Biomedical Engineering Reference
In-Depth Information
3. However, the vertical line which passes through the C.G. is displaced from the
center of the support base, being closer to the hips. Therefore, a general tendency
in this posture is to bend forward, thus increasing the
x
C.G.
, in an attempt to make
the vertical line through the C.G. approach the center of the base support.
Exercise 3.8
Now, analyze and justify the degree of difficulty in remaining lying
on the floor with the legs lifted until the toes are pointed straight up, as illustrated in
the figure of Exercise 3.8. This posture is the same as in Example 3.5, but with a 90
rotation. Which posture is easier, that of Example 3.5 or of this exercise?
3.6 Motion of the Center of Gravity
Consider a racket in translational motion, as shown in Fig.
3.13a
. When a body
experiences translational motion, every point of the body has the same displace-
ment in any small time interval as its center of gravity. Therefore, the motion of the
center of gravity of the body is called translational motion of a body.
However, when the same body rotates and whirls about an axis while in motion
as in the case of a racket thrown by one person to another, as shown in Fig.
3.13b
,
only the center of gravity moves through a simple parabolic trajectory. Each
elementary mass performs a different motion, and the trajectory is not as simple
as that of the center of gravity.
When a diver jumps off a trampoline, he changes the shape of the body during
the fall and each body part performs a different trajectory, complicated, but the
trajectory of the center of gravity is described by a parabola, which can be written
mathematically.
