At the conclusion of this chapter, the student should be able
- 1. Define the term center of gravity, and
explain the basis for its location in the human body.
- 2. Estimate the location of the center of gravity of individuals
in any position.
- 3. State the principles of equilibrium, and explain and demonstrate
applications of each.
- 4. Locate the center of gravity of an individual using either
the reaction board or the segmental method.
Center of Gravity
The center of gravity of a body
is sometimes described as its balance point or that point about
which a body would balance without a tendency to rotate. For this
reason, the center of gravity is often identified as the point where
all the weight of the body or object is concentrated. More accurately,
it is the point where the weight of the body may be said to act.
The ability to locate the center of gravity of a body is based
on the knowledge of what it takes for a system to be balanced, or
in equilibrium. Two conditions must be met:
- 1. All the linear forces acting on the body must be balanced.
- 2. All the rotary forces (torques) must be balanced.
Another way of expressing these necessary conditions for equilibrium
is to say that the sum of all the forces acting on the body must
equal zero. If there is a downward-directed linear force, there must
be an equal upward force so that the vector sum of these forces
equals zero. If there is a negative clockwise torque, it must be
canceled out by a positive counterclockwise torque of equal magnitude
The center of gravity of a body is the point where all
forces acting on the body equal zero. In this illustration it is
represented as the intersection of the x,
y, and z axes. It may be located
by application of the principle of torques.
A simple experiment to locate the center of gravity, or balance
point, consists of suspending an irregularly shaped object by a
string and letting it hang until it ceases to move (Figure 14.2).
A vertical line is drawn on the object from the point of suspension
in a line that is a continuation of the string. The object is then
suspended from another point, and the vertical string continuation line
is drawn again. This procedure is repeated one more time. The point
at which the three drawn lines intersect is the center of gravity.
If the object is suspended from this point, it will hang in whatever
position it is placed because the weight of the object is equally
distributed about this point and no unbalanced forces (torques)