back away from the wheel, would it be easier or
figure 4-14 applies his effort on the handles of
harder to lift?
the oars. The oarlock acts as the fulcrum, and the
water acts as the resistance to be overcome. In
Both first- and second-class levers are
this case, as in figure 4-12, the force is applied
commonly used to help in overcoming big
on one side of the fulcrum and the resistance to
resistances with a relatively small effort.
be overcome is applied to the opposite side, hence
this is a first-class lever. Crowbars, shears, and
pliers are common examples of this class of lever.
Occasionally you will want to speed up the
movement of the resistance even though you have
The second-class lever (fig. 4-13, view B) has
to use a large amount of effort. Levers that help
the fulcrum at one end; the effort is applied at
you accomplish this are third-class levers. As
the other end. The resistance is somewhere
shown in figure 4-13, view C, the fulcrum is at
between these points. The wheelbarrow in
figure 4-15 is a good example of a second-class
to be overcome is at the other end, with the
lever. If you apply 50 pounds of effort to the
effort applied at some point between. You can
handles of a wheelbarrow 4 feet from the fulcrum
always spot third-class levers because you will find
(wheel), you can lift 200 pounds of weight 1 foot
the effort applied between the fulcrum and the
from the fulcrum. If the load were placed further
resistance. Look at figure 4-16. It is easy to see
that while point E is moving the short distance
(e), the resistance (R) has been moved a greater
distance (r). The speed of R must have been
greater than that of E since R covered a greater
distance in the same length of time.
Your arm (fig. 4-17) is a third-class lever. It
is this lever action that makes it possible for you
to flex your arms so quickly. Your elbow is the
fulcrum. Your biceps muscle, which ties into your
forearm about an inch below the elbow, applies
the effort; and your hand is the resistance, located
some 18 inches from the fulcrum. In the split
second it takes your biceps muscle to contract an
inch, your hand has moved through an 18-inch
arc. You know from experience that it takes a big
Figure 4-14.--Oars are levers.
pull at E to overcome a relatively small resistance
at R. Just to remind yourself of this principle, try
closing a door by pushing on it about 3 or 4 inches
from the hinges (fulcrum). The moral is, you
don't use third-class levers to do heavy jobs, you
use them to gain speed.
Figure 4-16.--A third-class lever.
Figure 4-15.--This makes it easier.