FORCE, A SPACE RATE OF ENERGY 145 



for m = l g. and s = l cm. the change in energy is g 

 ergs. Since g is about 980 cm. / sec. 2 it follows that 1 

 gram-centimeter is about 980 ergs. 1 

 Considering equation (7) we see that 



(8) 



states the rate at which the energy changes as the 

 separation s is altered. This space 2 rate of change of 

 energy is defined as the force. The unit of energy 

 being the erg, the unit of force is 1 erg per cm. This 

 unit is called the "dyne" from the root which we 

 recognize hi our word "dynamic." It is also evident 

 that the dyne is the force which will accelerate 1 g. 

 unit amount, that is, change the velocity unit amount 

 (1 cm. per sec.) each second. 

 According to this definition forces are called into 



1 A pound is 454 g. and a foot is 30.5 cm. It therefore follows 

 that 1 ft. Ib. of work is (980) (454) (30.5) or 13,600,000 ergs. The 

 erg is obviously too small a unit for practical purposes. A multiple 

 known as the "joule" and equal to ten million (10 7 ) ergs is there- 

 fore used. 



2 Distinguish between the space rate of energy, which is force, 

 and the time rate, which is power. 



Frequently we are interested in the time rate at which energy 

 is expended. The unit of 1 erg per second is inconveniently small 

 and the practical unit of one joule per second is known as the watt. 

 A thousand joules per second (10 10 ergs) is the kilowatt, which is 

 familiar to all purchasers of electrical energy. In the 3600 seconds 

 of an hour during which energy is received at the rate of 1 kilowatt 

 there is received a total of 3.6 million joules or 36 million million ergs. 

 In such units one's household consumption of electrical energy appears 

 enormous. It might be noted, however, that the energy released 

 by the combustion of one pound of coal (and the necessary air) is 

 about 13 million joules or enough for almost 4 kw. hrs. if there were 

 no dissipation in the transformation. 



