151. The pulley forms a good introduction to the important subject of
the mechanical powers. But before entering on the discussions of the
next few chapters, it will be necessary for us to explain what is meant
in mechanics by “work,” and by “energy,” which is the capacity for
performing work, and we shall therefore include a short outline of this
subject in the present lecture.
152. The pulley is a machine which is employed for the purpose of
changing the direction of a force. We frequently wish to apply a
force in a different direction from that in which it is convenient to
exert it, and the pulley enables us to do so. We are not now speaking
of these arrangements for increasing power in which pulleys play an
important part; these will be considered in the next lecture: we
at present refer only to change of direction. In fact, as we shall
shortly see, some force is even wasted when the single fixed pulley
is used, so that this machine certainly cannot be called a mechanical
power.
153. The occasions upon which a single fixed pulley is used are
numerous and familiar. Let us suppose a sack of corn has to be elevated
from the lower to one of the upper stories of a building. It may of
course be raised by a man who carries it, but he has to lift his own
weight in addition to that of the sack, and therefore the quantity of
exertion used is greater than absolutely necessary. But supposing there
be a pulley at the top of the building over which a rope passes; then,
if a man attach one end of the rope to the sack and pull the other, he
raises the sack without raising his own weight. The pulley has thus
provided the means by which the downward force has been changed in
direction to an upward force.
154. The weights, ropes, and pulleys which are used in our windows for
counterpoising the weight of the sash afford a very familiar instance
of how a pulley changes the direction of a force. Here the downward
force of the weight is changed by means of the pulley into an upward
force, which nearly counterbalances the weight of the sash.