43 



MECHANICS 



OR 



THE EQUILIBRIUM OF BODIES (STATICS), AND 

 THE MOTION OF BODIES (DYNAMICS). 



8. Inertia. A body is said to be at rest when it con- 

 stantly keeps the same position in space ; if it changes 

 its position it is said to be in motion. No body with 

 which we are acquainted is absolutely at rest. The 

 earth moves round the sun, and turns at the same time 

 about its axis ; all bodies upon the earth are thus in 

 motion. Hence, when bodies are said to be at rest, we 

 mean that they maintain their position in relation to the 

 earth ; in this sense a building or a tree is at rest, while 

 a moving carriage or a running man is in motion ; and 

 it is in this sense that the term rest will have to be 

 understood in the succeeding articles. 



A body has a motion of translation, if all its particles 

 occupy successively different positions ; otherwise the 

 motion is rotatory or vibratory, the body as a whole 

 maintaining in these cases its position, while portions of 

 the body change their position in space in consecutive 

 times. The motion of a rolling carriage, of a running 

 man, of a cannon-ball fired from a gun, is a motion of 

 translation ; the motion of a wheel, of a mill-stone, of a 

 boy's top, is rotatory ; the motion of the pendulum of a 

 clock, of a sounding violin string, is vibratory. The 

 motion of the earth is at the same time a motion of 

 translation and a motion of rotation. 



