24. _Q._--What do you understand by centrifugal and centripetal forces?

_A._--By centrifugal force, I understand the force with which a revolving
body tends to fly from the centre; and by centripetal force, I understand
any force which draws it to the centre, or counteracts the centrifugal
tendency. In the conical pendulum, or steam engine governor, which consists
of two metal balls suspended on rods hung from the end of a vertical
revolving shaft, the centrifugal force is manifested by the divergence of
the balls, when the shaft is put into revolution; and the centripetal
force, which in this instance is gravity, predominates so soon as the
velocity is arrested; for the arms then collapse and hang by the side of
the shaft.

25. _Q._--What measures are there of the centrifugal force of bodies
revolving in a circle?

_A._--The centrifugal force of bodies revolving in a circle increases as
the diameter of the circle, if the number of revolutions remain the same.
If there be two fly-wheels of the same weight, and making the same number
of revolutions per minute, but the diameter of one be double that of the
other, the larger will have double the amount of centrifugal force. The
centrifugal force of the _same wheel_, however, increases as the square of
the velocity; so that if the velocity of a fly-wheel be doubled, it will
have four times the amount of centrifugal force.

26. _Q._--Can you give a rule for determining the centrifugal force of a
body of a given weight moving with a given velocity in a circle of a given
diameter?