MASS, WEIGHT, FORCE, WORK, AND POWER 535 



III. MASS, WEIGHT, FORCE, WORK, AND POWER 



626. Meaning of Terms. In all study and use of machines 

 we need to understand exactly the meaning of certain terms. 

 No two people can talk intelligently about machinery and its 

 operation unless both use the same terms to express exactly 

 the same thought; moreover, we must gain much of our knowl- 

 edge concerning machinery from reading and it is impossible 

 for us to understand what we read unless we know the exact 

 meaning of the terms used. The terms used in all text-books, 

 in reliable magazines, in all government reports, and in most 

 advertising circulars, are carefully chosen and have certain 

 definite meanings. If we are to be intelligent people and are to 

 speak accurately when referring to mechanical matters, we 

 must know the exact meaning of the terms we use. 



627. Mass. By MASS we mean the quantity of matter in an 

 object. We never mean its weight, its size, or its density. 

 When we buy a certain quantity of flour, sugar, eggs, bananas, 

 potatoes, or coal, we are paying for a certain mass of the article 

 purchased. We may determine and speak of the mass pur- 

 chased in several ways : We generally speak of buying a certain 

 number of pounds of sugar, or of a certain number of eggs or 

 bananas, or of a certain number of pecks or bushels of apples. 

 In every case, however, what we endeavor to do is to determine 

 the mass of the article purchased. It is becoming more and 

 more common for all such commodities to be bought and sold 

 by the POUND-MASS. A dozen bananas is an indefinite quan- 

 tity; likewise, a dozen eggs does not indicate clearly the 

 amount of mass because they vary so greatly in mass. Some 

 cities, states, and national governments have passed laws 

 obliging all dealers to buy and sell by the mass instead of the 

 dozen, or the peck, or the bushel. Nearly all commodities are 

 thus bought and sold when handled in large quantities. For 

 example, while we speak of buying oats or corn at a certain 

 price per bushel, we actually pay the price for 32 Ib.-mass of 

 oats or for 56 Ib.-mass of corn. We shall see soon that we 



