ENERGY AND FORCE 



39 



gram is the unit of mass in the study of science, although 

 we are accustomed to think of mass in terms of pounds 

 of the English system. The attraction, or pull, that the 

 earth exerts on a given mass is irrespective of the kind 

 of matter in the body. The pull on a pound of feathers 

 is the same as the pull on a pound of lead. 



Center of Gravity. A large body such as a block of 

 stone is made up of countless small particles of matter. 

 Now according to our law of 

 gravitation, each little particle 

 of matter attracts and will be 

 attracted by the others and by 



,, ,, T ,, i ,-u FIG. 30. Center of Gravity 



the earth. In other words there or Center of Mass of a Complex 

 will be a countless number of Form - 

 little pulls between these particles of matter in the stone 

 and the earth, and it is evident that the sum of these little 

 pulls between the particles and the earth will equal the 



total pull of the earth on that 

 body. The point in a body at 

 which a single force equal in 

 magnitude to the weight of the 

 body and directed upward can 

 be applied so that the body will 

 remain at rest in whatever posi- 

 tion it is placed is called the 

 center of gravity. It is the 

 place at which all the weight 

 may be said to be. To do this 

 mechanically we simply find a 

 place at which the body will 



FIG. 31 . Determining the Center 1 



of Gravity. balance (Figure 30). 



Experiment 3. The center of gravity of a piece of uniformly rolled 

 metal may be found in the following manner. Support it from one 



