This is illustrated in the above figure.  The body, A, is supposed to be revolving in the direction indicated by the arrow, in the circle, A B F G, around the center, O, to which it is held by the cord, O A. At the point, A, it is moving in the tangential direction, A D. It would continue to move in this direction, did not the cord, O A, compel it to move in the arc, A C. Should this cord break at the point, A, the body would move; straight on toward D, with whatever velocity it had.

You perceive now what centrifugal force is.  This body is moving in the direction, A D. The centripetal force, exerted through the cord, O A, pulls it aside from this direction of motion.  The body resists this deflection, and this resistance is its centrifugal force.

[Illustration:  Fig. 1]

Centrifugal force is, then, properly defined to be the disposition of a revolving body to move in a straight line, and the resistance which such a body opposes to being drawn aside from a straight line of motion.  The force which draws the revolving body continually to the center, or the deflecting force, is called the centripetal force, and, aside from the 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.