MOTION OF A POINT 3 



The present book deals with theoretical mechanics. We start 

 from the laws supplied by experimental mechanics, and have to 

 discuss how these laws can be used to predict the motion of bodies, 

 for instance, the falling of bodies to the ground, the firing of 

 projectiles, the motion of the earth and the planets round the sun. 

 An important class of problems which we shall have to discuss 

 will be those in which no motion takes place, the forces of nature 

 which tend to cause motion being so evenly balanced that no 

 motion occurs. Such problems are known as statical. 



MOTION OF A POINT 



4. State of rest. Before we can reason about the motion of a 

 body we have to determine what is meant by a body being at 

 rest. In ordinary language we say that a train is at rest when the 

 cars are not moving over the rails. We know, however, that the 

 train, in common with the rest of the earth, is not actually at rest, 

 but moving round the sun with a great velocity. Again, a fly 

 crawling on the wall of a railway car might in one sense be said to 

 be at rest, if it remained standing on the same spot of the wall. 

 The fly, however, would not actually be at rest ; it would share in 

 the motion of the train over the country, the country would share 

 in the motion of the earth round the sun, and the sun would 

 share in the motion of the whole solar system through space. 



These instances will show the necessity of attaching a clear 

 and exact meaning to the conceptions of rest and motion. Obvi- 

 ously our statements would have been exact enough if we had 

 said that in the first case the train was at rest relatively to the 

 earth, and that in the second case the fly was at rest relatively to 

 the car. 



5. Frame of reference. Thus we find it necessary, before dis- 

 cussing rest and motion, to introduce the conception of a frame of 

 reference. The earth supplied a frame of reference for the motion 

 of the train, and when a train is not moving over the rails we 

 may say that it is at rest, the earth being taken as frame of 



