# Old Lecture 16 sect ..

```ME 221 Statics
Lecture #16
Section 4.5
ME 221
Lecture 16
1
Homework #6
• Chapter 4 problems:
– 10, 11, 18, 24, 39, 42, 43 &amp; 47
– Must use integration methods to solve
– Due Monday, October 14
ME 221
Lecture 16
2
Quiz #4
• Wednesday, October 9
ME 221
Lecture 16
3
Last Lecture:
• Centroids of Areas, Volumes &amp; Lines
ME 221
Lecture 16
4
Centroid of Composite Bodies
• Basic shapes are added together to get the
composite centroid
– We look these up in Table 4.2
– This table will be supplied on future exams
• Problems
ME 221
Lecture 16
5
Area Centroids
This case is representative of line and volume
centroids.
Divide total area into simple shapes
(rectangles, semi-circles)
y
A2
x1
x2
A1
y1
ME 221
+
y2
A3
+
=
xc
x3
Lecture 16
yc
y3
x
6
May Also Use Subtraction
A1
x2
x1
y1
ME 221
y
A2
=
y2
Lecture 16
yc
xc
x
7
Composite Centroid of an Area
• Geometric center of the area
– Summation of the pieces (holes are negative)
1
xc   xiAi
A i
– Where:
1
yc   yiAi
A i
A   Ai
i
ME 221
Lecture 16
8
Composite Centroid of a Volume
• Geometric center of the volume
– Average of the first moment over entire volume
1
xc   xiVi
V i
1
yc   yiVi
V i
– Where:
V   Vi
1
zc   ziVi
V i
i
ME 221
Lecture 16
9
Examples
ME 221
Lecture 16
10
```