impelling and retarding forces which act in the direction of its motion,
the centripetal force is, dynamically speaking, the only force which is
exerted on the body.

It is true, the resistance of the body furnishes the measure of the
centripetal force. That is, the centripetal force must be exerted in a
degree sufficient to overcome this resistance, if the body is to move in
the circular path. In this respect, however, this case does not differ
from every other case of the exertion of force. Force is always exerted
to overcome resistance: otherwise it could not be exerted. And the
resistance always furnishes the exact measure of the force. I wish to
make it entirely clear, that in the dynamical sense of the term "force,"
there is no such thing as centrifugal force. The dynamical force, that
which produces motion, is the centripetal force, drawing the body
continually from the tangential direction, toward the center; and what
is termed centrifugal force is merely the resistance which the body
opposes to this deflection, _precisely like any other resistance to a
force_.

The centripetal force is exerted on the radial line, as on the line, A
O, Fig. 1, at right angles with the direction in which the body is
moving; and draws it directly toward the center. It is, therefore,
necessary that the resistance to this force shall also be exerted on the
same line, in the opposite direction, or directly from the center. But
this resistance has not the least power or tendency to produce motion in
the direction in which it is exerted, any more than any other resistance
has.

We have been supposing a body to be firmly held to the center, so as to
be compelled to revolve about it in a fixed path. But the bond which