176 EXPERIMENTAL GENERAL SCIENCE 



carpenters, and other artisans make use of this force by 

 means of the plumb-bob, a top-shaped piece of metal sus- 

 pended by a string. The string always takes a position 

 perpendicular to the earth's surface, and a line at right 

 angles to this is horizontal. 



147. Equilibrium. Since the attraction of gravity always 

 acts toward the center of its mass, the center of gravity in a 

 body is that point upon which it will exactly balance itself. 

 When this center is near the base of the body, as in a book 

 lying on its side, it resists any attempt to change its position 

 or upset it. It is therefore said to be in stable equilibrium. 

 If, however, the center of gravity is so located that moving 

 the body will lower the center of gravity, the body is in an 

 unstable equilibrium and easily upset. A meter stick standing 

 on end is in unstable equilibrium. In a few bodies, a ball for 

 instance, the center of gravity is so placed that moving them 

 neither raises nor lowers it. Such bodies are said to be in 

 neutral equilibrium. 



Practical Exercises 



1. Why cannot one fire a rifle around a tree? 



2. Why can one jump farther with a running start than he can 

 without? 



3. Why does a heavy flywheel cause machines to run more smoothly? 



4. Why may one escape a close pursuer by dodging? 



5. How does beating a carpet get the dust out of it? 



6. When a car suddenly goes around a curve in which direction are 

 the passengers thrown, toward or away from the center of the curve? 

 Why? 



