The center of gravity is a geometric property of any object.
The center of gravity is the average location of the
weight
of an object. We can completely describe the
motion
of any object through space in terms of the translation of
the center of gravity of the object from one place to another, and
the rotation of the object about its center of gravity if it
is free to rotate. If the object is confined to rotate about some
other point, like a hinge, we can still describe its motion.
In flight, both
airplanes
and
rockets
rotate about their centers of gravity.
A kite, on the other hand, rotates about the
bridle point.
But the
trim
of a kite still depends on the location of the
center of gravity
relative to the bridle point, because for every object the weight always acts through the
center of gravity.
Determining the center of gravity is very important
for any flying object.
How do engineers determine the location of the center of
gravity for an aircraft which they are designing?
In general, determining the center of gravity (cg) is a complicated
procedure because the mass (and weight) may not be uniformly distributed
throughout the object. The general case requires the use of calculus
which we will discuss at the bottom of this page.
If the mass is uniformly distributed, the problem is greatly simplified.
If the object has a line (or plane) of symmetry, the cg lies
on the line of symmetry.
For a
solid block of uniform material, the center of gravity is simply
at the average location of the
physical dimensions. (For a rectangular block, 50 X 20 X 10,
the center of gravity is at the point (25,10, 5) ).
For a triangle of height h, the cg is at h/3, and for a semicircle of radius
r, the cg is at (4*r/(3*pi)) where pi is ratio of the circumference of the
circle to the diameter. There are tables of the location of the center of gravity
for many simple shapes in math and science books. The tables were generated
by using the equation from calculus shown on the slide.
For a general shaped object, there is a simple mechanical way to
determine the center of gravity:
 If we just balance the object using
a string or an edge, the point at which the object
is balanced is the center of gravity. (Just like balancing a
pencil on your finger!)
 Another, more complicated way, is a two step method shown on
the slide. In Step 1, you hang the object from any
point and you drop a weighted
string from the same point. Draw a line on the object along the
string. For Step 2, repeat the procedure from another point on the object
You now have two lines drawn on the object which intersect.
The center of gravity is the point where the lines intersect. This
procedure works well for irregularly shaped objects that are hard
to balance.
If the mass of the object is not uniformly distributed, we must use calculus
to determine center of gravity.
We will use the symbol S dw to denote the integration of a continuous
function with respect to weight. Then the center of gravity can be determined from:
cg * W = S x dw
where x is the distance from a reference line, dw is an
increment of weight, and
W is the total weight of the object.
To evaluate the right side, we have to determine how the weight varies
geometrically. From the
weight equation, we know that:
w = m * g
where m is the mass of the object, and g is the gravitational
constant. In turn, the mass m of any object is equal to the
density, rho,
of the object times the
volume, V:
m = rho * V
We can combine the last two equations:
w = g * rho * V
then
dw = g * rho * dV
dw = g * rho(x,y,z) * dx dy dz
If we have a functional form for the mass distribution, we can solve the
equation for the center of gravity:
cg * W = g * SSS x * rho(x,y,z) dx dy dz
where SSS indicates a triple integral over dx. dy. and dz.
If we don't know the functional form of the mass distribution,
we can numerically integrate the equation using a spreadsheet.
Divide the distance into a number of small volume segments and
determining the average value of the weight/volume (density times gravity) over
that small segment. Taking the sum of the average value of the weight/volume
times the distance times the volume segment
divided by the weight will produce the center of gravity.
You can view a short
movie
of "Orville and Wilbur Wright" explaining how the center of gravity
affected the flight of their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.
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