ENERGY AND ITS TRANSFORMATIONS. 



17 



kilogrammetres : the total work so measured is then 

 divided by the time of operation and by the value of 

 the horse-power for the assumed unit of time and the 

 mean value of the power expended thus finally ex- 

 pressed in horse-powers."^ 



The forces acting in machines are distinguished into 

 driving and resisting forces. That component of the 

 force, acting to produce motion in any part which lies in 

 the line of motion only, is that which does the work ; 

 and this component is distinctly called the Effort." 

 Similarly, only that component of the resistance which 

 lies in the line of motion is considered in measuring 

 the work of resistance. In either case, if the angle 

 formed between the directions of the motion of the 

 piece and of the driving or the resisting force be called 



the effort is 



P = R cos a, (12) 



The other component, acting at right angles to the 

 path of the effort, is 



Q = Rs'ma, (13) 



and has no useful effect, but produces waste of power 

 by introducing lateral pressures and consequent friction. 



Energy, which is defined as capacity for performing 

 work, is either actual or potential. 



Actual or Kinetic Energy is the energy of an actually 

 moving body, and is measured by the work which it is 

 capable of performing while being brought to rest, 

 under the action of a retarding force ; this work is 



* Custom has not yet settled the proper form of the plural of this 

 word ; there is no reason why it should not follow the rule. 



