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Hence the idea of the conservation of force, as Opposed
to the destruction of force, which was supposed to occur
when inelastic bodies met in collision.

We now know that the principle of conservation holds
equally good with elastic and unelastic bodies. Perfectly
elastic bodies develop no heat on collision. They retain
their motion afterward, though its direction may be changed ;
and it is only when sensible motion is, in whole or in part,
destroyed that heat is generated. This always occurs in
unelastic collision, the heat developed being the exact
equivalent of the motion extinguished. This heat virtually
declares that the property of elasticity, denied to the masses,
exists among their atoms, and by their recoil and oscillation
the principle of conservation is vindicated.

But ambiguity in the use of the term “ force ” has been
for some time more and more creeping upon us. We called
the attraction of gravity a force without any reference to
motion. A body resting on a shelf is as much pulled by
gravity as when after having been pushed off the shelf it
falls toward the earth. We applied the term force also to
that molecular attraction which we called chemical affinity.
When, however, we spoke of the conservation of force in
the case of elastic collision, we meant neither a pull nor a
push, which, as just indicated, might be exerted upon inert
matter, but we meant the movingforce, if I may use the
term, of the colliding masses.

What I have called moving force has a definite me-
chanical measure in the amount of work that it can perform.
The simplest form of work is the raising of a weight. A
man walking up-hill or up-stairs with a pound weight in
his hand, to an elevation say of sixteen feet, performs a cer-
tain amount of work over and above the lifting Of his own
body. If he ascend to a height of thirty-two feet, he does
twice the work; if to a height of forty-eight feet, he does
three times the work; if to sixty-four feet, he does four

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