2. It is necessary to know the answer to this question, What is a
force? People who have not studied mechanics occasionally reply, A
push is a force, a steam-engine is a force, a horse pulling a cart
is a force, gravitation is a force, a movement is a force, &c., &c.
The true definition of force is _that which tends to produce or to
destroy motion_. You may probably not fully understand this until some
further explanations and illustrations shall have been given; but, at
all events, put any other notion of force out of your mind. Whenever I
use the word Force, do you think of the words “something which tends
to produce or to destroy motion,” and I trust before the close of the
lecture you will understand how admirably the definition conveys what
force really is.
3. When a string is attached to this small weight, I can, by pulling
the string, move the weight along the table. In this case, there is
something transmitted from my hand along the string to the weight in
consequence of which the weight moves: that something is a force. I
can also move the weight by pushing it with a stick, because force
is transmitted along the stick, and makes itself known by producing
motion. The archer who has bent his bow and holds the arrow between his
finger and thumb feels the string pulling until the impatient arrow
darts off. Here motion has been produced by the force of elasticity in
the bent bow. Before he released the arrow there was no motion, yet
still the bow was exerting force and _tending_ to produce motion. Hence
in defining force we must say “that which _tends_ to produce motion,”
whether motion shall actually result or not.
4. But forces may also be recognized by their capability or tendency to
prevent or to destroy motion. Before I release the arrow I am conscious
of exerting a force upon it in order to counteract the pull of the
string. Here my force is merely manifested by _destroying the motion_
that, if it were absent, the bow would produce. So when I hold a weight
in my hand, the force exerted by my hand destroys the motion that the
weight would acquire were I to let it fall; and if a weight greater
than I could support were placed in my hand, my efforts to sustain it
would still be properly called force, because they _tended_ to destroy
motion, though unsuccessfully. We see by these simple cases that a
force may be recognized either by producing motion or by trying to
produce it, by destroying motion or by tending to destroy it; and hence
the propriety of the definition of force must be admitted.