32 



FORCE AND ENERGY 



wood is lighter than a small lump of lead, but you would 

 mean that, taken volume for volume, wood is lighter than 

 lead. This is the same as saying that the density of wood 

 is less than the density of lead (Fig. 27). 



We have already learned that the mass of a body is 

 the quantity of matter in it (c/. 20). We may now de- 



n 



Gold. 



Lead. Copper. 



Coal. 



Aluminum. 



Fia. 27. 

 Cubes of Different Volumes, but of Equal Weight 



Wood. 



fine the density of a body as the quantity of matter 

 packed in a given volume of the body. To get the mass of 

 a body we weigh it. To get the density of a body we 

 divide its weight, in grams, by its volume, in cubic 

 centimeters. 



mass (in g.) 



Density = 



volume (in c.c.) 



Suppose that a piece of marble weighs 5 g. and has a volume of 2 

 c.c. ; it is plain that 1 c.c. of the marble would weigh 2.5 g. We say that 

 the marble has a density of 2.5 g. for each cubic centimeter. The den- 

 sity of water is 1 g. for each cubic centimeter (c/. 14). In the Eng- 

 lish system the density of water is about 62.5 Ibs. for each cubic foot. 



We often find the expression specific gravity used in place of density. 

 We get the specific gravity of a body by dividing the weight of the 

 body by the weight of an equal volume of water. 



Specific gravity = 



Wt. of body 



Wt. of equal vol. of water 



