MECHANICS 57 



Here, as before, when dealing with geometry,* what 

 we see in real life does not exactly correspond with 

 abstract scientific principles. All the motions we ob- 

 serve about us sooner or later come to an end, and no 

 body propelled from the earth's surface goes on long 

 in one direction, but sooner or later descends to 

 the earth. These facts are, of course, due to friction 

 which, in different degrees, retards motion, and to the 

 force of gravity which draws all things that may be 

 propelled from the earth's surface downwards again 

 towards its centre. Therefore in many dynamical 

 problems we have to neglect the consideration of 

 friction. But friction may not only be more or less 

 diminished, it may be actually neutralised by the action 

 of some other force. 



Thus a railway train once set in motion would, 

 according to our first law of motion, continue onwards 

 uniformly if its motions were not retarded by any 

 other force; but friction tends to prevent this, and it 

 would soon stop the train but that the force generated 

 by the engine is sufficient to overcome the impeding 

 influence of friction. The train will thus continue 

 onwards in uniform motion under the influence of 

 opposing forces. 



The motions which pertain to any separate body, con- 

 tinue unaffected by a motion common to it and other 

 bodies also e.g., a watch will continue its proper move- 

 ments while in the pocket of a man running a race. 

 This truth is connected with the second law of motion, 

 which affirms the effective independent action of forces 

 apparently combined. 



It may be illustrated by the fact that a weight 



* See ante, p. 36. 



