THE LAW OF MOTION 123 



waves by tugging at them, and clearly another repre- 

 sentation of the agency which deflects them must be 

 found. 



The Law of Motion. I must now ask you to let your 

 mind revert to the time of your first introduction to 

 mechanics before your natural glimmerings of the truth 

 were sedulously uprooted by your teacher. You were 

 taught the First Law of Motion — 



"Every body continues in its state of rest or uniform 

 motion in a straight line, except in so far as it may be 

 compelled to change that state by impressed forces." 



Probably you had previously supposed that motion 

 was something which would exhaust itself; a bicycle 

 stops of its own accord if you do not impress force to 

 keep it going. The teacher rightly pointed out the 

 resisting forces which tend to stop the bicycle; and he 

 probably quoted the example of a stone skimming over 

 ice to show that when these interfering forces are re- 

 duced the motion lasts much longer. But even ice offers 

 some frictional resistance. Why did not the teacher do 

 the thing thoroughly and abolish resisting forces alto- 

 gether, as he might easily have done by projecting the 

 stone into empty space? Unfortunately in that case 

 its motion is not uniform and rectilinear; the stone 

 describes a parabola. If you raised that objection you 

 would be told that the projectile was compelled to 

 change its state of uniform motion by an invisible force 

 called gravitation. How do we know that this invisible 

 force exists? Why! because if the force did not exist 

 the projectile would move uniformly in a straight line. 



The teacher is not playing fair. He is determined to 

 have his uniform motion in a straight line, and if we 

 point out to him bodies which do not follow his rule 



