Identity of Energy. 485 
The two forms of energy correspond to the factors in the 
product work *. 
“ Potential’’ energy correspond to I’, 
“ Kinetic ”’ energy correspond to s. 
But is this quite true and satisfactory? A strained bow is 
exerting force and possesses energy. A pillar supporting a 
roof is exerting force, but possesses no energy. What is the 
difference between the two cases? It is evident that some- 
thing more is needed than mere force. 
The difference of course is that the bow can recoil, it has a 
range or distance through which it will continue to exert a 
force: not the originai force, but still some force. The pillar 
is exerting a great force but it has no recoil in it; if released 
it would at once cease to exert any force; consequently its 
energy is minute. 
Thus, then, for a body to possess potential energy we must 
have two things—the exertion of a force, together with a 
guarantee that that force shall be exerted over a certain dis- 
tance; 7.¢. a continuance of the force even after motion is 
permitted. 
And this is quite analogous to what may be said of the 
other form of energy. “‘ A body in motion possesses energy ;” 
but is it so necessarily ? Cannot a body in motion be conceived 
as possessing no energy? Suppose it stops the instant you 
give it work to do—the instant you make it exert force. It 
is evident you must have not merely motion, you must have 
a guarantee of persistence of motion, the body must possess 
inertia; the motion must continue over a certain range even 
against resistance. Hence we may exhibit the relation be- 
tween the two forms thus :— 
Kinetic energy corresponds to motion combined with inertia, 
so that the motion shall continue even against some force; 
and 
Potential energy corresponds to force combined with 
elasticity (or something like it), so that the force shall 
continue even though motion be permitted. 
Both forms of energy are potential work, but, gua energy, 
one is as real and actual as the other. Hach has a factor 
missing, which if supplied, work will at once be done. 
Kinetic energy requires the Force factor todo work. Potential 
energy requires the Motion factor to do work. 
An important thing is now evident moreover, a_ thing 
which I have never seen accepted, though it has been pre- 
* Of. Lodge on “ Forms of Energy,’ Phil. Mag. October 1879, p. 281, 
and June 1881, p. 531. Also ‘Elementary Mechanics’ (Chambers), 
§ 84, 
