EQUILIBRIUM 



35 



B, the strip is in neutral equilibrium ; and when supported as at C, it is- 

 in unstable equilibrium, for the slightest movement lowers the centre of 

 gravity. 



Relation of centre of gravity to base of support. A circular 

 disc, in which it will be remembered the centre of gravity coincides 

 with the geometrical centre, will not rest upon a table if the centre 

 is beyond the edge of the table, but will topple over. In a similar way, 

 if. any plain figure lies flat upon a table, the centre of gravity of the 

 figure must be within the edge of the table. The same conditions 

 apply to any object resting upon a support. 

 For an object resting upon a base to be in 

 equilibrium, a vertical line drawn from the 

 centre of gravity downward must fall 

 within the base. When this vertical line 

 falls outside the base the body topples 

 over. 



Consider the case of an omnibus on level 

 ground. The centre of gravity is some- 

 where inside the omnibus, and a vertical 

 line drawn from it downward would fall 

 within a line traced around the omnibus 

 upon the ground. But, if the outside of 

 the omnibus is filled with people and the 

 vehicle happens to be running across a 

 sloping road, it might topple over, for 

 a jerk might cause so great a change of position of the centre of 

 gravity as to make the vertical line from the centre fall outside the 

 base of support, and in such a case an accident must happen (Fig. 27). 



Equilibrium. When a body is at rest, all the forces acting upon it 

 balance one another (or, what is the same thing, any force is equal 

 and opposite to the resultant of the remaining forces), and it is said 

 to be in equilibrium. It is in stable equilibrium when any turning 

 motion to which it is subjected raises the centre of gravity ; in unstable 

 equilibrium when a similar movement lowers the centre of gravity, 

 and in neutral equilibrium when the height of the centre of gravity 

 is unaffected by such movement. Consequently, if a body in stable 

 equilibrium is disturbed, it returns to its original position ; if in un- 

 stable equilibrium, it will, if disturbed, fall away from its original 

 position ; while if the condition of equilibrium is neutral it will, in 

 similar circumstances, stay where it is placed. 



Conditions of stability of suspended and resting objects. 

 The centre of gravity must in every case be below the point of support 



FIG. 27. If the vertical line 

 from the centre of gravity should 

 fall outside the base of support, 

 the omnibus would topple over. 



