Simple Machines - Biomechanics of the Human Body - page 78

Exercise 5.3 Draw a sketch of each of the quoted second class levers and the
corresponding diagram of forces, identifying the involved forces, their arms and the
rotation axis (fulcrum or pivot).

5.4.3 Third Class Levers

In these levers, the action force is applied between the fulcrum and the load.
Figure 5.3 shows the typical representation of levers of this category.
Observe that, in the representation of third class levers, the applied force has a
larger magnitude than that of the load because the arm of the action force is smaller
than that of the load. In other words, the effect of the action force is reduced.
Tweezers and salad or pasta tongs are good examples of this type of lever. Third
class levers are predominant in the human body. These lever systems are designed
for increasing the speed of motion rather than increasing load capabilities.

Exercise 5.4 Draw a sketch of each of the quoted third class levers and the
corresponding diagram of forces, identifying the involved forces, their arms, and
the rotation axis (fulcrum or pivot).

5.4.4 Mechanical Advantage

In representations utilized for levers, we tried to make explicit the configurations
that amplify or reduce the effect of the action force. We note that in the case of third
class levers, the magnitude of the action force is greater than that of the resistance
force. The contrary occurs in the case of second class levers. In case of first class
levers, the magnitude of forces will depend on its construction or the way it is used.
It is verified that, for the forces considered, the determinant parameter will be the
arm of each force.
Let us imagine the situation of equilibrium of a lever, that is, in which there is
equality between the torques of the action force and that of the resistance force ( 5.2 ):

F A d A ¼ F R d R :

(5.2)

This equality shows that a force with a smaller magnitude can produce the same
torque as that of a larger magnitude force, just by increasing its arm. Then, we
define what is called mechanical advantage, MA, which corresponds to the ratio
between the resistance force and the action force ( 5.3 ), that is, how many times the
resistance force is larger than the action force:

F R
F A :

¼

MA (5.3)

So, as the mechanical advantage becomes larger, the necessary effort for a
determined task becomes smaller and vice-versa. The mechanical advantage of