48 



WELLS'S NATURAL PHILOSOPHY. 



What is Indif- 

 ferent Equili- 

 brium ? 



"VATiat is Stable 

 Equilibrium ? 



long as this position is maintained, the wheel will remain at rest, but the mo- 

 meat the center of gravity, a, is moved a little to the right or left, so as to 

 throw it out of the vertical line joining a and c, the wheel will turn over, and 

 assume such a position as to bring the center of gravity immediately beneath 

 the point of support, as in the second case. 



Upon what 92. The stability of a body, therefore, de- 



puty oflbod^^ pends upon the manner in which it is sup- 

 djpend? ported, or in other words, upon the positica 



of its center of gravity. 



What are the 93, As a body may be supported in three 

 timsVE^quUi- positions, we have, as a consequence, three 

 brium? conditions of equihbrium, viz., Indifferent, 



Stable, and Unstable Equilibrium. 



Indifferent Equilibrium occurs when a body is supported 

 upon its center of gravity ; for then it remains at rest indiffer- 

 ently in every position. 



Stable Equilibrium occurs when the point of support is 

 above the center of gravity. If a bod}' be moved from this 

 position, it swings backward and forward for a time, and 

 finally returns to its original situation. 



. „ Unstable Equilibrium occurs when the point of support 13 



What 13 Un- . ,<>,/. 



Bt:ible Equili- beneath the center of gravity. The tendency ot the center of 



bnum ? gravity in such cases is to change, and take the lowest situation 



the support of the body will allow. 



94. The principle that when a body is suspended freely, it 



will have its center of gravity in a vertical line, immediately 



below the point of support, has been taken advantage of to 



determine experimentally the position of the center of gravity, 



in irregular shaped bodies. Sujipose we suspend, as in Fig. 



15, an irregular piece of board by means of cord. A plumb-line let fall from 



the point of support, or the prolongation of the cord, wUl 



pass through the center of gravity, G. If we now attach 



the cord to another point, and suspend the body anew, tha 



prolongation of the cord in this instance, also, will pas3 



through the center of gravity, G. The intersection of 



these two lines will be the center of gravity, and tha 



board, if suspended by a cord attached to this point, will 



hang evenly balanced. 



95. A line which connects the center of 

 gravity of a body with the center of the 

 earth, or, in other words, a line drawn from 

 the center of gravity perpendicularly downward, is called 

 the LixE of Direction. It is called the Line of Direction, 



How may we 



determine the 

 center of grav- 

 ity ill irrejjular 

 bodies ? 



Fig. 15. 



