Lifting heavy weights through great distances is not the only way in
which work is done. Painting, chopping wood, hammering, plowing,
washing, scrubbing, sewing, are all forms of work. In painting, the
moving brush spreads paint over a surface; in chopping wood, the
descending ax cleaves the wood asunder; in scrubbing, the wet mop
rubbed over the floor carries dirt away; in every conceivable form of
work, force and motion occur.

A man does work when he walks, a woman does work when she rocks in a
chair--although here the work is less than in walking. On a windy day
the work done in walking is greater than normal. The wind resists our
progress, and we must exert more force in order to cover the same
distance. Walking through a plowed or rough field is much more tiring
than to walk on a smooth road, because, while the distance covered may
be the same, the effort put forth is greater, and hence more work is
done. Always the greater the resistance encountered, the greater the
force required, and hence the greater the work done.

The work done by a boy who raises a 5-pound knapsack to his shoulder
would be 5x4, or 20, providing his shoulders were 4 feet from the
ground.

The amount of work done depends upon the force used and the distance
covered (sometimes called displacement), and hence we can say that

Work = force multiplied by distance,
or _W = f x d_.

151. Machines. A glance into our machine shops, our factories, and
even our homes shows how widespread is the use of complex machinery.
But all machines, however complicated in appearance, are in reality
but modifications and combinations of one or more of four simple
machines devised long ago by our remote ancestors. These simple
devices are known to-day, as (1) the lever, represented by a crowbar,
a pitchfork; (2) the inclined plane, represented by the plank upon
which barrels are rolled into a wagon; (3) the pulley, represented by
almost any contrivance for the raising of furniture to upper stories;
(4) the wheel and axle, represented by cogwheels and coffee grinders.

[Illustration: FIG. 95.--Primitive method of grinding corn.]

Suppose a 600-pound bowlder which is embedded in the ground is needed
for the tower of a building. The problem of the builder is to get the
heavy bowlder out of the ground, to load it on a wagon for
transportation, and finally to raise it to the tower. Obviously, he
cannot do this alone; the greatest amount of force of w