force bill, popular name for several laws in U.S. history, notably the act of Mar. 2, 1833, and the Reconstruction acts of May 31, 1870; Feb. 28, 1871; and Apr. 20, 1871. The first force bill, passed in response to South Carolina's ordinance of nullification, empowered President Jackson to use the army and navy, if necessary, to enforce the laws of Congress, specifically the tariff measures to which South Carolina had objected so violently. In the second set of force bills, or enforcement acts, as they were also called, the radical Republicans controlling Congress strengthened their Reconstruction program for the South by imposing severe penalties on those Southerners who tried to obstruct it. The act of May 31, 1870, designed to enforce the Fifteenth Amendment, provided heavy penalties of fine and imprisonment for anyone preventing qualified citizens (in this case African Americans) from voting. Such cases were to come under the jurisdiction of the federal courts. Congressional elections were placed exclusively under federal control, and the President was authorized to use the armed forces. In a similar vein but even more drastic was the act of Feb. 28, 1871. The act of Apr. 20, 1871, inspired by the activities of the Ku Klux Klan, declared the acts of armed combinations tantamount to rebellion and empowered the President to suspend the privilege of habeas corpus in lawless areas. President Grant did this in certain counties of South Carolina. Hundreds were indicted, fined, and imprisoned, and the act was partially responsible for the subsequent decline of the Klan.

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. The magnitude of a force is measured in units such as the pound, dyne, and newton, depending upon the system of measurement being used. An unbalanced force acting on a body free to move will change the motion of the body. The quantity of motion of a body is measured by its momentum, the product of its mass and its velocity. According to Newton's second law of motion (see motion), the change in momentum is directly proportional to the applied force. Since mass is constant at ordinary velocities, the result of the force is a change in velocity, or an acceleration, which may be a change either in the speed or in the direction of the velocity.

Two or more forces acting on a body in different directions may balance, producing a state of equilibrium. For example, the downward force of gravity (see gravitation) on a person weighing 200 lb (91 km) when standing on the ground is balanced by an equivalent upward force exerted by the earth on the person's feet. If the person were to fall into a deep hole, then the upward force would no longer be acting and the person would be accelerated downward by the unbalanced force of gravity. If a body is not completely rigid, then a force acting on it may change its size or shape. Scientists study the strength of materials to anticipate how a given material may behave under the influence of various types of force.

There are four basic types of force in nature. Two of these are easily observed; the other two are detectable only at the atomic level. Although the weakest of the four forces is the gravitational force, it is the most easily observed because it affects all matter, is always attractive and because its range is theoretically infinite, i.e., the force decreases with distance but remains measurable at the largest separations. Thus, a very large mass, such as the sun, can exert over a distance of many millions of miles a force sufficient to keep a planet in orbit. The electromagnetic force, which can be observed between electric charges, is stronger than the gravitational force and also has infinite range. Both electric and magnetic forces are ultimately based on the electrical properties of matter; they are propagated together through space as an electromagnetic field of force (see electromagnetic radiation). At the atomic level, two additional types of force exist, both having extremely short range. The strong nuclear force, or strong interaction, is associated with certain reactions between elementary particles and is responsible for holding the atomic nucleus together. The weak nuclear force, or weak interaction, is associated with beta particle emission and particle decay; it is weaker than the electromagnetic force but stronger than the gravitational force.

fifth force, postulated fifth basic force of nature (the four known forces of nature are gravity, electromagnetism, and the strong and weak interactions). Proposed in 1986 to account for gravitational discrepancies observed during some experiments, it was said to result in a repulsive effect about 1,000 times less powerful than gravity, and its strength was said to fall off quickly with distance, having a range of about 700 ft (200 m). The results of some initial experiments supported the possibility that the fifth force might exist, but later investigations, including a large-scale, highly accurate oceanic experiment, provided no evidence of such a force.

electromotive force, abbr. emf, difference in electric potential, or voltage, between the terminals of a source of electricity, e.g., a battery from which no current is being drawn. When current is drawn, the potential difference drops below the emf value. Electromotive force is usually measured in volts.

centripetal force and centrifugal force, action-reaction force pair associated with circular motion. According to Newton's first law of motion, a moving body travels along a straight path with constant speed (i.e., has constant velocity) unless it is acted on by an outside force. For circular motion to occur there must be a constant force acting on a body, pushing it toward the center of the circular path. This force is the centripetal ("center-seeking") force. For a planet orbiting the sun, the force is gravitational; for an object twirled on a string, the force is mechanical; for an electron orbiting an atom, it is electrical. The magnitude F of the centripetal force is equal to the mass m of the body times its velocity squared v ² divided by the radius r of its path: F=mv²/r. According to Newton's third law of motion, for every action there is an equal and opposite reaction. The centripetal force, the action, is balanced by a reaction force, the centrifugal ("center-fleeing") force. The two forces are equal in magnitude and opposite in direction. The centrifugal force does not act on the body in motion; the only force acting on the body in motion is the centripetal force. The centrifugal force acts on the source of the centripetal force to displace it radially from the center of the path. Thus, in twirling a mass on a string, the centripetal force transmitted by the string pulls in on the mass to keep it in its circular path, while the centrifugal force transmitted by the string pulls outward on its point of attachment at the center of the path. The centrifugal force is often mistakenly thought to cause a body to fly out of its circular path when it is released; rather, it is the removal of the centripetal force that allows the body to travel in a straight line as required by Newton's first law. If there were in fact a force acting to force the body out of its circular path, its path when released would not be the straight tangential course that is always observed.

atomic force microscope (AFM), device that uses a spring-mounted probe to image individual atoms on the surface of a material. Unlike the scanning tunneling microscope, which is also a scanning probe microscope, the AFM can be used on materials that do not conduct electricity. In the original AFM, the probe traverses the surface, moving upward due to bumps and downward due to depressions; a laser beam reflected from the tip of the probe measures the up and down movements, and the pattern of reflected light creates an image of the surface. Another type of AFM measures the sideways deflection of the tip caused by friction as the probe moves across the surface; differences in friction can be used distinguish different atoms and molecules on the material. A third variation employs a magnetic probe; this probe does not touch the material but moves up and down in reaction to the magnetic forces between the tip and the surface. In a microchip-size AFM, the electronic circuitry and multiple probes are integrated on a sliver of silicon; although less sensitive than a full-size AFM, the device has applications in microelectronics where the multiple probes make it possible to record images very quickly.

Wright-Patterson Air Force Base, U.S. military installation, 8,023 acres (3,247 hectares), W Ohio, NE of Dayton; est. 1917. One of the largest airport installations in the world, it is the air force's main research and development base, and the headquarters of the Air Force Logistics Command (national center for defense activities). The Aerospace Medical Laboratory and the Air Force Museum are also on the base.

Vandenberg Air Force Base, U.S. military installation, 3,456 acres (1,399 hectares), SW Calif., near Lompoc; chief Pacific coast launch site for military satellites. Commercial satellites are launched from the base as well, and it is also a site for test-firing intercontinental ballistic missiles.

United States Air Force Academy, at Colorado Springs, Colo.; for training young men and women to be officers in the U.S. air force; authorized in 1954 by Congress. Temporary quarters were opened at the former Lowry Air Force Base at Denver in 1955, and the permanent campus opened in 1958. The academy began admitting women in 1975 and they now make up some 13% of the cadet corps.

Candidates must be between 17 and 22 years old and meet special physical and educational qualifications. An applicant must obtain a nomination to be considered for an appointment to the academy. The sources of nomination are the President of the United States; the Vice President; U.S. Senators and Representatives; and the representatives of the District of Columbia and the U.S. territories. Special appointment categories include children of deceased and disabled veterans or of career military personnel, foreign students, regular U.S. air force, U.S. air force reserve, honor graduates of military and naval schools and ROTC, and children of Medal of Honor recipients. Approximately 4,000 cadets are enrolled in the four-year course of instruction that leads to a bachelor's degree and a position of second lieutenant. The curriculum includes standard academic subjects as well as military training, flight instruction, and athletics. Cadets receive free tuition and room and board and a monthly allotment to pay for supplies, clothing, and personal expenses.

Offutt Air Force Base, U.S. military installation, 1,907 acres (772 hectares), E Neb., S of Omaha; est. 1896 as Fort Crook, an army base. Converted to an airbase in the early 1900s and renamed in 1924, it is the headquarters of the Strategic Command, the successor to the Strategic Air Command. The Nebraska Museum of Aerospace History is on the base.

Malmstrom Air Force Base, U.S. military installation, 3,573 acres (1,446 hectares), W central Mont., E of Great Falls; est. 1942. During World War II, it was the takeoff point for Soviet-bound lend-lease materiel; after the war it was a training base for crews in the Berlin Airlift. The now-dissolved Strategic Air Command (SAC) assumed command in 1954; SAC's first Minuteman missile wing was established there in 1961. The missile complex adjoining the base is one of the largest in the world.

Langley Air Force Base, U.S. military installation, 3,195 acres (1,293 hectares), SE Va., N of Hampton; est. 1917 and named for aviation pioneer Samuel P. Langley. The facility, the oldest continuously active air force base in the United States, is the headquarters of the Air Combat Command and has air-defense missile units. NASA's Langley Research Center is adjacent.

Lackland Air Force Base, U.S. military installation, c.6,835 acres (2,766 hectares), S Tex., W of San Antonio; est. 1941. It is a major air force training center.

Force, Peter, 1790-1868, American journalist and historian, b. near Paterson, N.J. He served in the War of 1812 and afterward established himself in Washington, D.C., as a printer. Entering local politics, he was at different times president of both the city council and the board of aldermen and was mayor of Washington (1836-40). His National Journal, established in 1823 to support John Quincy Adams for the presidency, continued as a daily from 1824 to 1831. He issued for many years the National Calendar, a yearbook of historical and statistical information, and edited four volumes of rare documents, Tracts and Other Papers Relating Principally to the Origin, Settlement, and Progress of the Colonies in North America (1836-46). His project for publishing early American documents, national, state, and private, dealing with colonial and American history down to 1789, was authorized by Congress but was discontinued before completion. The resulting American Archives (9 vol., 1837-53), the work by which Force is chiefly known, covers only the years 1774-76 but has proved indispensable to students of the American Revolution. Force's large collections were purchased by the Library of Congress.

Edwards Air Force Base, U.S. military installation, 301,000 acres (121,805 hectares), S Calif., NE of Lancaster; est. 1933. It is one of the largest air force bases in the United States and has the world's longest runway. The base houses the Air Force Flight Test Center, which researches and develops aerospace weapons and rocket-propulsion systems, and the National Aeronautical and Space Administration's Flight Research Center. The base is also the proving ground for military aircraft. It has been the landing point for a number of space shuttle missions; on Sept. 5, 1983, the Challenger made the first nighttime landing of a spacecraft there.

Andrews Air Force Base, U.S. military installation, 4,279 acres (1,732 hectares), central Md., est. 1943. It is the chief military airport of Washington, D.C., as well as the headquarters for the air force's high-priority airlift command.

Air Force, United States Department of the, military department within the U.S. Dept. of Defense (see Defense, United States Department of). The Air Force traces its roots to the founding of the Aeronautical Division of the Army Signal Corps (1907), variously renamed before becoming a separate service under the National Security Act of 1947. In 1949 the National Security Act Amendments made the Air Force a military department within the newly organized Department of Defense. The chain of command goes directly from the President to the Secretary of Defense to the Secretary of the Air Force. The Air Force played an important role in World War I (see William Mitchell; Eddie Rickenbacker) and World War II (see H. H. Arnold; atomic bomb; James Harold Doolittle). After World War II, the Air Force quickly grew in importance, becoming the cornerstone of President Eisenhower's defense policy. The Air Force played a major part in the Korean War, the Vietnam War, and numerous cold war confrontations (see Berlin airlift, Cuban Missile Crisis). Its control of long-range, land-based guided missiles and the strategic bombers gives the Air Force monopolies on two major components of U.S. nuclear strategy. It has the leading role in the military exploration of space and uses aircraft and satellites to collect photo, video, and signal intelligence.

Semiskilled or unskilled workers who move from one region to another, offering their services on a temporary, usually seasonal, basis. In North America, migrant labour is generally employed in agriculture and moves seasonally from south to north following the harvest. In Europe and the Middle East, migrant labour usually involves urban rather than agricultural employment and calls for longer periods of residence. The migrant labour market is often disorganized and exploitative. Many workers are supervised by middlemen such as labour contractors and crew leaders, who recruit and transport them and dispense their pay. Labourers commonly endure long hours, low wages, poor working conditions, and substandard housing. In some countries, child labour is widespread among migrant labourers, and even in the U.S. those children who do not work often do not go to school, since schools are usually open only to local residents. Workers willing to accept employment on these terms are usually driven by even worse conditions in their home countries. Labour organizing is made difficult by mobility and by low rates of literacy and political participation, though some migrant labourers in the U.S. have been unionized. Seealso Cesar Chavez.

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Association of workers in a particular trade, industry, or plant, formed to obtain improvements in pay, benefits, and working conditions through collective action. The first fraternal and self-help associations of labourers appeared in Britain in the 18th century, and the era of modern labour unions began in Britain, Europe, and the U.S. in the 19th century. The movement met with hostility from employers and governments, and union organizers were regularly prosecuted. British unionism received its legal foundation in the Trade-Union Act of 1871. In the U.S. the same effect was achieved more slowly through a series of court decisions that whittled away at the use of injunctions and conspiracy laws against unions. The founding of the American Federation of Labor (AFL) in 1886 marked the beginning of a successful, large-scale labour movement in the U.S. The unions brought together in the AFL were craft unions, which represented workers skilled in a particular craft or trade. Only a few early labour organizers argued in favour of industrial unions, which would represent all workers, skilled or unskilled, in a single industry. The Congress of Industrial Organizations (CIO) was founded by unions expelled from the AFL for attempting to organize unskilled workers, and by 1941 it had assured the success of industrial unionism by organizing the steel and automotive industries (see AFL-CIO). The use of collective bargaining to settle wages, working conditions, and disputes is standard in all noncommunist industrial countries, though union organization varies from country to country. In Britain, labour unions displayed a strong inclination to political activity that culminated in the formation of the Labour Party in 1906. In France, too, the major unions became highly politicized; the Confédération Générale du Travail (formed in 1895) was allied with the Communist Party for many years, while the Confédération Française Démocratique du Travail is more moderate politically. Japan developed a form of union organization known as enterprise unionism, which represents workers in a single plant or multiplant enterprise rather than within a craft or industry.

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Body of law that applies to matters such as employment, wages, conditions of work, labour unions, and labour-management relations. Laws intended to protect workers, including children, from abusive employment practices were not enacted in significant numbers until the late 19th century in Europe and slightly later in the U.S. In Asia and Africa, labour legislation did not emerge until the 1940s and '50s. Employment laws cover matters such as hiring, training, advancement, and unemployment compensation. Wage laws cover the forms and methods of payment, pay rates, social security, pensions, and other matters. Legislation on working conditions regulates hours, rest periods, vacations, child labour, equality in the workplace, and health and safety. Laws on trade unions and labour-management relations address the status of unions, the rights and obligations of workers' and employers' organizations, collective bargaining agreements, and rules for settling strikes and other disputes. Seealso arbitration; mediation.

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Study of how workers are allocated among jobs, how their rates of pay are determined, and how their efficiency is affected by various factors. The labour force of a country includes all those who work for gain in any capacity as well as those who are unemployed but seeking work. Many factors influence how workers are utilized and how much they are paid, including qualities of the labour force itself (such as health, level of education, distribution of special training and skills, and degree of mobility), structural characteristics of the economy (e.g., proportions of heavy manufacturing, technology, and service industries), and institutional factors (including the extent and power of labour unions and employers' associations and the presence of minimum-wage laws). Miscellaneous factors such as custom and variations in the business cycle are also considered. Certain general trends are widely accepted by labour economists; for instance, wage levels tend to be higher in jobs that involve high risk, in industries that require higher levels of education or training, in economies that have high proportions of such industries, and in industries that are heavily unionized.

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In economics, the general body of wage earners. In classical economics, labour is one of the three factors of production, along with capital and land. Labour can also be used to describe work performed, including any valuable service rendered by a human agent in the production of wealth, other than accumulating and providing capital. Labour is performed for the sake of its product or, in modern economic life, for the sake of a share of the aggregate product of the community's industry. The price per unit of time, or wage rate, commanded by a particular kind of labour in the market depends on a number of variables, such as the technical efficiency of the worker, the demand for that person's particular skills, and the supply of similarly skilled workers. Other variables include training, experience, intelligence, social status, prospects for advancement, and relative difficulty of the work. All these factors make it impossible for economists to assign a standard value to labour. Instead, economists often quantify labour hours according to the quantity and value of the goods or services produced.

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Specialization in the production process. Complex jobs can usually be less expensively completed by a large number of people each performing a small number of specialized tasks than by one person attempting to complete the entire job. The idea that specialization reduces costs, and thereby the price the consumer pays, is embedded in the principle of comparative advantage. Division of labour is the basic principle underlying the assembly line in mass production systems. See Émile Durkheim.

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Employment of boys and girls in occupations deemed unfit for children. Such labour is strictly controlled in many countries as a result of the effective enforcement of laws passed in the 20th century (e.g., the United Nations Declaration of the Rights of the Child in 1959). In developing nations the use of child labour is still common. Restrictive legislation has proved ineffective in impoverished societies with few schools, although some improvements have resulted from global activism, such as boycotts of multinational firms alleged to be exploiting child labour abroad.

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or weak nuclear force

Fundamental interaction that underlies some forms of radioactivity and certain interactions between subatomic particles. It acts on all elementary particles that have a spin of 12. The particles interact weakly by exchanging particles that have integer spins. These particles have masses about 100 times that of a proton, and it is this relative massiveness that makes the weak force appear weak at low energies. For example, in radioactive decay, the weak force has a strength about 1/100,000 that of the electromagnetic force. However, it is now known that the weak force has intrinsically the same strength as the electromagnetic force, and the two are believed to be only different manifestations of a single electroweak force (see electroweak theory).

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or strong nuclear force

Fundamental force acting between elementary particles of matter, mainly quarks. The strong force binds quarks together in clusters to form protons and neutrons and heavier short-lived particles. It holds together the atomic nucleus and underlies interactions among all particles containing quarks. In strong interactions, quarks exchange gluons, carriers of the strong force, which are massless particles with one unit of intrinsic spin. Within its short range (about 10⁻¹⁵ m), the strong force appears to become stronger with distance. At such distances, the strong interaction between quarks is about 100 times greater than the electromagnetic force.

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Attraction or repulsion that arises between electrically charged particles that are in motion. While only electric forces exist among stationary electric charges, both electric and magnetic forces exist among moving electric charges. The magnetic force between two moving charges is the force exerted on one charge by a magnetic field created by the other. This force is zero if the second charge is traveling in the direction of the magnetic field due to the first and is greatest if it travels at right angles to the magnetic field. Magnetic force is responsible for the action of electric motors and the attraction between magnets and iron.

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Universal force of attraction that acts between all bodies that have mass. Though it is the weakest of the four known forces, it shapes the structure and evolution of stars, galaxies, and the entire universe. The laws of gravity describe the trajectories of bodies in the solar system and the motion of objects on Earth, where all bodies experience a downward gravitational force exerted by Earth's mass, the force experienced as weight. Isaac Newton was the first to develop a quantitative theory of gravitation, holding that the force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. Albert Einstein proposed a whole new concept of gravitation, involving the four-dimensional continuum of space-time which is curved by the presence of matter. In his general theory of relativity, he showed that a body undergoing uniform acceleration is indistinguishable from one that is stationary in a gravitational field.

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Statement that any particle of matter in the universe attracts any other with a force (math.F) that is proportional to the product of their masses (math.m₁ and math.m₂) and inversely proportional to the square of the distance (math.R) between them. In symbols: math.F = math.G(math.m₁math.m₂)/math.R², where math.G is the gravitational constant. Isaac Newton put forth the law in 1687 and used it to explain the observed motions of the planets and their moons, which had been reduced to mathematical form by Johannes Kepler early in the 17th century.

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In physics, the effect of any of the four fundamental forces—gravitational, electromagnetic, strong, and weak. All known natural forces can be traced to these fundamental interactions. Gravitation is the attractive force between any two objects that have mass; it causes objects to fall to the ground and maintains the orbits of planets around the Sun. Electromagnetic force is responsible for the attraction and repulsion between electric charges and explains the chemical behaviour of atoms and the properties of light. The strong force binds quarks together in protons, neutrons, and other hadrons and also holds the protons and neutrons of an atomic nucleus together, overcoming the repulsion of the positively charged protons for each other. The weak force is observed in certain forms of radioactive decay (see radioactivity) and in reactions that fuel the Sun and other stars.

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Agency that alters the direction, speed, or shape that a body would exhibit in the absence of any external influence. It is a vector quantity, having both magnitude and direction. Force is commonly explained in terms of Newton's laws of motion. All known natural forces can be traced to the fundamental interactions. Force is measured in newtons (N); a force of 1 N will accelerate a mass of 1 kg at a rate of 1 m/sec/sec. Seealso centrifugal force; Coriolis force; electromagnetic force; Coulomb force; magnetic force; strong force; weak force.

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or electric force

Force between two electric charges. The magnitude of the force math.F is proportional to the product of the two charges, math.q₁ and math.q₂, divided by the square of the distance math.r between them, or math.F = math.kmath.q₁math.q₂/math.r², where math.k is a constant that depends on the measurement system being used. The Coulomb force can be one of repulsion, such as the force between two objects having like charges, or it can be attractive, such as the force between two objects having opposite charges.

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Energy per unit electric charge that is imparted by an energy source, such as an electric generator or a battery. As the device does work on the electric charge being transferred within itself, energy is converted from one form to another. The work done on a unit of electric charge or the energy gained by the unit charge is the electromotive force emf (or math.E) and is characteristic of any energy source capable of driving electric charge around a circuit. A common unit of electromotive force is the volt math.V, a unit equal to the difference in electric potential between two points in a conductor carrying a current of one ampere and dissipating one watt of power between the two points.

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One of the four known basic forces in the universe. Electromagnetism is responsible for interactions between charged particles that occur because of their charge, and for the emission and absorption of photons (electromagnetic radiation). The phenomena of electricity and magnetism are consequences of this force, and the relationships between them were first described by James Clerk Maxwell in the 1860s. The physical description of electromagnetism has since been combined with quantum mechanics into the theory of quantum electrodynamics. The electromagnetic force is about 10³⁶ times as strong as the gravitational force (see gravitation), but significantly weaker than both the weak force and the strong force.

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Fictitious force, peculiar to circular motion, that is equal but opposite to the centripetal force that keeps a particle on a circular path (see centripetal acceleration). For example, a stone attached to a string and whirling in a horizontal circular path is accelerated toward the centre of its path by the tension in the string, the only force acting on the string. However, in a reference frame at rest with the stone, another force—the centrifugal force—must be introduced for Newton's laws of motion to apply. Centrifugal force is a useful concept in analyzing behaviour in rotating systems.

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Military organization that has the primary responsibility for conducting air warfare. The air force must gain control of the air, support ground forces (e.g., by attacking enemy ground forces), and accomplish strategic-bombing objectives. Its basic weapons platforms are fighters, bombers, attack aircraft, and early warning and control aircraft. Since the mid-20th century, some air forces have also been responsible for land-based nuclear missiles as well as nuclear-armed bombers. The army and naval branches of a state's armed forces may also operate aircraft.

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Institution for the training of commissioned officers for the U.S. Air Force, located in Colorado Springs, Colorado. Created by an act of Congress in 1954, it opened in 1955. Graduates receive a bachelor's degree and a second lieutenant's commission. Most physically qualified graduates go on to Air Force pilot-training schools. Candidates may come from the ranks of the U.S. Army or Air Force, may be children of deceased veterans of the armed forces, or may be nominated by U.S. senators or representatives or by the president or vice president. All applicants must take a competitive entrance examination.

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