MACHINES 65 



10. The nuts on one side of a wagon have right-handed threads 

 and on the other side left-handed threads. Why? 



11. State the general law of machines. 



12. What is the advantage of using iron rails on a railroad? 



13. Why do wide- tired wagon wheels make hauling over soft 

 fields easier? 



14. Why do we scatter sawdust on icy pavements? 



Power. - - The term power is one which is used quite 

 generally to designate sources of energy, but power in a 

 specific sense means the rate of doing work. Time is 

 not a factor in the determination of work. The same 

 amount of work will be done in moving a ton of coal 

 into the basement whether the work be done in two hours 

 or ten hours, but the rate at which energy is consumed 

 will be much greater when the work is done in two hours. 

 An engine which can do a certain piece of work in an 

 hour has twice as much power as one which can do but 

 half the work in an hour. The first engine liberates 

 energy faster than the second. 



Unit of Power. The unit of work in the English system 

 is the foot pound, the unit of time is the second, and the 

 unit of power is a foot pound of work in a second, or 



Work p W 



Power = TH; written P = = r 

 Iime 1 



If W and T are unity, then P equals one foot pound 

 per second. 



It is quite common now to speak of the rating of an 

 engine or motor in terms of horse power. Cf. page 44. 



Man has learned to use the energy that is stored in 

 nature to aid him in the operation of his machines. 

 The energy of the wind, the water, the sun, and that 

 stored in materials used as fuels and foods is utilized in 

 this way. 



