PART Il. 
THE INCOME AND EXPENDITURE OF ENERGY. 
CHAPTER VII. 
FORCE AND ENERGY. 
Force is defined as whatever is capable of changing the rate of 
motion of a mass of matter. When a force acts upon a mass, im- 
parting to it a certain velocity, it does work, the amount of work 
being measured by the product of the force into the distance through 
which it acts. Energy may be defined as the capacity to do work. 
Any mass of matter which can act upon another mass in such a 
way as to change its rate of motion is said to possess energy. 
Kinetic anD PorentTiaAL Enrercy.—In studying energy we 
distinguish between kinetic energy, or the energy due to motion, 
and potential energy, or the energy due to position. The falling 
weight of a pile-driver at the instant it strikes the pile possesses a 
certain amount of kinetic energy and does a corresponding amount 
of work on the pile. When it is raised again a certain amount of 
work is done on it, and when it comes to rest at the top of the ma- 
chine a corresponding amount of energy is stored up in it as poten- 
tial energy. As long as the weight is supported at this point it 
does no work, but simply possesses the possibility of doing work. 
When it is allowed to fall again, this potential energy due to its 
position is converted into the actual or kinetic energy of motion, 
and when it reaches the point from which it was raised and strikes 
the pile it does work upon the latter exactly equal to that formerly 
226 
