SIMPLE MACHINES 185 



left hand on the other end, which is the fulcrum, and 

 the right hand between, which is the force; (if the left 

 hand pushes downward and the right hand holds its 



fl 



LEVER OF THE THIRD CLASS 



position the pitchfork is a lever of the first class); the 

 forearm with the fulcrum at the elbow, the weight on 

 the hand, and the force applied just below the elbow 

 joint; the whole arm with the fulcrum at the shoulder 

 joint. In the lever of the third class the weight arm is 

 the entire lever bar. The weight arm is the distance 

 from the weight to the fulcrum. The mechanical ad- 

 vantage of the lever of the third class is less than one; 

 the force is greater than the weight lifted. It is used to 

 move small objects rapidly and with high speed, while the 

 levers of the first and second class are used to move 

 heavy objects with a small force. The bones of the 

 body are used as levers of the third class, with the ful- 

 crum at the joints, and powerful muscles with their 

 tendons attached just across the joints act as forces to 

 move the parts of the body with rapidity. 



The lever of the first class can also be used for speed 

 and for throwing objects, if the force arm is made short 

 and the weight arm made long. This would require a 

 large force to lift a small weight and the mechanical ad- 

 vantage would be less than one. 



Problems. 1. In which of the three classes of levers do the 

 following belong: a boat oar, grocer's scales, sugar tongs, a claw 

 hammer, and a hatchet pulling a nail? 



2. How would you arrange a ruler to use it as a lever of the first 

 class, and then as a lever of the second class, in lifting a book? 



3. If a lever of the second class is 10 feet long, what is the median- 



