in philosophy, denotes the cause of the change in the state of a body, when, being at rest, it begins to move, or has a motion which is either not uniform or not direct. While a body remains in the same state, either of rest or of uniform and rectilinear motion, the cause of its remaining in such a state is in the nature of the body, and it cannot be said that any extrinsic force has acted on it. This internal cause or principle is called inertia.
Mechanical forces may be reduced to two sorts; one of a body at rest, the other of a body in motion. The force of a body at rest, is that which we conceive to be in a body lying still on a table, or hanging by a rope, or supported by a spring, &c; and this is called by the names of pressure, tension, force, or vis mortua, solicitation, conatus movendi, obnemum, &c. To this class also of forces we must refer centripetal and centrifugal forces, though they reside in a body in motion; because these forces are homogeneous to weights, pressures, or tensions of any kind.
The force of a body in motion is a power residing in that body so long as it continues its motion; by means of which it is able to remove obstacles lying in its way; to lessen, destroy, or overcome the force of any other moving body, which meets it in an opposite direction; or to surmount any dead pressure or resistance, as tension, gravity, friction, &c., for some time; but which will be lessened or destroyed by such resistance as lessens or destroys the motion of the body. This is called moving force, vis motrix, and by some late writers vis viva, to distinguish it from the vis mortua spoken of before; and by these appellations, however different, the same thing is understood by all mathematicians; namely, that power of displacing, of withstanding opposite moving forces, or of overcoming any dead resistance, which resides in a moving body, and which, in whole or in part, continues to accompany it, so long as the body moves. See Mechanics.
We have several curious as well as useful observations in Defagulier's Experimental Philosophy, concerning the comparative forces of men and horses, and the best way of applying them. A horse draws with the greatest advantage when the line of direction is level with his breast; in such a situation, he is able to draw 200 lb. eight hours a day, walking about two miles and an half an hour. And if the same horse is made to draw 240 lb. he can work but six hours a day, and cannot go quite so fast. On a carriage, indeed, where friction alone is to be overcome, a middling horse will draw 1000 lb. But the best way to try a horse's force, is by making him draw up out of a well, over a single pulley or roller; and in such a case, one horse with another will draw 200 lb. as already observed.
Five men are found to be equal in strength to one horse, and can, with as much ease, push round the horizontal beam of a mill, in a walk 40 feet wide; whereas three men will do it in a walk only 19 feet wide.
The worst way of applying the force of a horse, is to make him carry or draw up hill: for if the hill be steep, three men will do more than a horse, each man climbing up faster with a burden of 100 lb. weight, than a horse that is loaded with 300 lb. a difference which is owing to the position of the parts of the human body being better adapted to climb than those of a horse.
On the other hand, the best way of applying the force of a horse, is an horizontal direction, wherein a man can exert least force; thus a man, weighing 140 lb. and drawing a boat along by means of a rope coming over his shoulders, cannot draw above 27 lb. or exert above one-seventh part of the force of a horse employed to the same purpose.
The very best and most effectual posture in a man, is that of rowing; wherein he not only acts with more muscles at once for overcoming the resistance, than in any other position; but as he pulls backwards, the weight of his body assists by way of lever. See Defagulier, Exp. Phil. vol. i. p. 241, where we have several other observations relative to force acquired by certain positions of the body, from which that author accounts for most feats of strength and activity. See also Memoire on this subject by M. de la Hire, in Mem. Roy. Acad. Sc. 1629; or in Defagulier, Exp. &c. p. 267, &c. who has published a translation of part of it with remarks.
law, signifies any unlawful violence offered to things or persons, and is divided into simple and compound. Simple force is what is so committed, that it has no other crime attending it; as where a person by force enters on another's possession, without committing any other unlawful act. Compound force, is where some other violence is committed with such an act which of itself alone is criminal; as if one enters by force into another's house, and there kills a person, or ravishes a woman. There is likewise a force implied in law, as in every trespass, felony, or distress, and an actual force with weapons, number of persons, &c.—Any person may lawfully enter a tavern, inn, or victualling-house; so may a landlord, his tenant's house to view repairs, &c. But if, in these cases, the person that enters commits any violence or force, the law will intend that he entered for that purpose.