Tenth Edition

VECTOR MECHANICS FOR ENGINEERS: STATICS

Tenth Edition

Ferdinand P. Beer E. Russell Johnston, Jr. David F. Mazurek Lecture Notes: John Chen California Polytechnic State University

CHAPTER

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5 Distributed Forces: Centroids and Centers of Gravity

© 2013The McGraw-Hill Companies, Inc. All rights reserved.

Vector Mechanics for Engineers: Statics

Tenth Edition

Contents

5 - 2

Introduction

Center of Gravity of a 2D Body

Centroids and First Moments of Areas and Lines

Sample Problem 5.4

Centroids of Common Shapes of Areas

Centroids of Common Shapes of Lines

Composite Plates and Areas

Sample Problem 5.1

Determination of Centroids by Integration

Theorems of Pappus-Guldinus Sample Problem 5.7

Distributed Loads on Beams

Sample Problem 5.9 Center of Gravity of a 3D Body:

Centroid of a Volume

Centroids of Common 3D Shapes

Composite 3D Bodies

Sample Problem 5.12



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Application

5 - 3

There are many examples in engineering analysis of distributed loads. It is convenient in some cases to represent such loads as a concentrated force located at the centroid of the distributed load.



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Introduction

5 - 4

•  The earth exerts a gravitational force on each of the particles forming a body – consider how your weight is distributed throughout your body. These forces can be replaced by a single equivalent force equal to the weight of the body and applied at the center of gravity for the body.

•  The centroid of an area is analogous to the center of gravity of a body; it is the “center of area.” The concept of the first moment of an area is used to locate the centroid.

•  Determination of the area of a surface of revolution and the volume of a body of revolution are accomplished with the Theorems of Pappus-Guldinus.



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Center of Gravity of a 2D Body

5 - 5

•  Center of gravity of a plate

∫

∑∑∫

∑∑

=

Δ=

=

Δ=

dWy

WyWyMdWx

WxWxM

y

y

•  Center of gravity of a wire



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First Moments of Areas and Lines

5 - 6

•  An area is symmetric with respect to an axis BB’ if for every point P there exists a point P’ such that PP’ is perpendicular to BB’ and is divided into two equal parts by BB’.

•  The first moment of an area with respect to a line of symmetry is zero.

•  If an area possesses a line of symmetry, its centroid lies on that axis

•  If an area possesses two lines of symmetry, its centroid lies at their intersection.

•  An area is symmetric with respect to a center O if for every element dA at (x,y) there exists an area dA’ of equal area at (-x,-y).

•  The centroid of the area coincides with the center of symmetry.



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Centroids of Common Shapes of Areas

5 - 7



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Centroids of Common Shapes of Lines

5 - 8



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Composite Plates and Areas

5 - 9

•  Composite plates

∑∑∑∑

=

=

WyWYWxWX

•  Composite area

∑∑∑∑

=

=

AyAYAxAX



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Sample Problem 5.1

5 - 10

For the plane area shown, determine the first moments with respect to the x and y axes and the location of the centroid.

SOLUTION:

•  Divide the area into a triangle, rectangle, and semicircle with a circular cutout.

•  Compute the coordinates of the area centroid by dividing the first moments by the total area.

•  Find the total area and first moments of the triangle, rectangle, and semicircle. Subtract the area and first moment of the circular cutout.

•  Calculate the first moments of each area with respect to the axes.



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Sample Problem 5.1

5 - 11

33

33

mm107.757

mm102.506

×+=

×+=

y

x

Q

Q•  Find the total area and first moments of the triangle, rectangle, and semicircle. Subtract the area and first moment of the circular cutout.



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Sample Problem 5.1

5 - 12

23

33

mm1013.828mm107.757

×

×+==

∑∑AAxX

mm 8.54=X

23

33

mm1013.828mm102.506

×

×+==

∑∑AAyY

mm 6.36=Y

•  Compute the coordinates of the area centroid by dividing the first moments by the total area.



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Centroids and First Moments of Areas and Lines

5 - 13

( ) ( )

xQdAyAy

y

QdAxAx

dAtxAtx

dWxWx

x

y

respect toh moment witfirst

respect toh moment witfirst

=

==

=

==

=

=

∫

∫

∫

∫γγ

•  Centroid of an area

( ) ( )

∫

∫

∫

∫

=

=

=

=

dLyLy

dLxLx

dLaxLax

dWxWx

γγ

•  Centroid of a line



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5 - 14

( )

( )ydxydAyAy

ydxx

dAxAx

el

el

∫

∫

∫

∫

=

=

=

=

2

( )[ ]

( )[ ]dxxay

dAyAy

dxxaxadAxAx

el

el

−=

=

−+

=

=

∫

∫

∫

∫

2

⎟⎠⎞

⎜⎝⎛=

=

⎟⎠⎞

⎜⎝⎛=

=

∫

∫

∫

∫

θθ

θθ

drr

dAyAy

drr

dAxAx

el

el

2

2

21sin

32

21cos

32

∫∫∫∫

∫∫∫∫===

===

dAydydxydAyAy

dAxdydxxdAxAx

el

el •  Double integration to find the first moment may be avoided by defining dA as a thin rectangle or strip.



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Sample Problem 5.4

5 - 15

Determine by direct integration the location of the centroid of a parabolic spandrel.

SOLUTION:

•  Determine the constant k.

•  Evaluate the total area.

•  Using either vertical or horizontal strips, perform a single integration to find the first moments.

•  Evaluate the centroid coordinates.

First, estimate the location of the centroid by inspection. Discuss with a neighbor where it is located, roughly, and justify your answer.



Tenth Edition

Sample Problem 5.4

5 - 16

SOLUTION:

•  Determine the constant k.

2121

22

22

2

ybaxorx

aby

abkakb

xky

==

=⇒=

=

•  Evaluate the total area.

3

30

3

20

22

ab

xabdxx

abdxy

dAAaa

=

⎥⎥⎦

⎤

⎢⎢⎣

⎡===

=

∫∫

∫



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Sample Problem 5.4

5 - 17

•  Using vertical strips, perform a single integration to find the first moments.

€

Qy = x el dA∫ = xydx∫ = x ba2

x 2⎛

⎝ ⎜

⎞

⎠ ⎟ dx

0

a∫

=b

a2x 4

4⎡

⎣ ⎢

⎤

⎦ ⎥ 0

a

=a2b4

Qx = y el dA∫ =y2

ydx∫ =12

ba2

x 2⎛

⎝ ⎜

⎞

⎠ ⎟ 2

dx0

a∫

=b2

2a4x 5

5⎡

⎣ ⎢

⎤

⎦ ⎥ 0

a

=ab2

10



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Sample Problem 5.4

5 - 18

•  Or, using horizontal strips, perform a single integration to find the first moments. Try calculating Qy or Qx by this method, and confirm that you get the same value as before.

€

Qy = x el dA∫ =a + x2

a − x( )dy∫ =a2 − x 2

2dy

0

b∫

=12

a2 − a2

by

⎛

⎝ ⎜

⎞

⎠ ⎟

0

b∫ dy =

a2b4

Qx = y el dA∫ = y a − x( )dy∫ = y a − ab1 2

y1 2⎛

⎝ ⎜

⎞

⎠ ⎟ dy∫

= ay − ab1 2

y 3 2⎛

⎝ ⎜

⎞

⎠ ⎟ dy

0

b∫ =

ab2

10



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Sample Problem 5.4

5 - 19

•  Evaluate the centroid coordinates.

43

2baabx

QAx y

=

=

ax43

=

103

2ababy

QAy x

=

=

by103

=

Is this “center of area” close to where you estimated it would be?



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5 - 20

Usually, the choice between using a vertical or horizontal strip is equally good, but in some cases, one choice is much better than the other. For example, for the area shown below, is a vertical or horizontal strip a better choice, and why? Think about this and discuss your choice with a neighbor.

x

y



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Theorems of Pappus-Guldinus

5 - 21

•  Surface of revolution is generated by rotating a plane curve about a fixed axis.

•  Area of a surface of revolution is equal to the length of the generating curve times the distance traveled by the centroid through the rotation.

LyA π2=



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Theorems of Pappus-Guldinus

5 - 22

•  Body of revolution is generated by rotating a plane area about a fixed axis.

•  Volume of a body of revolution is equal to the generating area times the distance traveled by the centroid through the rotation.

AyV π2=



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Sample Problem 5.7

5 - 23

The outside diameter of a pulley is 0.8 m, and the cross section of its rim is as shown. Knowing that the pulley is made of steel and that the density of steel is determine the mass and weight of the rim.

33 mkg 1085.7 ×=ρ

SOLUTION:

•  Apply the theorem of Pappus-Guldinus to evaluate the volumes of revolution of the pulley, which we will form as a large rectangle with an inner rectangular cutout.

•  Multiply by density and acceleration to get the mass and weight.



Tenth Edition

Sample Problem 5.7

5 - 24

SOLUTION:

•  Apply the theorem of Pappus-Guldinus to evaluate the volumes or revolution for the rectangular rim section and the inner cutout section.

€

m = ρV = 7.85×103 kg m3( )7.65×106mm3( )10−9m3 /mm3( ) kg 0.60=m

€

W =mg= 60.0 kg( )9.81 m s2( ) N 589=W

•  Multiply by density and acceleration to get the mass and weight.



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Distributed Loads on Beams

5 - 25

•  A distributed load is represented by plotting the load per unit length, w (N/m) . The total load is equal to the area under the load curve.

∫∫ === AdAdxwWL

0

( )

( ) AxdAxAOP

dWxWOPL

==

=

∫

∫

0

•  A distributed load can be replace by a concentrated load with a magnitude equal to the area under the load curve and a line of action passing through the area centroid.



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Sample Problem 5.9

5 - 26

A beam supports a distributed load as shown. Determine the equivalent concentrated load and the reactions at the supports.

SOLUTION:

•  The magnitude of the concentrated load is equal to the total load or the area under the curve.

•  The line of action of the concentrated load passes through the centroid of the area under the curve.

•  Determine the support reactions by (a) drawing the free body diagram for the beam and (b) applying the conditions of equilibrium.



Tenth Edition

Sample Problem 5.9

5 - 27

SOLUTION:

•  The magnitude of the concentrated load is equal to the total load or the area under the curve.

kN 0.18=F

•  The line of action of the concentrated load passes through the centroid of the area under the curve.

kN 18mkN 63 ⋅

=X m5.3=X



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Sample Problem 5.9

5 - 28

•  Determine the support reactions by applying the equilibrium conditions. For example, successively sum the moments at the two supports:

€

MA∑ = 0: By 6 m( )− 18 kN( )3.5 m( )= 0

kN 5.10=yB

€

MB∑ = 0: −Ay 6 m( )+ 18 kN( )6 m−3.5 m( )= 0

kN 5.7=yA

€

Fx∑ = 0 : Bx = 0

•  And by summing forces in the x-direction:



Tenth Edition

Center of Gravity of a 3D Body: Centroid of a Volume

5 - 29

•  Center of gravity G

( )∑ Δ−=− jWjW!!

( ) ( )[ ]( ) ( ) ( )jWrjWr

jWrjWr

G

G !!!!

!!!!

−×Δ=−×

Δ−×=−×

∑∑

∫∫ == dWrWrdWW G!!

•  Results are independent of body orientation,

∫∫∫ === zdWWzydWWyxdWWx

∫∫∫ === zdVVzydVVyxdVVx

dVdWVW γγ == and

•  For homogeneous bodies,



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Centroids of Common 3D Shapes

5 - 30



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Composite 3D Bodies

5 - 31

•  Moment of the total weight concentrated at the center of gravity G is equal to the sum of the moments of the weights of the component parts.

∑∑∑∑∑∑ === WzWZWyWYWxWX

•  For homogeneous bodies,

∑∑∑∑∑∑ === VzVZVyVYVxVX



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Sample Problem 5.12

5 - 32

Locate the center of gravity of the steel machine element. The diameter of each hole is 1 in.

SOLUTION:

•  Form the machine element from a rectangular parallelepiped and a quarter cylinder and then subtracting two 1-in. diameter cylinders.



Tenth Edition

Sample Problem 5.12

5 - 33



Tenth Edition

Sample Problem 5.12

5 - 34

€

X = x V∑ V∑ = 3.08 in4( ) 5.286 in3( )

€

Y = y V∑ V∑ = −5.047 in4( ) 5.286 in3( )

€

Z = z V∑ V∑ = 1.618 in4( ) 5.286 in3( )

in. 577.0=X

in. 577.0=Y

in. 577.0=Z

Tenth Edition

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