(282.) It will in all cases be found, that that quantity by which the
weight exceeds the power is supported by fixed points; and therefore,
although it be commonly stated that a small power supports a great
weight, yet in the pulley, as in all other machines, the power supports
no more of the weight than is exactly equal to its own amount. It
will not be necessary to establish this in each of the examples which
have been given: having explained it in one instance, the student
will find no difficulty in applying the same reasoning to others. In
_fig. 126._, the fixed pulley sustains a force equal to twice the
power, and by it the power giving tension to the first rope sustains a
part of the weight equal to itself. The first hook sustains a portion
of the weight equal to the tension of the first string, or to the
power. The second hook sustains a force equal to twice the power; and
the third hook sustains a force equal to four times the power. The
three hooks therefore sustain a portion of the weight equal to seven
times the power; and the weight itself being eight times the power, it
is evident that the part of the weight which remains to be supported by
the power is equal to the power itself.
(283.) When a weight is raised by any of the systems of pulleys which
have been last described, the proportion between the velocity of
the weight and the velocity of the power, so frequently noticed in
other machines, will always be observed. In the system of pulleys
represented in _fig. 126._ the weight being eight times the power,
the velocity of the power will be eight times that of the weight. If
the power be moved through eight feet, that part of the rope between
the fixed pulley and the first moveable pulley will be shortened by
eight feet. And since the two parts which lie above the first moveable
pulley must be equally shortened, each will be diminished by four feet;
therefore the first pulley will rise through four feet while the power
moves through eight feet. In the same way it may be shown, that while
the first pulley moves through four feet, the second moves through two;
and while the second moves through two, the third, to which the weight
is attached, is raised through one foot. While the power, therefore, is
carried through eight feet, the weight is moved through one foot.
By reasoning similar to this, it may be shown that the space through
which the power is moved in every case is as many times greater than
the height through which the weight is raised, as the weight is greater
than the power.
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