# APPENDIX A MOMENTS OF AREAS

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APPENDIX A MOMENTS OF AREAS
A.1 First Moment of An Area; Centroid First Moments of the Area A About the x- and y-Axis are Defined As The centroid of the area A is defined as the point C of coordinates and which satisfy the relations

APPENDIX A MOMENTS OF AREAS
The first moment of an area about its symmetric axis is zero, so, the centroid of the area must be on the symmetric axis. When an area possesses a center of symmetry O, the first moment of the area about any axis through O is zero. In other words, O is the centroid of the area.

APPENDIX A MOMENTS OF AREAS
If an area has two symmetric axes, the inter-section of the two axes must be the centroid of the area.

APPENDIX A MOMENTS OF AREAS
Sample Problem A.1 For the triangular area of Fig. (a), determine (a) the first moment Qx of the area with respect to the x-axis, (b) the coordinate of the centroid of the area. Fig. (a) Fig. (b)

APPENDIX A MOMENTS OF AREAS
dA=udy,

APPENDIX A MOMENTS OF AREAS
Complement Problem Determine the first moment of a semi-circular area about the x-axis, (b) the coordinate of the centroid of the semi-circle.

APPENDIX A MOMENTS OF AREAS
A.1 Determination of The First Moment And Centroid of A Composite Area

APPENDIX A MOMENTS OF AREAS
Sample Problem A.2 Locate the Centroid C of the area A shown in Fig. (a). Fig. (a) Fig. (b)

APPENDIX A MOMENTS OF AREAS
Solution: A1=80×20=1600 mm2, A2=60×40=2400 mm2

APPENDIX A MOMENTS OF AREAS
Sample Problem A.3 Referring to the area A of Sample Problem A.2, we consider the horizontal x axis which is through its centroid C. (Such an axis is called a centroidal axis.) Denoting by A the portion of A located above that axis (Fig. a), determine the first moment of A with respect to the x axes. Fig. (a) Fig. (b) Fig. (c)

APPENDIX A MOMENTS OF AREAS
Solution: In fact

c(19.7;39.7) Complement Problem
Determine the centroid of the L-shape area. x y 10 C2 c(19.7;39.7) 10 C1 80

Alternative method: Negative area method
80 120 10 x y Alternative method: Negative area method x

APPENDIX A MOMENTS OF AREAS
A.3 Second Moment, or Moment of Inertia, of An Area; Radius of Gyration Moment of Inertia of A With Respect To the And x Axis And y Axis are Defined, Respectively, As Define the Polar Moment of Inertia of the Area A With Respect To Point O As the Integral :

APPENDIX A MOMENTS OF AREAS
Radii of Gyration of An Area A with respect to the x and y axis: Radii of Gyration With Respect To the Origin O

APPENDIX A MOMENTS OF AREAS
Sample Problem A.4 For the rectangular area of Fig.(a). determine (a) the moment of inertia Ix of the area with respect to the centroidal x axis. (b) the corresponding radius of gyration rx. Fig. (b) Fig. (a)

APPENDIX A MOMENTS OF AREAS
Sample Problem A.5 For the circular area of Fig. (a), determine (a) the polar moment of inertia JO, (b) rectangular moments of inertia Ix and Iy. Fig. (a) Fig. (b)

A.4 Parallel-Axis Theorem
APPENDIX A MOMENTS OF AREAS A.4 Parallel-Axis Theorem

APPENDIX A MOMENTS OF AREAS
A.5 Determination of The Moment of Inertia of a Composite Area.

APPENDIX A MOMENTS OF AREAS
Sample Problem A.6 Determine the moment of inertia of the area shown with respect to the centroidal x axis (Fig. a). Fig. (a) Fig. (b)

APPENDIX A MOMENTS OF AREAS
Solution:

APPENDIX A MOMENTS OF AREAS
A.6 Product of Inertia for An Area. product of inertia for an element of area located at point (x, y) is defined as If either x or y axis is a symmetric axis, Ixy=0.

Parallel-Axis theorem
APPENDIX A MOMENTS OF AREAS Parallel-Axis theorem

APPENDIX A MOMENTS OF AREAS
Sample Problem A.7 Determine the product of inertia Ixy of the triangle shown in Fig. (a).

APPENDIX A MOMENTS OF AREAS
Sample Problem A.7

APPENDIX A MOMENTS OF AREAS
Solution:

APPENDIX A MOMENTS OF AREAS
Sample Problem A.8 Compute the product of inertia of the beam’s cross-sectional area, shown in Fig. (a), about the x and y centroidal axes.

APPENDIX A MOMENTS OF AREAS
Solution: Rectangle A mm4 Rectangle B Rectangle D mm4 mm4 The product of inertia for the entire cross section is mm4

APPENDIX A MOMENTS OF AREAS
A.7 Moments of Inertia for An Area About Inclined Axes

APPENDIX A MOMENTS OF AREAS

APPENDIX A MOMENTS OF AREAS
Principal Moments of Inertia

APPENDIX A MOMENTS OF AREAS

APPENDIX A MOMENTS OF AREAS
Sample Problem A.9 Determine the principal moments of inertia for the beam’s cross-sectional area shown in Fig. (a) with respect to an axis passing through the centroid.

APPENDIX A MOMENTS OF AREAS
A.8 Mohr’s Circle for Moments of Inertia

APPENDIX A MOMENTS OF AREAS
Sample Problem A.10 Using Mohr’s circle, determine the principal moments of inertia for the beam’s cross-sectional area, shown in Fig. (a), with respect to an axis passing through the centroid.

APPENDIX A MOMENTS OF AREAS