Biomedical Engineering Reference
In-Depth Information
Fig. 5.3
Representation
of the third class lever,
denoting the position of the
axis of rotation in relation to
the force of action and of
resistance, and their arms.
An example of a third class
lever used to support a
heavy object is also
represented in a diagram
F
A
d
A
axis
d
R
F
R
of the bar was not taken into account, being considered negligible, because
otherwise the torque due to the weight of the bar must be considered a pair of
scissors, a pair of pliers, a seesaw, a nail puller etc. are examples of first class levers.
To make explicit the way these levers are constructed, we represent in a typical
model the force of action with a smaller magnitude than that of the resistance
because the arm of the applied force was designed to be larger than that of the load.
In other words, there is an amplification of the effect of the action force.
Exercise 5.1
Draw a sketch of each one of the quoted first class levers and the
corresponding diagram of forces, identifying not only the forces involved but their
arms and the rotation axis (fulcrum or pivot).
Exercise 5.2
Indicate in the example to the right of Fig.
5.1
the corresponding
distances of the arms of force of action and of resistance.
5.4.2 Second Class Levers
In this category of levers, the load is between the fulcrum and the action force.
Its generic representation can be seen in Fig.
5.2
.
Normally we want to amplify the effect of our action force by the use of this type
of lever. Therefore, in a typical representation, the magnitude of the action force
is drawn smaller than that of the load, because the arm of the applied force has been
designed to be larger than the arm of the load. Second class levers are probably the
more numerous. Some examples are: screwdrivers, doorknobs, tire-irons, door
keys, wheelbarrows, can opener, pedals, flywheels etc.
