Machines 245 



the effort, the fulcrum, and the weight. In the first class the 

 fulcrum is between the weight and the effort. Examples of this 

 class are common shears, pliers, ordinary steelyards or scales, 

 and seesaws. In the shears the fulcrum is at the hinge, the 

 effort at the handle, and the weight is at the point where the 

 cutting is done. Can you locate the fulcrum, the weight, and the 

 effort in the other objects referred to? 



In the second class, the weight is between the fulcrum and 

 the effort. Examples of this class are the oar of a boat, the 

 wheelbarrow, and the long-handled lemon squeezer. In the use 

 of the oar, the arm of the rower is the effort, the boat is the 

 weight and the water pressing against the blade of the oar is the 

 fulcrum. Locate the fulcrum, the weight, and the effort in the 

 other objects named. 



In a lever of the third class the effort is between the weight 

 and the fulcrum. Examples of this class are the leg and claws 

 of a bird, the treadle on a sewing machine, and the fire tongs. 

 When a bird rests upon a perch, its body is the weight, the muscles 

 of the legs are the force, and the perch is the fulcrum. Locate 

 the effort, the weight, and the fulcrum in the treadle and in the 

 fire tongs. 



The mechanical advantage of any lever may be found by 

 dividing the weight lifted by the effort applied. It may also be 

 found by dividing the distance from the fulcrum to the effort 

 by the distance from the fulcrum to the weight. If it is 5 feet 

 from the fulcrum to the effort and 1 foot from the fulcrum to 

 the weight, the mechanical advantage is 5-i-l, or 5. 



Levers of the first class have a mechanical advantage of 

 greater or less than 1, depending on the location of the fulcrum. 



Levers of the second class always have a mechanical advantage 

 greater than 1. 



Levers of the third class always have a mechanical advantage 

 of less than 1. While they exert less force than the effort applied 

 to them, they offer advantages in other ways. Your forearm 

 and the muscles that move it form a third class lever. The 

 elbow is the fulcrum. The point on the forearm where the muscle 

 of the upper arm is attached is the point where the effort is 



