SIMPLE MACHINES 179 



119. A Machine is a device used to transform or 

 transfer energy and to apply force for doing useful work. 

 Illustration of how a machine can transform energy: 

 when coal is burned in the fire-box of a boiler, the heat 

 of the coal makes steam of the water, and the steam in 

 running the engine develops mechanical energy, which 

 can be made to develop electricity or electrical energy 

 by turning a dynamo. The electrical energy can be 

 changed back to mechanical energy and drive street cars 

 along the tracks. All kinds of steam and gas engines and 

 electrical machines are devices for transforming energy as 

 well as for transferring it; while the simple machines are 

 either devices for transferring energy or devices to which 

 force can be applied and useful work- result. 



The simple machines are six in number: The (i) lever, 

 (2) inclined plane, (3) wedge, (4) screw, (5) pulley, and 

 (6) wheel and axle. Of these the lever and inclined plane 

 are basic types. The pulley and the wheel and axle are 

 modified forms of the lever, while the wedge and screw 

 are modified inclined planes. All complex machines are 

 only combinations of two or more simple machines. 



120. The Mechanical Advantage of a machine is the ratio 

 of the resistance (as, for instance, the weight lifted) to the 

 force applied. The weight in pounds divided by the force 

 in pounds gives the mechanical advantage. The mechanical 

 advantage of a machine is also the ratio of the distance 

 that the force moves to the distance that the resistance 

 moves. That is, force distance divided by weight distance 



a distance of one centimeter. If we lift a gram one centimeter, we do a 

 gram centimeter of work. The smallest unit of work in the metric system 

 is the erg. The gram centimeter is equal to 980 ergs. (An erg is the 

 work done by a force of one dyne moving through a distance of one centi- 

 meter. A dyne is a force that can give to a gram mass an increase in 

 speed of one centimeter per second.) 



