Biomedical Engineering Reference
In-Depth Information
had their leg or their arm amputated, have to retrain their equilibrium, because the
position of their C.G. has been changed.
The difficulties in performing abdominals, with the arms much closer to the
head and at the beginning of the yoga posture called sarvanga-asana with the legs
closer to the ground, were discussed in Exercises 2.2 and 2.3. The main parameter
involved in these situations is the change of the position of the C.G., which
produces longer lever arms.
3.5 Stable, Unstable, and Neutral Equilibrium
The considerations already presented in this chapter allow us to classify briefly the
conditions of equilibrium of a body related to the action of its weight force.
Physically, the degree of body stability depends on four factors, namely:
(a) Height of the center of gravity with regard to the ground—the nearer to the
support area is the C.G., the more stable it is.
(b) Size of the support area—the larger the base area, the more stable it is.
(c) Location of the vertical imaginary line passing through the C.G. in relation to
the support area—the closer the line to the center of the base area, the greater is
the stability.
(d) Weight of the body—a heavier body has greater stability.
The equilibrium of a body depends on the existence, or lack, of a torque of the
weight force that acts at its center of the gravity, as already discussed. If this torque
is zero, the body is said to be in equilibrium. For this, it is only necessary that the
imaginary line passing through the center of gravity passes also through the reaction
point at the support base. There are three types of equilibrium.
When the body and its center of gravity are slightly displaced and both tend to
return to their original position, the body is said to be in stable equilibrium.
This occurs because, in the intermediate situation, the weight force exerts a torque
which causes the clockwise rotation of the body, as can be seen in Fig.
3.10
, which
restores the original position. In this position, the vertical line passing through
the center of gravity passes through the support base which, in this case, is not a
point but the correspondent base area.
If the body and its center of gravity were displaced from their original positions
and did not tend to return, but assume a new position, the body is said to be in
unstable equilibrium. In this case, the torque of the weight force at the intermediate
situation causes clockwise rotation, which will make the body assume a new position,
which is the final one, as shown in Fig.
3.11
. In the intermediate situation, the vertical
line passing through the center of gravity falls outside the base support, rotating the
body and driving it to the final position, caused by the torque of the weight force.
Neutral equilibrium refers to the displacement of the body and its center of
gravity in a way that the torque due to the weight force is always zero about the
support point. This means that the body will remain in any similar position in which
it was placed. This situation is illustrated in Fig.
3.12
.
