1. Chapter 9 – Center of Gravity and Centroids (9.2 only)
Note, center of gravity location = centroid location for homogenous material (density or specific weight is constant throughout the body).

2. APPLICATIONS (continued)
One concern about a sport utility vehicle (SUV) is that it might tip over while taking a sharp turn.
One of the important factors in determining its stability is the SUV’s center of mass.
Should it be higher or lower to make a SUV more stable?
How do you determine the location of the SUV’s center of mass?

3. APPLICATIONS (continued)
To design the ground support structure for the goal post, it is critical to find total weight of the structure and the center of gravity location.
Integration must be used to determine total weight of the goal post due to the curvature of the supporting member.
How do you determine the location of its center of gravity?

4. CONCEPT OF CENTER OF GRAVITY (CG)
A body is composed of an infinite number of
particles, and so if the body is located within a
gravitational field, then each of these particles will have a weight dW.
The center of gravity (CG) is a point, often shown as G, which locates the resultant weight of a system of particles or a solid body.
From the definition of a resultant force, the sum of moments due to individual particle weighst about any point is the same as the moment due to the resultant weight located at G.
Also, note that the sum of moments due to the individual particle’s weights about point G is equal to zero.

5. CONCEPT OF CG (continued)
The location of the center of gravity, measured from the y axis, is determined by equating the moment of W about the y axis to the sum of the moments of the weights of the particles about this same axis. ~
x W =  x dW _
If dW is located at point (x, y, z), then
Similarly,
y W =  y dW
z W =  z dW
Therefore, the location of the center of gravity G with respect to the x, y,z axes becomes

6. CONCEPT OF CENTROID ~ x = ( A x dA ) / ( A dA ) ~
The centroid, C, is a point which defines the geometric center of an object.
The centroid coincides with the center of mass or the center of gravity only if the material of the body is homogenous (density or specific weight is constant throughout the body).
If an object has an axis of symmetry, then the centroid of object lies on that axis.
In some cases, the centroid is not located on the object (u-shape).
Equations: ~
x = ( A x dA ) / ( A dA ) ~
y = ( A y dA ) / ( A dA )

7. Centroids of Common Areas
Note: Centroid = CG for homogenous body!!!
Centroids of Common Areas

8. Centroids of Common Areas

9. COMPOSITE BODIES
The I-beam or tee-beam shown are commonly used in building various types of structures.
When doing a stress or deflection analysis for a beam, the location of the centroid is very important.
How can we easily determine the location of the centroid for a given beam shape?
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Section 9.3

10. CONCEPT OF A COMPOSITE BODY
Many industrial objects can be considered as composite bodies made up of a series of connected “simple” shaped parts or holes, like a rectangle, triangle, and semicircle.
Knowing the location of the centroid, C, or center of gravity, G, of the simple shaped parts, we can easily determine the location of the C or G for the more complex composite body.
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Section 9.3

11. CONCEPT OF A COMPOSITE BODY (continued)
This can be done by considering each part as a “particle” and following the procedure as described in Section 9.1.
This is a simple, effective, and practical method of determining the location of the centroid or center of gravity of a complex part, structure or machine.
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Section 9.3

12. STEPS FOR ANALYSIS
1. Divide the body into pieces that are known shapes Holes are considered as pieces with negative weight or size.
2. Make a table with the first column for segment number, the second column for weight, mass, or size (depending on the problem), the next set of columns for the moment arms, and, finally, several columns for recording results of simple intermediate calculations.
3. Fix the coordinate axes, determine the coordinates of the center of gravity of centroid of each piece, and then fill in the table.
4. Sum the columns to get x, y, and z. Use formulas like
x = (  xi Ai ) / (  Ai ) or x = (  xi Wi ) / (  Wi )
This approach will become clear by doing examples! 
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Section 9.3

13. Find: The centroid of the part.
EXAMPLE
Given: The part shown.
Find: The centroid of the part.
Plan: Follow the steps for analysis.
Solution:
1. This body can be divided into the following pieces: rectangle (a) + triangle (b) + quarter circular (c) – semicircular area (d). Note the negative sign on the hole!
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Section 9.3

14. EXAMPLE (continued)
Steps 2 & 3: Make up and fill the table using parts a, b, c, and d.
39.83
76.5
28.0 
- 2/3
– 9 0
(3) / (3 ) 4(1) / (3 )
3 7 – 4(3) / (3 ) 0
 / 4 –  / 2
Rectangle Triangle
Q. Circle Semi-Circle
y A ( in3)
x A ( in3)
y (in)
x (in)
Area A (in2)
Segment 
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Section 9.3

15. x = (  x A) / (  A ) = 76.5 in3/ 28.0 in2 = 2.73 in
EXAMPLE (continued)
4. Now use the table data results and the formulas to find the coordinates of the centroid.
x = (  x A) / (  A ) = in3/ 28.0 in = in
y = (  y A) / ( A ) = in3 / 28.0 in2 = in  C
Area A x A y A 
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Section 9.3

16. CONCEPT QUIZ Answers: 1. C 2. C 3cm 1 cm
1. Based on the typical centroid information, what are the minimum number of pieces you will have to consider for determining the centroid of the area shown at the right?
A) B) C) D) 4
2. A storage box is tilted up to clean the rug underneath the box. It is tilted up by pulling the handle C, with edge A remaining on the ground. What is the maximum angle of tilt (measured between bottom AB and the ground) possible before the box tips over?
A) 30° B) 45 ° C) 60 ° D) 90 °
30º G C A B
Answers:
1. C
2. C
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Section 9.3

17. Examples: Find Centroids for these cross-sections:
Side, CG?

18. Centroid of Composite Areas
Missing Areas can be calculated into the Centroid by analyzing it as a negative area.

19. Sample Problem - Hole

20. Sample Problem - Hole
Compute the coordinates of the area centroid by dividing the first moments by the total area.

21. CG / CM OF A COMPOSITE BODY
Consider a composite body which consists of a series of particles(or bodies) as shown in the figure. The net or the resultant weight is given as WR = W.
Summing the moments about the y-axis, we get
x WR = x1W1 + x2W2 + ……… xnWn
where x1 represents x coordinate of W1, etc.. ~
Similarly, we can sum moments about the x- and z-axes to find the coordinates of G.
By replacing the W with a M in these equations, the coordinates of the center of mass can be found.

22. Example: Center of Gravity, 3D with different densities:
Given: Two blocks of different materials are assembled as shown.
The densities of the materials are:
A = lb / ft3 and B = lb / ft3.
Find: The center of gravity of this assembly.
Plan: Follow the steps for analysis.
Solution
1. In this problem, the blocks A and B can be considered as two pieces (or segments).

23. GROUP PROBLEM SOLVING (continued)
Weight = w =  (Volume in ft3)
wA = (0.5) (6) (6) (2) / (12)3 = lb
wB = (6) (6) (2) / (12)3 = lb
56.25
53.12
29.17
19.79 
6.25
50.00
3.125
12.5
16.67 2 3 1 4 A B
zA (lb·in)
yA (lb·in)
xA (lb·in)
z (in)
y (in)
x (in)
w (lb)
Segment 

24. GROUP PROBLEM SOLVING (continued)
Table Summary
W (lb) x w y w z w
(lb·in) (lb·in) (lb·in) 
Substituting into the equations: ~
x = ( x w) / ( w ) = /19.79 = 1.47 in
y = ( y w) / ( w ) = / = 2.68 in
z = ( z w) / ( w ) = / = in
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Section 9.3