MACHINES 57 



In the lever of the third class (Figure 50), the power 

 is between the fulcrum and the weight, as in sugar tongs, 

 sheep shears, and the treadle on a sewing machine. 



FIG. 50. Lever of the Third Class. 



EXERCISES 



1. What kind of lever is a claw hammer when used to pull a 

 nail? The oar of a boat? Wire cutters? 



2. A man places a fulcrum 1 foot from the end of a 6-foot 

 crowbar. If he presses down on the other end with a force equal 

 to 150 pounds, what weight can he raise? 



3. With the same power and bar, what weight can he raise if 

 the fulcrum is placed 6 inches from the end ? 



4. On a balanced meter bar a weight of 300 g. is placed 30 

 cm. from the point of support. Where must a weight of 450 g. 

 be placed to balance it ? 



5. Where is the fulcrum in a wheelbarrow? The weight? 

 The power? 



6. What kind of lever is the forearm? Why is it better than 

 another class of lever? 



7. What advantage is gained in a lever when the power is 

 quite close to the fulcrum, as in the case of a boat oar ? 



8. Name three uses of the lever not mentioned in this 

 book? 



9. A 200-pound weight is placed 1 foot from the axle of a 

 wheelbarrow. How much force must be exerted on the handles 

 5 feet from the axle to lift the weight ? (The weight of the 

 wheelbarrow is not considered in this problem.) 



10. An oar is 1\ feet long and the oarlock is 1^ feet from the 

 handle. If a man pulls on the oar with a force of 100 pounds, 

 what force is exerted on the water? 



