INERTIA, (Lat.. sluggishness, from biers, idle, from in-, not ars, art). A general property of matter of •hieh we become conscious through our niusele-senses whenever by our ninselns we •hange the motion of matter. e.g. throw or slim a ball, apt n a door, stop a revolv ing wheel, etc. lt is noticed that the mtensity of the sensation depends both on the material body itself and on the rate at which its motion is changed. Similarly, if the motion of the body is stopped by sonic inanimate object, the effect on the latter is proportional to the rate of de crease and varies with the body stopped; e.g. if a moving body meets a spiral spring and com presses it until it is brought to rest. If two mov ing bodies are brought to rest by a given spring in the same time and produce the same amount of compression, they may be said to have the same inertia. (On this definition is based the system for measuring quantities of matter, q.v.).
if it takes longer to bring the first body to rest, it is said to have the greater inertia. It is be lieved that, if there were nothing external to the moving body affecting it, it would continue to maintain its motion unaltered forever. Galileo was the first to advance this idea ; and it is now assumed as one of the principles of mechanics. On the other hand, if there is sonic external ac tion on the body it takes time to produce a change in its motion, and the change produced in a given time will vary inversely as the inertia ; or, to produce a given change in motion, the ex ternal action required varies directly as the in elfin. It is to Newton that We owe our ideas of inertia as a measurable fundamental property of matter. Up to his time weight was the only property of matter used as a measure of the 'Twin ity of matter.'