# force

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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**vector,**

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.

**.....**Click the link for more information. quantity, having both magnitude and direction. The magnitude of a force is measured in units such as the pound, dyne

**dyne**

, unit of force in the cgs system of units, which is based on the metric system; an acceleration of 1 centimeter per second per second is produced when a force of 1 dyne is exerted on a mass of 1 gram.

**.....**Click the link for more information. , and newton

**newton,**

abbr. N, unit of force in the mks system of units, which is based on the metric system; it is the force that produces an acceleration of 1 meter per second per second when exerted on a mass of 1 kilogram. The newton is named for Sir Isaac Newton.

**.....**Click the link for more information. , depending upon the system of measurement being used. An unbalanced force acting on a body free to move will change the motion

**motion,**

the change of position of one body with respect to another. The rate of change is the speed of the body. If the direction of motion is also given, then the velocity of the body is determined; velocity is a vector quantity, having both magnitude and direction, while speed

**.....**Click the link for more information. of the body. The quantity of motion of a body is measured by its momentum

**momentum**

, in mechanics, the quantity of motion of a body, specifically the product of the mass of the body and its velocity. Momentum is a vector quantity; i.e., it has both a magnitude and a direction, the direction being the same as that of the velocity vector.

**.....**Click the link for more information. , the product of its mass

**mass,**

in physics, the quantity of matter in a body regardless of its volume or of any forces acting on it. The term should not be confused with weight, which is the measure of the force of gravity (see gravitation) acting on a body.

**.....**Click the link for more information. and its 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. . According to Newton's second law of motion (see motion

**motion,**

the change of position of one body with respect to another. The rate of change is the speed of the body. If the direction of motion is also given, then the velocity of the body is determined; velocity is a vector quantity, having both magnitude and direction, while speed

**.....**Click the link for more information. ), 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

**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. , which may be a change either in 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. 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**equilibrium,**

state of balance. When a body or a system is in equilibrium, there is no net tendency to change. In mechanics, equilibrium has to do with the forces acting on a body.**.....** Click the link for more information. . For example, the downward force of gravity (see 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 is often called**.....** Click the link for more information. ) 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**strength of materials,**

measurement in engineering of the capacity of metal, wood, concrete, and other materials to withstand stress and strain. Stress is the internal force exerted by one part of an elastic body upon the adjoining part, and strain is the deformation or change in**.....** Click the link for more information. 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**field,**

in physics, region throughout which a force may be exerted; examples are the gravitational, electric, and magnetic fields that surround, respectively, masses, electric charges, and magnets. The field concept was developed by M.**.....** Click the link for more information. of force (see electromagnetic radiation**electromagnetic radiation,**

energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.**.....** Click the link for more information. ). 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**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. The atom in turn is made from the proton, neutron, and electron.**.....** Click the link for more information. and is responsible for holding the atomic nucleus**nucleus,**

in physics, the extremely dense central core of an atom. **The Nature of the Nucleus****Composition**

Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons.**.....** Click the link for more information. together. The weak nuclear force, or weak interaction, is associated with beta particle**beta particle,**

one of the three types of radiation resulting from natural radioactivity. Beta radiation (or beta rays) was identified and named by E. Rutherford, who found that it consists of high-speed electrons.**.....** Click the link for more information. emission and particle decay; it is weaker than the electromagnetic force but stronger than the gravitational force.

## 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 *FT*^{2} *L*^{-1}. *See* Free fall

## force

Symbol:*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.

## Force

## Force

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 (*see*DYNAMOMETER). 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

[fȯrs]## force

**1.**

*Physics*

**a.**a dynamic influence that changes a body from a state of rest to one of motion or changes its rate of motion. The magnitude of the force is equal to the product of the mass of the body and its acceleration

**b.**a static influence that produces an elastic strain in a body or system or bears weight.

**2.**

*Physics*any operating influence that produces or tends to produce a change in a physical quantity

**3.**

*Criminal law*violence unlawfully committed or threatened

**4.**

*Philosophy*

*Logic*that which an expression is normally used to achieve

**5.**

**in force**(of a law) having legal validity or binding effect