FORCE, the impulse which tends to impart motion to a body at rest, or to increase or di minish the velocity or change the direction of a body already in motion. This impulse is attribut able to the action of some other body. It is to be noted that while force tends to produce motion, that may not be the result. The force acting may be inadequate to overcome existing forces, and it becomes simply pressure ;— as, for exam ple, a force of 10 pounds acting against a mass of stone weighing 10 tons. In this case the attraction of the earth is the greater force which the impulse of 10 pounds is insufficient to overcome. But, in general, force is con sidered in its visible effect motion. In this connection Newton's Laws of Motion become applicable. They are: (1) If a body be at rest, it will remain at rest; or if it is in motion, it will move uniformly in a straight line until it is acted upon by some force. (2) If a body be acted on by several forces at once, it will be influenced by each as though the others did not exist, whether the body be at rest or in motion. (3) If a force act to change the state of a body with respect to rest or motion, the body will offer a resistance equal to and directly op posed to the force: in other words, to every action of a force upon a body is opposed an equal and opposite reaction.
The quantity of motion possessed by a body is called its momentum. The value of the mo mentum of a body is found by multiplying its mass by its velocity. The change in quantity of motion possessed by a body under the action of a force is proportional to the magnitude of the force and to the time during which the force acts.
Forces are classified as (1) Momentary, or impulsive forces; and (2) Continuous, or per manent forces. If a continuous force is uni form in its action it is called a constant force, and is measured by the momentum it imparts to the body upon which it acts.
A force is said to be "central,* when it acts always toward a definite centre, which may be either fixed or in motion. The gravitative forces with which the heavenly bodies act upon one another are of this character, and are often popularly called ncentripetaP (that is, "centre seeking)) forces for this reason. When a body is caused to move in a curved path, it exerts a force which acts along the radius of curvature of the path, and in a direction away from the centre of curvature. Forces of this nature are called ((centrifugal) (or “centre-fleeing)), and familiar examples are afforded by the pressure of swiftly moving water against the curved vanes of a turbine water-wheel, and by the ten sion produced in a string when a stone that is attached to the string is whirled rapidly about in a circle. The nature of centrifugal force has been the subject of more or less controversy, some authorities maintaining that it should not be classed as a true force, since it does not produce any acceleration in the direction in which it acts; — that is, a particle on the rim of a swiftly revolving wheel does not fly off radially when it is liberated, but merely con tinues its motion with unaltered speed, in the direction in which it was moving at the instant of its liberation; — or tangentially to the wheel.
The subject is too technical for extended dis cussion in this place, but it may be pointed out that such a particle is actually subject to a radial acceleration, if its motion is considered relatively to the wheel.
Forces are said to be when the principle of the conservation of energy holds true for the system in which they occur. (See ENERGETICS). All forces of nature are believed to be fundamentally conservative, al though this has not yet been rigorously proved for the forces that prevail within animals and plants. • A "field of force) is any region in which a given ;orce has a sensible magnitude. A. con ductor charged with electricity, for example, exerts an attractive (or repulsive) force upon all bodies that are exterior to it, and, from the point of view of theoretical physics, this force still exists at an infinite distance from the charged body, though at such a distance it becomes infinitesimal in intensity. A familiar illustration is the operation of the earth's at traction upon the moon. From apractical standpoint, however, the afield of due to the charged body can be considered to be limited by an indefinite but finite boundary, whose distance from the body depends upon the intensity of the charge, and also upon the order of minuteness of the forces that can be re garded as negligible, so far as any effect upon the problem that happens to be under considera tion is concerned. Within a closed conductor there is no field of electric force, so long as the charges upon the conductor itself, and in the region external to it, are in equilibrium. This fact may be demonstrated mathematically, and it was abundantly proved experimentally by Faraday.
The unit of force among English-speaking people is the "pound.) This term is used to designate that impulse which would give to a body weighing one pound a motion of 32.1740 feet per second, or, if the body were already in motion, would accelerate its motion by so much, for each second of time during which it acts. Suppose, for example, that a uniform force of magnitude represented by F acts for T seconds upon a body having the mass M, thereby in creasing (or diminishing) its velocity by V units. The magnitude of the force F will be equal to MV divided by T, and by substituting numerical values for the symbolic letters a con crete result may be obtained. If M is ex pressed in grammes and V in centimeters per second, the resulting value for F will be in dynes. Consult Poorman. A. P., Mechanics' (New York 1917). See ELAsncrry;