LEVERS
A lever is a rigid bar that rotates around a fixed point called the fulcrum. The bar may
be either straight or curved. In use, a lever has both an applied force and a resistance
force.
The mechanical advantage of a lever is the ratio of the length of the lever on the
applied force side of the fulcrum to the length of the lever on the resistance force side
of the fulcrum. The mechanical advantage of the lever below is 10:1. Therefore, an
applied force of 10 pounds will balance a resistance force of 100 pounds. However,
the applied force end of the lever must move 10 feet for every one foot the resistance
force s raised.
There are three different classes of levers. The class of a lever is determined by the
location of the applied and resistance forces relative to the fulcrum. Each of the three
classes of levers will be discussed next.
First-Class Lever. In a first-class lever the fulcrum is located at some point between
the effort and resistance forces. Common examples of first-class levers include
crowbars, scissors, pliers, tin snips and seesaws.
A first-class lever always changes the direction of force (I.e. a downward effort force
on the lever results in an upward movement of the resistance force). A first-class lever
is illustrated below:
With a first-class lever, when the fulcrum is closer to the resistance, the output force
is increased. However, there is a corresponding decrease in both output speed and
distance. Conversely, when the fulcrum is closer tot he effort, the output force is
decreased and there is a corresponding increase in both output speed and distance.
Second-Class Levers. With a second-class lever, the resistance is located between the
fulcrum and the effort force. Common examples of second-class levers include nut
crackers, wheel barrows, and bottle openers. A second-class lever does not change the
direction of force. When the fulcrum is located closer to the resistance than to the
force, an increase in force (mechanical advantage) results. A second-class lever is
illustrated below:
Third-Class Lever. With a third-class lever, the effort force is applied between the
fulcrum and the resistance force. Examples of third-class levers include tweezers, ice
tongs, and shovels. A third-class lever does not change the direction of force; thirdclass levers always produce a gain in speed and distance and a corresponding decrease
in force. A third-class lever is illustrated below.