MATTER AND ITS PROPERTIES 



35 



are placed on the other side. The sum of the weights 

 necessary to balance the beam is the weight of the object. 

 For heavier masses a system of levers is used, as in a 

 platform scale where the weights 

 used are fractional multiples of the 

 reading on the scale beam. 



Density and Specific Gravity. - 

 Since equal volumes of different sub- 

 stances such as iron, wood, lead, 

 aluminum, and granite do not have 

 the same mass or weight, we use the 

 term density to indicate the differ- 

 ences in the weights of definite vol- 

 umes of these various substances. 

 Before we are very old we learn that 

 some substances are heavier than 

 others. A piece of iron is heavier 

 than a piece of wood of the same 

 size, and we say that the density of 

 iron is greater than that of wood. 



To get the maSS Of a body We Weigh the . determination of small 

 , 11- T i weights. 



it and to get the density we divide 

 its weight in grams by its volume in cubic centimeters. 

 Density determined by this procedure also gives the 

 numerical value of the specific gravity of the substance. 

 Specific gravity is the ratio of the weight of a substance 

 to the weight of an equal volume of water. 



FIG. 28. The Spring 



Balance. 

 Quite commonly used in 



Density 

 Specific gravity = 



mass m grams 

 volume in c.c. 



weight of body 



weight of equal volume of water 



Density in grams per cubic centimeter is specific grav- 

 ity, but density in terms of any other system of weights 



