60 UNIVERSITY OF MISSOURI BULLETIN 



Whenever a force produces a motion, work is said to be done, 

 and the magnitude is defined and measured by the product of 

 the force times the distance through which this force acts. 

 Work then is defined and measured in terms of force and 

 distance. Energy in turn may be defined in terms of work. 

 In any physical operation where work is done, we may say 

 that the condition of the body that did the work has changed 

 and that the condition of the body that had the work done 

 on it has also changed. In some cases this condition of a 

 body may involve its velocity ; in other cases it may involve 

 the position of the body, or, as in the case of a spring, it may 

 involve its length or shape. But it is something about the 

 body which changes when the body does work, or when work 

 is done on the body. Further, if we so define and measure 

 this condition that its change shall be numerically equal to the 

 work done, then we may call this condition energy. Hence 

 we may say that whenever we do work on a body, its energy 

 increases by an amount equal to the work done. When we 

 raise a weight we do work and add energy to that body. 

 When one throws a ball, he does work and adds energy to 

 the ball. On the other hand, when a body does work it loses 

 an amount of energy precisely equal to the amount of that 

 work. A weight in falling will do work, and will lose energy. 

 Energy then is a condition of a body measured in terms of 

 the work that it can do. 



Within the last half century the doctrine of the law of 

 conservation of enery has become universally accepted. This 

 doctrine states that energy cannot be created and cannot be 

 destroyed; that the sum total of energy in the universe is 

 constant. The acceptance of this doctrine almost revolution- 

 ized physical science. Since the days of Newton no other 

 principle or doctrine has done so much towards the correlat- 

 ing and explaining of various physical facts. By the aid of 

 this dectrine one can deduce laws and principles that before 

 seemed independent of all other physical laws. But we re- 



