Machines 243 



gradual substitution of machine for manual labor. It is largely 

 by means of machines that the transformation of energy is 

 effected. 



There are a few words and phrases whose meanings we 

 should know in order to understand the principles involved in 

 machines. Important among these are energy, work, foot pound, 

 force, effort, power, friction and mechanical advantage. 



Energy is the capacity to do work. 



Work is the overcoming of resistance. It is the result of motion 

 caused by force. 



The foot pound is the unit of work. It represents the work done 

 in lifting a pound avoirdupois a distance of one foot. 



Force is the use of energy in the effort to do work. It is usually 

 measured in pounds. A pound of force is equal to the pull of gravity on a 

 one pound weight. 



Effort is force applied to move a weight or overcome resistance. 



Power is the rate of doing work. It is measured in terms of horse 

 power. One horse power represents the ability to do 550 foot pounds of 

 work per second, or 33,000 foot pounds per minute. 



Friction is the resistance which a body meets with in rubbing 

 against another body. It hinders the motion of machines and causes loss 

 of work, the amount of loss being dependent on the conditions of the sur- 

 faces that come in contact. 



The mechanical advantage of a machine is its efficiency as measured 

 by the force it exerts divided by the force applied to it to make it work. 

 If a machine lifts a 4 pound weight when 1 pound of force is applied to 

 it, its mechanical advantage is 4 -^ 1, or 4. 



You are more or less familiar with machines. You have seen 

 sewing machines, mowing machines, washing machines, automo- 

 biles, airplanes, bicycles, steam engines, dynamos, and many other 

 complex machines, as well as such simpler ones as nut crackers, 

 tongs, and hammers. There are six simple machines the lever, 

 the inclined plane, the wedge, the screw, the pulley, and the wheel 

 and axle. Since the pulley and the wheel and axle are only varia- 

 tions of the lever, the study of the lever will help us to understand 

 their action. Since the wedge and the screw are only variations of 

 the inclined plane, the study of the plane will help us to understand 

 the action of the wedge and the screw. All complex machines 

 are merely two or more simple machines combined in such a way 

 that they work together. 



