=(c) Neutral.=--A body is in neutral equilibrium if its center of gravity is neither raised nor lowered whenever the body is moved. Familiar examples are a ball lying on a table (Fig. 75 _b_) and a wagon moving on a level street (referring to its forward motion). [Illustration: FIG. 76.--_B_ is more stable than _A_.] =93. Stability.=--When a body is in stable equilibrium, effort must be exerted to overturn it, and the degree of stability is measured by the effort required to overturn it. To overturn a body, it must be moved so that the vertical line through its center of gravity will pass outside of its supporting base. This movement in stable bodies necessitates a raising of the center of gravity. The higher this center of gravity must be raised in overturning the body, the more stable it is, _e.g._, see Fig. 76. Thus a wagon on a hillside will not overturn until its weight falls outside of its base, as in Fig. 77 _B_. The stability of a body depends upon the position of its center of gravity and the area of its base. _The lower the center of gravity and the larger the base_, the more stable the body. What means are employed to give stability to bodies, in every-day use (such as clocks, ink-stands, pitchers, vases, chairs, lamps, etc.)? [Illustration: FIG. 77.--_B_ will overturn; _A_ will not.] Important Topics 1. Gravitation; law of gravitation, gravity, weight. 2. Center of gravity. 3. The three states of equilibrium. Stability. Exercises 1. Why is a plumb-line useful in building houses? 2. What is the center of gravity of a body? 3. Explain the action of a rocking chair that has been tipped forward. 4. Is the stability of a box greater when empty or when filled with sand? Explain. 5. How can you start yourself swinging, in a swing, without touching the ground? 6. Is the center of gravity of the beam of a balance above, below, or at the point of a support? How did you find it out? 7. Why are some ink bottles cone shaped with thick bottoms? 8. Would an electric fan in motion on the rear of a light boat move it? Would it move the boat if revolving under water? Explain. 9. What turns a rotary lawn sprinkler? 10. Why, when you are standing erect against a wall and a coin is placed between your feet, can you not stoop and pick it up unless you shift your feet or fall over? 11. What would become of a ball dropped into a large hole bored through the center of the earth? 12. When an apple falls to the ground, does the earth rise to meet it? 13. How far from the earth does the force of gravity extend? 14. Why in walking up a flight of stairs does the body bend forward? 15. In walking down a steep hill why do people frequently bend backward? 16. Why is it so difficult for a child to learn to walk, while a kitten or a puppy has no such difficulty? 17. Explain why the use of a cane by old people makes it easier for them to walk? (6) FALLING BODIES