VIS

, a Latin word, signifying force or power; adopted by writers on physics, to express divers kinds of natural powers or faculties.

The term Vis is either active or passive: the Vis activa is the power of producing motion; the Vis passiva is that of receiving or losing it. The Vis activa is again subdivided into Vis viva and Vis mortua.

Vis Absoluta, or absolute force, is that kind of centripetal force which is measured by the motion that would be generated by it in a given body, at a given distance, and depends on the efficacy of the cause producing it.

Vis Acceleratrix, or accelerating force, is that centripetal force which produces an accelerated motion, and is proportional to the velocity which it generates in a given time; or it is as the motive or absolute force directly, and as the quantity of matter moved inversely.

Vis Impressa is defined by Newton to be the action exercised on any body to change its state, either of rest or moving uniformly in a right line.

This force consists altogether in the action; and has no place in the body after the action is ceased: for the body perseveres in every new state by the Vis inertiæ alone.

This Vis impressa may arise from various causes; as from percussion; pression, and centripetal force.

Vis Inertiæ, or power of inactivity, is defined by Newton to be a power implanted in all matter, by which it resists any change endeavoured to be made in its state, that is, by which it becomes difficult to alter its state, either of rest or motion.

This power then agrees with the Vis resistendi, or power of resisting, by which every body endeavours, as much as it can, to persevere in its own state, whether of rest or uniform rectilinear motion; which power is still proportional to the body, or to the quantity of matter in it, the same as the weight or gravity of the body; and yet it is quite different from, and even independent of the force of gravity, and would be and act just the same if the body were devoid of gravity. Thus, a body by this force resists the same in all directions, upwards or downwards or obliquely; whereas gravity acts only downwards.

Bodies only exert this power in changes brought on their state by some Vis impressa, force impressed on them. And the exercise of this power is, in different respects, both resistance and impetus; resistance, as the body opposes a force impressed on it to change its state; and impetus, as the same body endeavours to change the state of the resisting obstacle. Phil. Nat. Princ. Math. lib. 1.

The Vis inertiæ, the same great author elsewhere observes, is a passive principle, by which bodies persist in their motion or rest, and receive motion, in proportion to the force impressing it, and resist as much as they are resisted. See Resistance.

Vis Insita, or innate force of matter, is a power of resisting, by which every body, as much as in it lies, endeavours to persevere in its present state, whether of rest or of moving uniformly forward in a right line. This force is always proportional to the quantity of matter in the body, and differs in nothing from the Vis inertiæ, but in our manner of conceiving it.

Vis Centripeta. See Centripetal Force.

Vis Motrix, or moving force of a centripetal body, is the tendency of the whole body towards the centre, resulting from the tendency of all the parts, and is proportional to the motion which it generates in a given time; so that the Vis motrix is to the Vis acceleratrix, as the motion is to the celerity: and as the quantity of motion in a body is estimated by the product of the velocity into the quantity of matter, so the Vis motrix | arises from the Vis acceleratrix multiplied by the quantity of matter.

The followers of Leibnitz use the term Vis motrix for the force of a body in motion, in the same sense as the Newtonians use the term Vis inertiæ; this latter they allow to be inherent in a body at rest; but the former, or Vis motrix, a force inherent in the same body only whilst in motion, which actually carries it from place to place, by acting upon it always with the same intensity in every physical part of the line which it describes.

Vis Mortua, and Vis Viva, in Mechanics, are terms used by Leibnitz and his followers for force, which they distinguish into two kinds, Vis mortua, and Vis viva; understanding by the former any kind of pressure, or an endeavour to move, not sufficient to produce actual motion, unless its action on a body be continued for some time; and by the latter, that force or power of acting which resides in a body in motion.