i8o GENERAL SCIENCE 



is the mechanical advantage. (The above statements do 

 not take into consideration the loss by friction.) For 

 instance, if by the use of a machine a 5oo-pound piano is 

 lifted 10 feet by a force of 100 pounds moving 50 feet, the 

 . mechanical advantage of this machine is 500 pounds di- 

 vided by 100 pounds or 5. Again, the mechanical advan- 

 tage is 50 feet -f- 10 feet =5. (Friction not considered.) 

 121. Efficiency. Machines, especially the simple 

 machines, are often thought of as devices for saving 

 work. If a heavy object that could not be moved with- 

 out a machine can be moved by the use of one, we are 

 willing to waste a little work to accomplish our purpose. 

 By the use of a machine the force applied does not have 

 to be so great as it would have to be without the machine, 

 but the force has to move much farther than the weight 

 lifted to accomplish the desired result. Hence the work 

 put into a machine is always equal to or greater than the 

 work got out of the machine. Work = force X distance, 

 so the work put into a machine will be the force X the 

 force distance, and the work got out of a machine will be 

 the weight X the weight distance. The work got out of a 

 machine will be less than the work put into it, if there is 

 any loss by friction. All machines have more or less fric- 

 tion; when friction resists man's force in machines the ef- 

 ficiency of the machine is less than one, but if friction is 

 assisting in mechanical work the efficiency is greater than 

 one. The efficiency of a machine is the ratio of the work 

 done by the machine to the work spent on the machine. 



Work accomplished 



Efficiency - 



Work spent 



Efficiency is expressed in per cent; 90 per cent efficiency 

 means that one part of the work spent on a machine is 



