Sir Isaac Newton 9 



Take the case of " rest " and " motion." At first sight 

 it seems obvious that the former is the simpler phenomenon ; 

 but our trouble begins when we try to define " rest." Dis- 

 regarding this difficulty, let us ask ** What is the simplest 

 kind of motion ? " Every schoolboy now could give the" 

 answer : "A uniform motion in a straight line " ; but he 

 would be sorely puzzled if he were required to give an example 

 of a body moving with uniform motion in a straight line, for 

 such a thing does not exist. The Greek philosophers kept 

 more in touch with realities when they considered motion 

 in a circle to be the simplest of its kind, because they had 

 observed that the stars describe circles in the sky, and they 

 could artificially produce circular motion by tying a weight 

 to a string and whirling it round. As astronomy advanced, 

 and the motion of the planets were further investigated, 

 it became more and more difficult to reduce everything to 

 circular motion. All efforts to persevere in such attempts 

 finally broke down when the laws regulating the fall of 

 bodies from a height were discovered. The straight line 

 motion although never directly brought within the range of 

 observation then took its place as the simpler basic idea. 



Sir Isaac Newton (1643-1727) formulated the laws of 

 motion ; they - have formed ever since the foundation of 

 physical science, and a few words must be said as to their 

 significance. Our first idea of " force " is derived from 

 muscular sensation. We push a body, and see it change its 

 place, and are conscious that we can ourselves be made to 

 move by an application of muscular force from outside. 

 From this it is natural, though perhaps not altogether logical, 

 to conclude that every change of motion which we observe 

 in a body is due to some push or pull on that body. This 

 imaginary push or pull we call a force. The first law, 

 originally due to Galileo, asserts that absence of force does 

 not necessarily imply that a body is at rest ; it may be moving, 

 but, if so, it continues to move in a straight line with unaltered 

 velocity. The second law allows us to measure a force, and 

 may be said to have been first applied by Huygens. The 

 third law asserts that whenever we observe a change of 

 motion in a body there must be an equal and opposite 

 change of motion in another body or system of bodies. This 



