306 
Now I grant that Kinetic Energy and Vis vim have the same meaning ; 
but motion is neither the one nor the other. It is perfectly true that if you 
have two equal bodies moving, one at the rate of one foot in a second, and 
the other at the rate of two feet in a second, the one moving two feet will 
have four times the Kinetic Energy or Vis viva of the other ; but that is a 
different thing from having four times the motion of the other. Motion 
appears to me to have a meaning perfectly distinct from that of Kinetic 
Energy, or Vis viva. Professor Birks says : — 
“ Let us admit the power of this name, Energy, to fuse into one total 
unchangeable and indestructible, these unlike elements, Potential Energy, or 
force, and Kinetic Energy, or motion.” 
Now, potential energy is not force, and force is not potential energy. 
Kinetic energy is not motion, and motion is not kinetic energy. If you 
assume that potential energy and force are interchangeable terms, and that 
kinetic energy and motion are interchangeable terms, you get into a confu- 
sion from which it is very easy to show contradictions ; but as a matter of 
fact they are totally different things, and I cannot illustrate this more 
forcibly to your minds than by giving an example. Suppose I have two 
balls of equal size in my hand, and let them drop together ; they reach the 
earth at the same instant of time, if they are dropped at the same instant. 
We should say that those balls had the same motion. They reach the earth 
at the same instant, travelling side by side, in exactly the same time. But 
let us vary the experiment, and put a sheet of glass on the ground under my 
hand. Let me drop one ball, and it rebounds harmlessly ; then let me drop 
the other, and it breaks the glass. That is not the effect of the motion, but 
of the kinetic energy which the balls respectively possessed : the first hap- 
pened to be a ball of soft wood, and the other a ball of iron or lead. Now, 
although those balls may have had the same motion, they possess very 
different amounts of kinetic energy, or, according to my own definition, a 
very different power of doing work. One has power of doing work in 
smashing the glass which the other has not, and that depends on the amount 
of energy or work which it has acquired. Energy — ivipycia — simply means 
work, and the amount of work in each of these bodies is measured by the mass 
multiplied by the square of its velocity, and inasmuch as there is much more 
mass in the leaden than in the wooden ball, it has in the same proportion so 
much more kinetic energy, and does work which the wooden ball is incapable 
of. This, I think, points out a clear mental conception of the difference 
between motion and energy. Motion, as I conceive it, is one thing ; energy 
is a totally distinct thing. Professor Birks, at page 291, gives us a humorous 
illustration of “ kinetic energy” and “ potential energy” as applied to street 
population. He will forgive mo if I quote in reply the saying of a German 
author “If wisdom be attired in the parti-coloured garb of folly, for the 
purpose of exciting ridicule, the ridicule is due to the garb and not to the 
wearer.” Then Professor Birks says : — 
“ Fallacy the third. This total Energy, said to be invariable, is the sum 
of the actual motions.” 
