Center of Mass
Can Mass be Located at a Point?
Definitions
Center of Gravity is the point at which the
entire mass or weight of the body may be
considered to be concentrated.
Center of Mass is the point in a body or
system of bodies at which, for certain
purposes, the entire mass may be
assumed to be concentrated.
Simply, CoM (COG) is the point around
which the body’s mass is equally
distributed in all directions.
Center of Mass (COM)
Place where all the
moments/torques
acting on the
system are
balanced.
Place where all the
linear forces are
balanced.
Case Studies
Braune and Fischer (1889)
Dempster (1955)
Clauser (1969)
Braune and Fischer (1889)
Classic case
4 cadavers, frozen, steel spits.
Using saw, sliced cadavers in three
planes.
Determined the COG of the adult
human in the anatomic position to
be slightly anterior to the second
sacral vertebra.
Did the same with segments of
body.
Dempster (1955)
8 Males
Age: 52-83 (two ages unknown)
Heights: 155.3 to 186.6 cm (61.1-
73.5 in)
Weights: 486 to 709 N (109-195.5
lbs)
Hips and Shoulders frozen in semi
flexed position
Centers of Mass determined
Clauser (1969)
Purpose: Transfer findings of
interrelationships in cadavers to
living persons.
13 male cadavers dissected into 14
segments
Locating the CoG
Balance Method
Reaction Board Method
Body Segment Inertial Parameters
Method
Location of COG in Humans
Since the human body is not rigid,
the location of the COG is
dependent on the location of the
limbs.
Intersection of three planes locates
COG.
The location is about 55-57% of
your standing height.
Body Segment Inertial Parameters
Examples of CoM
COM and Stability
Stability is the capacity of an object
to return to equilibrium or to its
original position after being
displaced.
Question
What factors affect stability?
Factors that affect stability
Base of Support: Described by
those parts of the body in contact
with supporting surfaces and the
intervening area.
Height of the CoG above the base of
support.
Weight of the object
The relationship between the BoS
and the Line of Gravity.
Questions to Ponder
How does your stability change with
the use of crutches? How does the
BoS change?
Using concepts from this lecture,
which hand would you recommend
that a person put a cane in, given
that his right knee is injured and he
cannot fully put his weight on that
limb.