Biomedical Engineering Reference
In-Depth Information
Fig. 3.3
Bird in
equilibrium, with the center
of gravity below its body
masses placed at the edge of the wings, which displaces its C.G. to outside of the
body, immediately below the beak, more or less at the height of the lead masses.
Now it is possible to understand why a baby does not begin by walking, but
crawls on all fours: one of the reasons is because, with the hands and feet on
the ground, the area where the vertical line passing through the C.G. may fall is the
greatest possible. Besides this, its C.G. will stay closer to the ground in the posture
of crawl rather than in standing, reinforcing the control of equilibrium.
Persons who practice yoga know that the lotus posture (padmasana) is ideal for
meditation, as the body contact area with the ground is large and its C.G. stays near
the ground, allowing great stability.
Exercise 3.1
Analyze each situation illustrated in Fig.
3.1
and determine whether
the body will remain in the represented position or will rotate. In the case of
rotation, specify whether the direction is clockwise or counterclockwise.
Exercise 3.2
Explain why it is impossible to bend over to touch the toes without
bending the knees if, initially, the heels and the back are against the wall. Discuss,
justifying, how this exercise can be done.
Example 3.1
Describe the movements of the center of gravity of a person's body,
initially seated, as this person rises from a chair.
When seated on a chair, the line of action of the weight force, which passes
through the center of gravity, is found to be near the geometric center of the area
delimited by the legs of the chair, which is the area of support, plus the area of the
feet on the ground. To rise, as the line that passes through the C.G. must intercept
the base support that now will be the feet, the person has to bend forward. This can
be facilitated, if the arms are outstretched forward.
3.3 Practical Method to Locate the Center of Gravity
After justifying the importance of knowing the position of the center of gravity, let
us learn how to locate it in any body. Initially, let us consider a practical method
also called the method of the plumb line.
