64 MOTION AND FORCE 



substitute smooth steel screw eyes may be used with small hard 

 thread. 



Examples of levers are always at hand. In the following, name 

 the class to which each belongs and state whether we gain in 

 force or in speed by using it : scissors ; a common pump handle ; 



FIG. 51 



pincers; sugar tongs; steelyards; nutcrackers; a crowbar when 

 its fulcrum is on the ground beneath a weight ; ice tongs ; a tin- 

 smith's shears ; a wheelbarrow ; a claw hammer (Fig. 50). 



73. Other Simple Machines. The screw is generally 

 used to gain intensity of force. As the force is applied 

 to the circumference of a wheel b (Fig. 51), the sur- 

 face on which the resistance acts will move ahead only 

 the small width of one thread of the screw. Since the 

 force is applied through a far greater space than the 

 resistance, the gain in force is great. 



Fig. 52 shows a wheel and axle. The axle is much 

 smaller than the wheel and turns with it. A small 

 force F applied on the wheel may move a much greater 

 weight E on the axle; but E moves proportionally 

 slower than F, that is, a great force at E will move 



