536 MACHINES, WORK, AND ENERGY 



generally determine the mass by first-determining the weight, 

 but we must never confuse the weight of an object with its 

 mass. 



628. The Units of Mass. The common UNIT OF MASS used 

 in the United States is the POUND. Originally the old English 

 pound-mass was the mass of 7680 grains of wheat. During 

 the reign of Henry VIII (1509 to 1547) the standard pound was 

 reduced somewhat till it represented the mass of 7000 grains 

 of wheat, hence we say there are 7000 grains in 1 Ib. The 

 English government, many years ago, prepared a piece of 

 platinum equaling this mass and declared that to be the 

 STANDARD POUND. Since colonial days we have always 

 accepted this mass as the mass of a STANDARD POUND. 



Two other standard UNITS OF MASS are the GRAM and the KILO- 

 GRAM of the metric system. The metric system of weights and 

 measures is now used by all civilized nations except Great 

 Britain and the United States. The gram-mass is the mass in 

 1 c.c. of water at 4C. The kilogram equals 1000 grams. In 

 1893 the United States government defined the avoirdupois 

 pound as equal to 453.6 grams. The kilogram, then, is ap- 

 proximately equal to 2.2 Ib. Calculate this. 



629. Weight, Gravitation and Gravity. By the WEIGHT 

 of an object we mean the pull of the earth upon that object. 

 We all know that any object which is free to fall does fall 

 toward the earth. This is because both earth and object 

 attract each other. Physicists and astronomers have proved 

 that every body in the universe attracts every other body. 

 In general, this attraction of bodies for other bodies is called 

 the force of GRAVITATION. When, however, we are speaking 

 of the attraction between the earth and any object near its 

 surface we speak of the force as the PULL OF GRAVITY. 



Now, if we support a body so that it is not free to fall toward 

 the earth, it then exerts a push or a pull upon the support. It 

 is this push or pull which we call the WEIGHT of the object. 

 The weight of an object, then, is merely the measure of the 

 force of gravity upon it. We shall see later that weight is 



