CENTER OF MASS 



35 



Lead Wood 



halves together, the center of mass of the ball would not 



be the same as its center of volume. Lead ie so much 



denser than wood that the center of mass would be 



somewhere in the 



lead half of the 



ball (Fig. 29). In 



an egg the center 



of mass is nearer 



the larger than 



the smaller end. 



A body is in its TheBall Takes Position (2) The Heavy Knife Handles (3) 

 most Stable pOSl- Bring the Center of Mass Below the Tip of the Pencil. 



tion, that is, is best 



able to stand, when the center of its mass is lowest, or nearest the 

 earth's center. A ball of wood is at rest in any position, because its 

 center of mass is as low in one position as in another, but a ball 

 half wood and hah 7 lead will be in a stable position only when the 

 lead half is the lower. An egg lies on its side but not on end because 

 when it is on its side its center of mass is lower. To make it stand on 

 end we would need to keep its center of mass exactly above the point at 

 which it rests upon the table or other support. As we cannot do this, 

 ' the egg rolls over. We ourselves stand when we keep the center of 

 mass of our bodies above the space bounded by 'our feet, but we fall 

 when we lean over so far that our center of mass is no longer vertically 

 above this space. 



When an irregular body, such as a log or tree, lies on the ground, it 

 cannot shift itself so that it can bring, its center of mass to the lowest 

 position, but if we float the body upon water, it turns over until it 

 finds this position. 



36. Summary. Gravitation is the pull, or attraction, existing be- 

 tween all bodies of matter. 



Gravity is the earth's pull upon bodies at or near its surface. Grav- 

 ity pulls vertically downward, that is, toward the earth's center. 



