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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.
Force may be briefly described as that influence on a body which causes it to accelerate. In this way, force is defined through Newton's second law of motion.
This law states in part that the acceleration of a body is proportional to the resultant force exerted on the body and is inversely proportional to the mass of the body. An alternative procedure is to try to formulate a definition in terms of a standard force, for example, that necessary to stretch a particular spring a certain amount, or the gravitational attraction which the Earth exerts on a standard object. Even so, Newton's second law inextricably links mass and force. See Acceleration, Mass
One may choose either the absolute or the gravitational approach in selecting a standard particle or object. In the so-called absolute systems of units, it is said that the standard object has a mass of one unit. Then the second law of Newton defines unit force as that force which gives unit acceleration to the unit mass. Any other mass may in principle be compared with the standard mass (m) by subjecting it to unit force and measuring the acceleration ( a ), with which it varies inversely. By suitable appeal to experiment, it is possible to conclude that masses are scalar quantities and that forces are vector quantities which may be superimposed or resolved by the rules of vector addition and resolution.
In the absolute scheme, then, the equation F = m a is written for nonrelativistic mechanics; boldface type denotes vector quantities. This statement of the second law of Newton is in fact the definition of force. In the absolute system, mass is taken as a fundamental quantity and force is a derived unit of dimensions MLT-2 (M = mass, L = length, T = time).
The gravitational system of units uses the attraction of the Earth for the standard object as the standard force. Newton's second law still couples force and mass, but since force is here taken as the fundamental quantity, mass becomes the derived factor of proportionality between force and the acceleration it produces. In particular, the standard force (the Earth's attraction for the standard object) produces in free fall what one measures as the gravitational acceleration, a vector quantity proportional to the standard force (weight) for any object. It follows from the use of Newton's second law as a defining relation that the mass of that object is m = w/g, with g the magnitude of the gravitational acceleration and w the magnitude of the weight. The derived quantity mass has dimensions FT2 L-1. See Free fall
forceSymbol: F . According to Newton's laws of motion, any physical agency that alters or attempts to alter a body's state of rest or of uniform motion. The force required to accelerate a body of mass m is given by ma , where a is the acceleration imparted. There are many kinds of forces, including the gravitational force. The SI unit of force is the newton. See also fundamental forces; field.
in mechanics, a quantity that is a measure of the mechanical action on a given physical body by other bodies. This action causes a change in the velocities of points of the body or produces deformation of the body. The action can occur through direct contact—as in friction or the pressures exerted by bodies that are pressed against each other—or through fields generated by the bodies—such as a gravitational field or an electromagnetic field.
A force is a vector quantity and at every moment of time is characterized by a magnitude, direction in space, and point of application. Forces are added according to the parallelogram law. The straight line along which a force is directed is called the line of action of the force. In the case of a nondeformable rigid body, the force can be considered to be applied at any point on its line of action. A force acting on a particle can be constant or variable. The force of gravity is an example of a constant force. A variable force can be dependent on time (for example, an alternating electromagnetic field), the position of the particle in space (a gravitational force), or on the particle’s velocity (the resisting force of the medium).
Forces are measured by static or dynamic methods. The static method is based on the balancing of the force being measured by another, known force (seeDYNAMOMETER). The dynamic method is based on the law of dynamics mw = F. If the mass m of the body is known and the acceleration w of the body’s free translational motion with respect to the inertial frame of reference is measured, the force F can be found from this law. Frequently used units of force are the newton (N) and dyne(dyn): 1 dyn = 10–5 N, and 1 kilogram-force ≈ 9.81 N.
S. M. TARG
Force(1) See force quit and force touching.
(2) An earlier dBASE compiler developed by Sophco, Inc. that combined C and dBASE structures. Force was noted for generating very small executable programs.