FORCE AND MOTION 59 



The cable attached to the beam slips off and the beam falls 

 downward, crashing through obstacles and doing great de- 

 structive work. While suspended and motionless, the beam 

 had energy; it was capable of falling and doing work. The 

 ordinary use of the word work implies necessary or useful mo- 

 tion, but in scientific use the act of overcoming any resistance 

 is work. 



61. Machines. A machine is any instrument by means 

 of which work is advantageously done. It may be very 

 simple; as the lever, a stiff bar by which a man can move a 

 much heavier weight th^n he could move without a machine. 

 Another simple machine is the pulley, which consists of a 

 wheel over which a rope, chain, or belt is moved, so as to 

 lift a weight at one end when a force pulls on the other end. 

 The wheel and axle consists of a cylinder and a wheel fastened 

 together and moving upon the same axis. The cylinder, 

 which is the "axle," winds up a rope or cable that pulls or 

 lifts heavy weights. The wheel is often furnished with a crank 

 or spokes by which the whole machine is turned. Houses 

 are moved by this machine, operated by horses. 



Another simple machine is the screw, which is generally 

 made of metal or wood. It is a cylinder having a ridge, 

 called the thread, winding spirally around it. The screw 

 works in a nut that has a groove to fit the thread of the 

 screw. The screw is turned by hand or other power, and is 

 used in small presses and in machines for raising weights. 

 It is sometimes used to raise buildings. Several parts of 

 the underpinning of a building are removed and a screw is 

 put in each of these places. Every turn of the screw lifts 

 the part above it by the width between the threads. The 

 original screw propeller (1850) of a steamship really screwed 

 its way through the water, pushing the water to one side 

 as a small screw pushes wood. Later modifications have 

 produced a propeller which does not resemble a screw in 

 form. 



