Figure 91 (p. 154), the force arm will be the distance from the stone
or fulcrum to the end of the bar, and the weight arm will be the
distance from the fulcrum to the bowlder itself. The man pushes down
with a force of 100 pounds, but with that amount succeeds in prying up
the 600-pound bowlder. If, however, you look carefully, you will see
that the force arm is 6 times as long as the weight arm, so that the
smaller force is compensated for by the greater distance through which
it acts.

At first sight it seems as though the man's work were done for him by
the machine. But this is not so. The man must lower his end of the
lever 3 feet in order to raise the bowlder 6 inches out of the ground.
He does not at any time exert a large force, but he accomplishes his
purpose by exerting a small force continuously through a
correspondingly greater distance. He finds it easier to exert a force
of 100 pounds continuously until his end has moved 3 feet rather than
to exert a force of 600 pounds on the bowlder and move it 6 inches.

By the time the stone has been raised the man has done as much work as
though the stone had been raised directly, but his inability to put
forth sufficient muscular force to raise the bowlder directly would
have rendered impossible a result which was easily accomplished when
through the medium of the lever he could extend his small force
through greater distance.

154. The Wheelbarrow as a Lever. The principle of the lever is
always the same; but the relative position of the important points may
vary. For example, the fulcrum is sometimes at one end, the force at
the opposite end, and the weight to be lifted between them.

[Illustration: FIG. 98.--A slightly different form of lever.]

Suspend a stick with a hole at its center as in Figure 98, and hang a
4-pound weight at a distance of 1 foot from the fulcrum, supporting
the load by means of a spring balance 2 feet from the fulcrum. The
pointer on the spring balance shows that the force required to balance
the 4-pound load is but 2 pounds.

The force is 2 feet from the fulcrum, and the weight (4) is 1 foot
from the fulcrum, so that

Force x distance = Weight x distance,
or 2 x 2 = 4 x 1.

Move the 4-pound weight so that it is very near the fulcrum, say but 6
inches from it; then the spring balance registers a force only one
fourth as great as the weight which it suspends. In other words a
force of 1 at a distance of 24 inche