52 MOTION AND FORCE 



the force had been applied to that point alone. This 

 point is the center of mass of the body ; it is the point 

 about which the matter of the body seems to be evenly 

 t distributed. If now the 



force of gravity acts upon 

 a body, whether it be sup- 

 ported or whether it be falling freely, the body behaves 

 as if the force were applied at its center of mass. The 

 point may then be called the center of gravity (e.g.) of 

 the body. We may say that it is the point in a body at 

 which the force of gravity seems to be applied. 



Experiment 45. Try to balance a ruler on your finger 

 (Fig. 32). Where is the center of mass of the ruler ? Try to bal- 

 ance it upon a pencil point; mark the point. Is this the center 

 of mass ? If not, where is it ? Compare the quantity of matter on 

 both sides of this point. How do you think the action of gravity 

 upon one side of this spot compares with that upon the other? 

 Where is the e.g. of the 



ruler ? Now hang unequal J>~ SJ~ 



weights on the ruler, as in A 

 Fig. 33. Try to find the Fl(J 33 



e.g. of the whole. Where 



is it ? Compare the matter upon both sides of the point. Where 

 is the center of mass? 



62. Position of the Center of Gravity. A body acted 

 upon by gravity behaves as if the force were applied at 

 its e.g. alone. If gravity really did act only upon the 

 e.g., that point would, of course, move toward the e.g. 

 of the earth until stopped by some other force. And 

 we find it to be true that any body on earth that is free 

 to move takes such a position that its center of gravity 

 shall be as low as possible. 



