EXERCISES 187 



Levers are of three classes : 



(1) The fulcrum is between the weight and the power. 



(2) The weight is between the fulcrum and the power. 



(3) The power is between the fulcrum and the weight. 

 Pulleys are wheels over which ropes can be pulled. 



The wheel and axle is a form of the lever used for circular motion. 



The wedge and the screw are really forms of the inclined plane. 



Friction prevents us from getting the full amount of work out of a 

 machine; but some is necessary in order that machines may work at all. 



A sailboat is a device for getting some forward motion out of any 

 wind except one that is " dead ahead." 



The kite gets upward motion out of a horizontal wind. 



The aeroplane gets upward motion by the pushing of its obliquely- 

 set planes horizontally against the air. 



The windmill converts a horizontal wind into circular motion. 



212. Exercises. 



1. What kind of a machine is used for lifting window awnings? 

 A door transom? A weighted window sash? 



2. How could you arrange a system of pulleys so that by pulling 

 downward with a force of 20 Ibs. you could raise 40 Ibs.? 



3. If the crank of a winch (Fig. 179) makes a circle of 36 in. while 

 the rope is wound up 6 in. for each complete revolution of the crank, 

 how much force must I exert on the crank to lift a pail of water weigh- 

 ing 100 Ibs.? 



4. What kind of a machine is a nail, a coffee-grinder, a carpenter's 

 brace and bit, a pendulum bob, a snow plow, a pin-wheel, a door knob, 

 a spoon, a spade, a chisel? 



5. How do you hold your knife and your fork when cutting a tough 

 piece of meat? What kind of machine are these implements then? 

 What machine are the tines of the fork and the blade of the knife? 



6. A man who can exert a force of 100 Ibs. wishes to raise a 300 Ib. 

 barrel into a wagon 3 ft. above the ground. He uses an inclined board; 

 how long must it be? 



7. In loading a wheelbarrow, where should you put the load in order 

 to make the force needed for lifting as small as possible? 



