=90. Center of Gravity.=--A body is composed of a great many particles each of which is pulled toward the center of the earth by the force of gravity. A single force that would exactly equal the combined effect of the pull of the earth for all the particles of a body would be their resultant. The _magnitude_ of this resultant is the weight of the body. The _direction_ of this resultant is in a line passing toward the earth's center, while the _point of application_ of this resultant is called the _center of gravity_ of the body. The center of gravity of a body may also be briefly defined as _the point about which it may be balanced_. As the location of this point depends upon the distribution of matter in the body, the center of gravity is also sometimes called the _center of mass_ of the body. The earth's attraction for a body is considered for the sake of simplicity, not as a multitude of little forces, but as a single force applied at its center of gravity. To find the center of gravity of a body find two intersecting lines along which it balances, see Fig. 72, and the center of gravity will be at the intersection. A vertical line through this point is sometimes called the _line of direction of the weight_. [Illustration: FIG. 72.--The center of gravity is at the intersection of the lines of direction.] =91. Equilibrium of Bodies.=--Equilibrium means equally balanced. A body at rest or in uniform motion is then in equilibrium. An object is in equilibrium under gravity when a vertical line through its center of gravity passes through the point of support. A trunk is an example of a body in equilibrium since a vertical line from its center of gravity falls within the base formed by the area upon which it rests. Work will be necessary to tip the trunk from its position. The amount of work required will depend upon the weight of the body and the location of the center of gravity. =92. Kinds of Equilibrium.--(a) Stable.=--A body is in stable equilibrium under gravity if its center of gravity is raised whenever the body is displaced. It will return to its first position if allowed to fall after being slightly displaced. In Fig. 73, _a_ and _b_ if slightly tipped will return to their first position. They are in stable equilibrium. Other examples are a rocking chair, and the combination shown in Fig. 74. [Illustration: FIG. 73.--Stable equilibrium.] =(b) Unstable.=--A body is in unstable equilibrium under gravity if its center of gravity is lowered whenever the body is slightly displaced. It will fall farther from its first position. A pencil balanced on its point or a broom balanced on the end of the handle are in unstable equilibrium. The slightest disturbance will make the line of direction of the weight fall outside of (away from) the point of support (Fig. 75 _a_). [Illustration: FIG. 74--An example of stable equilibrium. Why?] [Illustration: FIG. 75.--Unstable equilibrium _a_, neutral equilibrium _b_.]