=vo + at. The distance covered during this time is s=vot + 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 forceforce,
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.
and motion was expressed by Sir Isaac NewtonNewton, Sir Isaac,
1642–1727, English mathematician and natural philosopher (physicist), who is considered by many the greatest scientist that ever lived. Early Life and Work
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 lawsconservation 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.
, in physics). Newton's laws of motion, together with his law of gravitationgravitation,
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 is often called
, 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 lightlight,
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.
must be described by the theory of relativityrelativity,
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.
. 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 particleselementary 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. The atom in turn is made from the proton, neutron, and electron.
) are described, the wavelike properties of matter must be taken into account (see quantum theoryquantum 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.
). 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 etherether
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.
, 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

If the position of a material system as measured by a particular observer changes with respect to time, that system is said to be in motion with respect to the observer. Absolute motion, then, has no significance, and only relative motion may be defined; what one observer measures to be at rest, another observer in a different frame of reference may regard as being in motion. See Frame of reference, Relative motion

The time derivatives of the various coordinates used to specify the system may be used to prescribe the motion at any instant of time. How the motion develops in subsequent instants is then determined by the laws of motion. In classical dynamics it is supposed that in principle the motion and configuration of the system may be specified to an arbitrary precision, although in quantum mechanics it is recognized that the measurement of the one disturbs the other.

The most general theory of motion that has yet been developed is quantum field theory, which combines both quantum mechanics and relativity theory, as well as the experimentally observed fact that elementary particles can be created and annihilated. See Degree of freedom (mechanics), Dynamics, Hamilton's equations of motion, Harmonic motion, Kinematics, Kinetics (classical mechanics), Lagrange's equations, Newton's laws of motion, Oscillation, Periodic motion, Quantum field theory, Quantum mechanics, Rectilinear motion, Relativity, Rotational motion

## Motion

in geometry, a transformation of space that preserves the properties of a figure (dimensions, shape, and so forth). The concept of a motion was formulated as an abstraction of actual motions of rigid bodies.

A motion of Euclidean space is a geometric transformation that preserves the distance between points. It is called proper or improper depending on whether it preserves or changes orientation. Apart from a translation, a motion is an orthogonal transformation.

A proper motion in a plane can be specified relative to a rectangular system of coordinates (x, y) by the formulas

x̄ = x cos φ - y sin φ + a
ȳ = y sin φ + y cos φ + b

that show that the set of all proper motions in a plane depends on the three parameters a, b, and φ. By assigning particular values to a and b we determine a translation of the plane by the vector (a, b), and by assigning a particular value to φ we determine a rotation of the plane about the origin by the angle φ. A proper motion can be represented as a translation or as a rotation about some point. An improper motion can be represented as a reflection in a line or as a product (the result of successive application) of a translation in some direction and a reflection in a line having the same direction.

A proper motion in space can be represented as a rotation about an axis, or a translation, or a screw motion (a rotation about an axis followed by a translation in the direction of this axis). An improper motion in space can be represented as a reflection in a plane, or a product of a reflection in a plane and a rotation about an axis perpendicular to this plane, or a product of a reflection in a plane and a translation determined by a vector parallel to this plane.

Apart from a translation, a motion in space can be represented analytically by a linear transformation with an orthogonal matrix having a determinant equal to 1 or -1 depending on whether the motion is proper or improper.

The concept of a motion is carried over into Riemannian spaces and spaces of affine connection. The concept of a motion plays an important role in the Riemannian spaces associated with the theory of relativity (the strong asymmetry of gravitational fields imposes restrictions on the motion of rigid bodies in such spaces).

Motion may be used as an undefined term in the axiomatic development of geometry. In this case, the axioms of a motion are substituted for the axioms of congruence. The congruence of segments, angles, and other figures is then defined in terms of motions (two figures are said to be congruent if there exists a motion that carries one figure into the other). The totality of motions forms a group.

### REFERENCES

Adamar, J. Elemenlarnaia geometriia.Part 1, 3rd ed., Moscow, 1948; part 2, 2nd ed., Moscow, 1951. (Translated from French.)
Rashevskii, P. K. Rimanova geometriia i tenzornyi analiz,3rd ed. Moscow, 1967.
Aleksandrov, P. S. Lektsii po analiticheskoi geometrii.Moscow, 1968.

E. G. POZNIAK

## Motion

a mode of matter’s existence and its most important attribute. In its most general aspect motion is “change in general” (F. Engels, in K. Marx and F. Engels, Soch.,2nd ed., vol. 20, p. 563), any kind of interaction between material objects.

The concept of the universality of motion originated in remote antiquity among the thinkers of China, India, and Greece. The ancient Greek philosophers, including the Milesian school, Heraclitus, Democritus, and Epicurus, regarded the primal principles of all things—water, apeiron, air, fire, and atoms—as being in a state of constant motion and change. Aristotle believed that an “ignorance of motion of necessity means an ignorance of nature” (Physics,III 1, 200 b; Russian translation, Moscow, 1936). The conception of motion as a mode of matter’s existence was precisely formulated in the 18th century by the British philosopher J. Toland and subsequently by the French materialist P. Holbach. However, they conceived of motion itself as being merely mechanical displacement and interaction. Profound ideas concerning the interpretation of motion were expressed by the objective idealists G. W. Leibniz, G. Hegel, and others. Thus, Hegel transcends the conception of motion as merely mechanical displacement and formulates general laws of motion—the law of the transition from quantitative changes to qualitative ones, the law of the conflict of opposites, and the law of the negation of the negation.

A new and higher stage in the understanding of motion as a mode of matter’s being was attained with the creation of dialectical materialism by K. Marx and F. Engels. This doctrine concerning motion was further developed during the 20th century in the works of V. I. Lenin. Dialectical materialism has substantiated the link between matter and motion in a new way and has established the principle that matter in motion can be neither created nor destroyed. “Matter without motion is just as inconceivable as motion without matter. Motion, therefore, can no more be created or destroyed than matter itself (F. Engels, in K. Marx and F. Engels, Soch.,2nd ed., vol. 20, p. 59). The principles of the connection between matter and motion and of the inability of moving matter to be destroyed or created acquired a special significance in the light of the great discoveries in the natural sciences of the 19th and 20th centuries. Lenin put forward the principle of the unity of matter and motion in order to counter the attempts of the so-called energeticism school to reduce matter to energy. Lenin emphasized that matter is not something inert, something to which motion “is added,” nor is it the empty “subject,” of the predicate “to move.” Rather, matter is the foundation and universal vehicle of all states of motion and development. “Whether we say the world is moving matter or that the world is material motion makes no difference whatever” (Poln. sobr. soch.,5th ed., vol. 18, p. 286). Dialectical materialism holds that, along with its material quality, the principal characteristics of motion are its absoluteness and its contradictory quality. The motion of matter is absolute, whereas all rest is relative and represents one of the aspects of motion. It determines all the characteristics and manifestations of the surrounding world and the inner content of all objects and phenomena. The contradictory nature of motion consists in the unbroken unity of two opposing factors—changeability and stability, motion and rest. In fact, the concept of change makes sense only in connection with the idea of a relatively stable, continuously fixed state. This very change, however, is at the same time also a fixed state, which continues and maintains itself; that is, it also possesses stability. In this contradictory unity of changeability and stability the leading role is played by changeability, for everything new in the world first appears by means of it, whereas stability and rest merely fix what has been attained through this process.

The motion of matter is diverse in its manifestations and exists in various forms. Classification of the principal forms of motion involves making a distinction between inorganic matter and the biological and social spheres. Motion may occur in an ascending line, advancing from simple forms to more complex ones, from what is lower to what is higher; such motion is called development. Motion may also proceed along a descending line, toward simpler forms; that is, it may be regressive.

Motion occurs in space and time, which, as the theory of relativity has established, are merely relative “aspects” of a single form of matter’s existence, that is, space-time.

### REFERENCES

Lenin, V. I. Poln. sobr. soch.,5th ed., vol. 29. (See the subject index.)
Hegel, G. W. F. “Filosofiia prirody.” Soch.,vol. 2. Moscow-Leningrad, 1934.
Sviderskii, V. I. Protivorechivost’ dvizheniia i ee proiavleniia.Leningrad, 1959.
Sviderskii, V. I. Nekotorye voprosy dialektiki izmeneniia i razvitiia.Moscow, 1965.
Meliukhin, S. T. Materiia v ee edinststve, beskonechnosti i razvitii.Moscow, 1966.
Ovchinnikov, N. F. Printsipy sokhraneniia. Moscow, 1966.
Struktura i formy materii. Moscow, 1967. (Collected essays.)

V. I. SVIDERSKII

## motion

[′mō·shən]
(mechanics)
A continuous change of position of a body.

## motion

1. the process of continual change in the physical position of an object; movement
2. a movement or action, esp of part of the human body; a gesture
3.
a. the capacity for movement
b. a manner of movement, esp walking; gait
4. a mental impulse
5. Politics a formal proposal to be discussed and voted on in a debate, meeting, etc.
6. Law an application made to a judge or court for an order or ruling necessary to the conduct of legal proceedings
7. Brit
a. the evacuation of the bowels
b. excrement
8.
a. part of a moving mechanism
b. the action of such a part
9. Music the upward or downward course followed by a part or melody. Parts whose progressions are in the same direction exhibit similar motion, while two parts whose progressions are in opposite directions exhibit contrary motion

## Motion

Andrew. born 1952, British poet and biographer; his collections include Pleasure Steamers (1978) and Public Property (2002): poet laureate from 1999
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