94-97] PROPERTY OF MASS. 99 



It is inconsistent with the idea of " the quantity of matter in 

 a body " that this quantity should be liable to increase or diminu- 

 tion when the body is transported from place to place. Rather 

 the idea of the quantity of matter in a body is the idea of some- 

 thing which cannot be increased or diminished so long as the 

 body remains the same. This idea with the rule that the quantity 

 of matter in a body is the sum of the quantities of matter in its 

 parts combine to point to mass as the measure of quantity of 

 matter. 



The common use of the word " weight " covers two notions 

 which are essentially distinct, the notion of pressure which a 

 heavy body exerts on a support, and the notion of quantity of 

 matter. In scientific speaking or writing, different words must be 

 used to express distinct notions. 



97. Inertia. Besides serving as a measure of the quantity of 

 matter in a body, the mass of a body is connected with a property 

 of bodies which is a subject of common observation. The postulate 

 that all changes of motion arise from the mutual actions of particles 

 includes the statement that if any body could be freed entirely 

 from the action of all other bodies it would have no acceleration. 

 To produce any particular change of motion in a body, a particular 

 set of forces must be applied to it, and these must be exerted on 

 the particles of the body by the particles of some other body. 

 The tendency of the body, apart from the action of other bodies, 

 to persist in its state of motion at any instant is known as its 

 inertia. It is evident that, the greater the mass of the body, the 

 greater must be the force applied to it to produce in a given 

 interval of time any particular change of velocity. The mass of 

 the body can thus be taken to measure the tendency in question, 

 and for this reason the mass of a body is sometimes called its 

 inertia. 



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