BUOYANT FORCE OF LIQUIDS 



33 



When the density of a body is given as so many grams for 1 c.c., 

 the number representing the density is the same as the number for the 

 specific gravity. 



34. Buoyant Force of Liquids. If we attach a string 

 to a block of wood, and lift the wood by the string, we 

 need to put forth a muscular effort equal to the weight of 

 the wood. But if we let the block rest on water, we do 

 not put forth any effort; the water supports the block. 

 We say that the block floats on the water. 



If we hold a piece of stone by a string, first in air, and 

 then in water, we find that the stone is lighter (weighs 

 less) in water than in the air. Here the water does not 

 support all of the weight, but it does support part of it. 

 It is well known that a stone can be lifted much more 

 easily when it is under water 

 than when it is out of the water. 



Suppose that some lead is cut or cast 

 in the form of a cube having a volume of 

 exactly 1 c.c., and is attached to one arm 

 of a balance, as in Fig. 28. If we weigh 

 it in air and then in water we shall find 

 that it weighs 1 gram less in water than 

 in the air. One cubic centimeter of iron, 

 glass, or marble, or of any solid which is 

 more dense than water, would lose the 

 same amount 1 gram when weighed 



in water. Since the 1 c.c. of water displaced by each of the cubes 

 weighs just what the cube seems to lose, we conclude that a body put 

 under water is pushed, or buoyed, up by just the amount of the 

 water it displaces. A mass of iron, stone, glass, etc., having a volume 

 of a cubic foot would weigh 62.5 pounds less in water than in the air, 

 since this is the weight of a cubic foot of water (cf. 33). 



FIG. 28. 



A Solid Immersed in a Liquid is 

 Buoyed up by the Liquid. 



