weight puts it in a position where gravity can cause it to move (Fig. 81).
As a consequence _the raising of bodies above the earth’s surface is a
means of storing energy_—the energy remaining stored until the bodies
fall. As they fall, the stored-up (potential) energy becomes kinetic and
can be made to do work.

2. _Storing of Energy through Elasticity._—Energy is stored also by doing
work in opposition to elasticity, as in bending a bow or in winding a
clock spring. The bending, twisting, stretching, or compressing of elastic
substances puts them in a condition of _strain_ which causes them to exert
a pressure (called elastic force) that tends to restore them to their
former condition. Energy stored by this means becomes active as the
distorted or compressed substance returns to its former shape or volume.

These simple methods of storing energy will serve to illustrate the
general principles upon which such storage depends:

1. To store energy, energy must be expended, or work done.

2. The work must be against some force, such as gravity or elasticity,
which can undo the work, i.e., bring about an effect opposite to that of
the work.

3. The stored energy becomes active (kinetic) as the force through which
the energy was stored undoes the work, or puts the substance upon which
the work was done into its former condition (gravity causing bodies to
fall, etc.).

These principles are further illustrated by the