24 PHYSICAL SCIENCE 



the result of our experiment on the motion of the 

 two bodies connected with each other by means 

 of a string. We can assert that no body has an 

 acceleration unless another body is acting on it. 

 Thus, we cannot form a complete picture of the 

 motion unless we consider both bodies. But it 

 is often necessary to concentrate our attention on 

 one of them, and it is then convenient to find some 

 quantity which measures correctly the effect of the 

 other body on the first. This quantity is not the 

 acceleration, for that depends on the mass of the 

 moving body, but it is the product of the mass 

 and the acceleration, and is independent of both. 

 This product records completely the mechanical 

 effect of the second body ; it may be taken as an 

 accurate definition of that quantity, of which a 

 rough measure is given by our sense-perception 

 of force. Philosophically, force is the more 

 fundamental concept, but for physics it may be 

 defined as mass-acceleration, and instead of saying 

 that one body is acted on by another, we may, if 

 more convenient, say that it is acted on by a 

 force. If a force moves its point of application, 

 work is done, and the quantity of work is measured 

 by the product of the force and the displacement 

 in the direction of the force. The capacity for 

 doing work is known as energy. A clear dis- 

 tinction is to be made between the ideas of force 

 and energy. 



Together with the conceptions of length, 

 time, and mass, the conception of force also was 

 employed by Newton in his development of 

 mechanical theory. A simultaneous and parallel 

 development of the science was led by Huygens, 

 who used the conception which we now call 



