MECHANICAL ENERGY AND HEAT 185 



is effective for purposes of destruction. The hammer and the 

 sled also owe their energy to motion. Energy of motion is 

 called kinetic energy. 



200. Measurement of energy. Since energy is simply the 

 capacity to do work, it can be measured in the same units 

 as are used to measure work; that is, energy is measured 

 in foot-pounds. 



A 5-pound weight at a height of 2 feet possesses 5x2 = 10 

 foot-pounds of energy, since in order to place it there a 

 force of 5 pounds had to be applied through a distance of 

 2 feet. Thus the energy which a body possesses is in general 

 equal to the work which has been done upon it. 



201. Work against friction produces heat. It is a common 

 experience that heat is produced when work is done against 

 friction. Anyone who has attempted to break a wire by 

 bending it back and forth has had occasion to know that it 

 becomes very hot at the place of bending, and the harder 

 and longer one works, the hotter the wire becomes. The 

 same thing results from hammering a nail, and a saw be- 

 comes much heated when in use, particularly if there is 

 much friction. If a wheel is allowed to run for some time 

 without oil on the axle, the latter becomes very much heated. 

 When a match is rubbed on a smooth surface, it is lighted 

 by the heat which is produced by friction. Primitive people 

 started their fires by use of friction between two pieces of 

 wood. In all these cases work is done, and a part of its 

 result appears only in the form of heat. 



202. Energy transformed into heat. Careful studies have 

 shown that in cases like the above the amount of heat devel- 

 oped is in proportion to the amount of energy lost in over- 

 coming friction ; that is, it is proportional to the amount of 

 wasted work. In lifting a weight by means of pulleys a part 

 of the energy which is exerted at the end of the rope goes to 

 the direct raising of the weight and makes its appearance in 

 the form of potential energy in the weight which has been 



