INERTIA of matter, in philosophy, is defined by Sir Isaac Newton to be a pas sive principle, by which bodies persist in a state of motion or rest, receive motion in proportion to the force impressing it, and resist as much as they are resisted. It is also defined by the same author to be a power implanted in all matter, whereby it resists any change endeavoured to be made in its state. See 'MECHANICS.
All bodies preserve or continue, as of themselves, in their state of rest, or of uniform motion in a right line, in such manner, that a body at rest cannot move without being solicited or urged by some force ; neither can the rectilinear and uniform motion of a body be changed without the action of a foreign cause.— That want of aptitude which bodies have, of producing in themsleves a change in their actual state, is called inertia. Now it is known that a body, whose state may be changed by the action of a foreign force, cannot give way to that effect, otherwise than by itself altering the state of that force ; that is to say, by itself tak ingaway apart of its motion. It has hence been concluded, that the continuance of a body in its state of repose, or of uniform motion, was itself the effect of a real force which resided in that body ; and this force has been viewed, sometimes, as a resistance, in so far as it opposed itself to the action of the other force, which changed the- state of that body, and some times as an effort, in so far as it tended to carry with it the change in the state of the other force.
The celebrated Laplace has proposed a more precise and natural manner of con templating inertia. To conceive in what it consists, suppose a body in motion to meet with a body at rest: it will commu nicate to it a part of its motion; in such manner, that if the first have, for example, a mass double to that of the second, in which case its mass will be two-thirds of the sum of the masses, the velocity which it will retain will be also two-thirds of that which it hadat first; and as the other third, which it has yielded to the second body, employs itself upon a mass of only half the magnitude of the former, the two bodies will both have the same velocity after the shock.
The effect of inertia is reduced, there fore, to the communication made by one of these bodies to the other, of a part of its motion; and since this latter cannot receive, but in consequence of the other'a losing, this loss has been attributed to a resistance exercised by the body receiv ing the motion. B ut in the instance before us, it is very nearly as in the motion of an elastic fluid, contained in a vessel from which we would open a communication to another vessel which should be empty; this fluid would introduce itself by its ex pansive force into the second vessel, un til it became uniformly distributed in the capacities of the two vessels : in like man ner, a body when it strikes another does nothing else, if we may so express our selves, than pour into this latter a part of its motion ; and there is no more reason to suppose a resistance in this case than in the examples we have just cited. It is true, that, when we strike with the hand a body at rest, or whose motion is less ra pid than that of the hand, we imaginethat we experience a resistance ; but the illu sion proceeds from this, that the .,ffect is the same with regard to the hand, as though it were at rest, and was struck by the body with a motion in a contrary di rection.