FORCE AND ENERGY. 227 
stored up in the weight as potential energy, which again was equal 
to the energy expended in raising it. 
An even simpler example of the conversion of potential into 
kinetic energy and vice versa is a swinging pendulum. When at 
rest for an instant at the end of a vibration it possesses a certain 
amount of potential energy, corresponding to its vertical height 
above the lowest point of its arc. When it reaches this lowest 
point, so far as the mechanism of which it forms part is concerned 
it has no more potential energy because it cannot fall any farther. 
In place of this, however, neglecting mechanical resistances, it con- 
tains an exactly equivalent amount of kinetic energy, due to its 
motion. During the second half of the swing this kinetic energy 
is expended in again raising the pendulum, and when it has all been 
expended the pendulum will (in the absence of external resistance) 
have been raised to exactly the same height as before above its 
lowest point. In other words, its kinetic energy will have been re- 
converted into an equivalent amount of potential energy and so 
the alternate conversion and re-conversion goes on as long as the 
pendulum continues to swing. 
The same facts which have been illustrated above in the case of 
the motion of visible masses of matter are likewise true of molecular 
and atomic motions. When molecules of carbon dioxide and water 
are converted into starch in the green leaves of the plant, work is 
done upon them by the energy of the sun’s rays. Their constituent 
atoms are forced apart and compelled to assume new groupings. In 
this process a certain amount of kinetic energy has disappeared 
and the resulting system of starch molecules and oxygen molecules 
contains a corresponding amount of potential energy. Under 
suitable conditions the reverse process may also take place. The 
atoms may, so to speak, fall together and resume their old positions, 
producing the original amounts of carbon dioxide and water and 
giving off in the process the exact amount of kinetic energy which 
was originally absorbed. This energy may appear in the form of 
heat, as in ordinary combustion, or in any other of the various 
forms of energy, according to circumstances. 
The last example is but an illustration of the general fact that 
in every chemical reaction there occurs a transformation of energy 
which most commonly takes the form of an evolution or absorption 
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