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1 © 2020 Montogue Quiz
Quiz SM102
Moment of Inertia Lucas Montogue
Problems
PROBLEM â¶A
The moments of inertia of the area shown about the x-axis and the y-axis are:
A) ðŒðŒð¥ð¥ = 213 (106) mm4 and ðŒðŒðŠðŠ = 457 (106) mm4
B) ðŒðŒð¥ð¥ = 213 (106) mm4 and ðŒðŒðŠðŠ = 588 (106) mm4 C) ðŒðŒð¥ð¥ = 331 (106) mm4 and ðŒðŒðŠðŠ = 457 (106) mm4 D) ðŒðŒð¥ð¥ = 331 (106) mm4 and ðŒðŒðŠðŠ = 588 (106) mm4
PROBLEM â¶B
The polar moment of inertia of the area presented in the previous part about the origin of the coordinate frame is:
A) ðœðœ0 = 549 (106) mm4
B) ðœðœ0 = 670 (106) mm4 C) ðœðœ0 = 801 (106) mm4 D) ðœðœ0 = 919 (106) mm4 PROBLEM â¶C
The product of inertia of the area introduced in Part A with respect to the x- and y-axes is
A) ðŒðŒð¥ð¥ðŠðŠ = 155 (106) mm4
B) ðŒðŒð¥ð¥ðŠðŠ = 267 (106) mm4 C) ðŒðŒð¥ð¥ðŠðŠ = 372 (106) mm4
D) ðŒðŒð¥ð¥ðŠðŠ = 480 (106) mm4
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PROBLEM â·
True or false?
1. ( ) The moment of inertia of the shaded area about the x-axis is Ix = (4 7â )ððâ3. 2. ( ) The moment of inertia of the shaded area about the y-axis is Iy = (2 15â )âðð3.
3. ( ) The moment of inertia of the shaded area about the x-axis is Ix = (4ðð4) (9ðð)â . 4. ( ) The moment of inertia of the shaded area about the y-axis is Iy = [ðð4(ðð2 â 2)] ðð3â .
PROBLEM âž
True or false?
1. ( ) The moment of inertia of the shaded area about the x-axis is Ix = ðððð04 8â . 2. ( ) The moment of inertia of the shaded area about the y-axis is Iy = ðððð04 4â .
3 © 2020 Montogue Quiz
3. ( ) The moment of inertia of the shaded area about the x-axis is greater than 1 m4.
4. ( ) The moment of inertia of the shaded area about the y-axis is greater than 1 m4.
PROBLEM â¹A
Determine the radius of gyration relative to the y-axis for the area shown.
A) ð ð ðŠðŠ = 1.95 u.l.
B) ð ð ðŠðŠ = 4.93 u.l. C) ð ð ðŠðŠ = 8.44 u.l.
D) ð ð ðŠðŠ = 12.1 u.l.
PROBLEM â¹B
Determine the radius of gyration relative to the y-axis for the shaded area.
A) ð ð ðŠðŠ = 2.77 u.l.
B) ð ð ðŠðŠ = 4.90 u.l. C) ð ð ðŠðŠ = 8.20 u.l.
D) ð ð ðŠðŠ = 11.8 u.l.
4 © 2020 Montogue Quiz
PROBLEM ⺠(Hibbeler, 2010, w/ permission)
Determine the moment of inertia of the area of the channel with respect to the y-axis.
A) ðŒðŒðŠðŠ = 273 in.4
B) ðŒðŒðŠðŠ = 388 in.4. C) ðŒðŒðŠðŠ = 476 in.4
D) ðŒðŒðŠðŠ = 545 in.4
PROBLEM â» (Bedford & Fowler, 2008, w/ permission)
Determine the moment of inertia of the section relative to the x-axis.
A) ðŒðŒð¥ð¥ = 109.6 (109) mm4
B) ðŒðŒð¥ð¥ = 163.6 (109) mm4 C) ðŒðŒð¥ð¥ = 224.0 (109) mm4 D) ðŒðŒð¥ð¥ = 298.5 (109) mm4
PROBLEM ⌠(Bedford & Fowler, 2008, w/ permission)
Determine the moment of inertia of the section relative to the x-axis.
A) ðŒðŒð¥ð¥ = 6.0 (106) mm4
B) ðŒðŒð¥ð¥ = 9.0 (106) mm4 C) ðŒðŒð¥ð¥ = 12.0 (106) mm4 D) ðŒðŒð¥ð¥ = 15.0 (106) mm4
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PROBLEM ✠(Bedford & Fowler, 2008, w/ permission)
Determine the moment of inertia relative to the y-axis.
A) ðŒðŒðŠðŠ = 8945 in.4
B) ðŒðŒðŠðŠ = 16,565 in.4 C) ðŒðŒðŠðŠ = 24,860 in.4
D) ðŒðŒðŠðŠ = 32,235 in.4
PROBLEM ⟠(Hibbeler, 2010, w/ permission)
Consider the following rectangular beam section and the hypothetical axes u and v. True or false?
1. ( ) The moment of inertia Iu of the section about the u-axis is Iu = 52.5 in.4 2. ( ) The moment of inertia Iv of the section about the v-axis is Iv = 23.6 in.4 3. ( ) The product of inertia Iuv of the section relative to axes u and v is Iuv = 17.5 in.4
PROBLEM â¿ (Hibbeler, 2010, w/ permission)
Locate the centroid ï¿œÌ ï¿œð¥ and ðŠðŠï¿œ of the cross-sectional area and then determine the orientation of the principal axes, which have their origin at the centroid C of the area.
A) ðððð = 15o and ðððð = â15o
B) ðððð = 30o and ðððð = â30o C) ðððð = 45o and ðððð = â45o
D) ðððð = 60o and ðððð = â60o
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Additional Information Figure 1 Moments of inertia for selected geometries
Solutions
P.1 â Solution
Part A: The moments of inertia in question are such that, for Ix,
ðŒðŒð¥ð¥ = ï¿œ ï¿œ ðŠðŠ2ðððŠðŠððð¥ð¥â2ð¥ð¥
12
0
2
0= 2.13 m4 = 213 (106) mm4
and, for Iy,
ðŒðŒðŠðŠ = ï¿œ ï¿œ ð¥ð¥2 ðððŠðŠððð¥ð¥â2ð¥ð¥
12
0
2
0= 4.57 m4 = 457 (106) mm4
â The correct answer is A.
Part B: The polar moment of inertia, J0, is obtained with the equation
ðœðœ0 = ï¿œðð2ðððð
ðŽðŽ
where r is the radial distance from the origin of the coordinate system to the elemental area dA. Instead of evaluating this integral, however, we could use the relation r2 = x2 + y2, so that
ðœðœ0 = ï¿œðð2ðððð
ðŽðŽ= ï¿œ (ð¥ð¥2 + ðŠðŠ2)ðððð
ðŽðŽ= ï¿œð¥ð¥2ðððð
ðŽðŽ+ ï¿œðŠðŠ2ðððð
ðŽðŽ= ðŒðŒðŠðŠ + ðŒðŒð¥ð¥
That is to say, the polar moment of inertia about the origin equals the sum of the moment of inertia with respect to the x-axis and the moment of inertia with respect to the y-axis. Using the results from Part A, we obtain
ðœðœ0 = 213 (106) + 457 (106) = 670 (106) mm4
â The correct answer is B.
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Part C: The product of inertia of the area in question is given by
ðŒðŒð¥ð¥ðŠðŠ = ï¿œð¥ð¥ðŠðŠðððð
ðŽðŽ= ï¿œ ï¿œ ð¥ð¥ðŠðŠ ðððŠðŠððð¥ð¥
â2ð¥ð¥12
0
2
0= 267 (106)mm4
â The correct answer is B.
P.2 â Solution
1. False. The area of the differential element parallel to the x-axis is
ðððð = (â â ðŠðŠ)ððð¥ð¥ = ï¿œâ ââðð2 ð¥ð¥
2ï¿œ ððð¥ð¥
Applying the equation for Ix and performing the integration, we have
ðŒðŒð¥ð¥ = ï¿œðŠðŠ2ðððð
ðŽðŽ= ï¿œ ðŠðŠ2 Ã
ððââ
ðŠðŠ12ðððŠðŠ
â
0
⎠ðŒðŒð¥ð¥ =ððââ
Ã2ðŠðŠ7 2â
7 ï¿œðŠðŠ=0
ðŠðŠ=â
=ðð
â12
Ã2â7 2â
7
⎠ðŒðŒð¥ð¥ =27ððâ
3
2. True. The area of the differential element parallel to the y-axis is
ðððð = (â â ðŠðŠ)ððð¥ð¥ = ï¿œâ ââðð2 ð¥ð¥
2ï¿œ ððð¥ð¥
so that, substituting in the expression for Iy and performing the integration, we get
ðŒðŒðŠðŠ = ï¿œ ð¥ð¥2 ï¿œâ ââðð2 ð¥ð¥
2ï¿œ ððð¥ð¥
ðŽðŽ= ï¿œ ï¿œâð¥ð¥2 â
âðð2 ð¥ð¥
4ï¿œ ððð¥ð¥ðð
0
⎠ðŒðŒðŠðŠ = ï¿œâð¥ð¥3
3 ââ
5ðð2 ð¥ð¥5ᅵᅵ
ðŠðŠ=0
ðŠðŠ=ðð
=âðð3
3 ââðð3
5
⎠ðŒðŒðŠðŠ =2
15 âðð3
3. True. The differential area element parallel to the y-axis is shown in blue.
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The contribution of this element to the moment of inertia Ix is given by
ðððŒðŒð¥ð¥ =ðŠðŠ3
12ððð¥ð¥ + (ðŠðŠððð¥ð¥) ï¿œðŠðŠ2ï¿œ
2=ðŠðŠ3
12ððð¥ð¥ +ðŠðŠ3
4 ððð¥ð¥ =13 ðŠðŠ
3ððð¥ð¥
or, substituting y,
ðððŒðŒð¥ð¥ =13 ï¿œðð sin
ðððð ð¥ð¥ï¿œ
3ððð¥ð¥ =
ðð3
3 sin3ðððð ð¥ð¥ ððð¥ð¥
Integrating on both sides, we have
ï¿œðððŒðŒð¥ð¥ = ï¿œðð3
3 sin3ðððð ð¥ð¥
ðð
0ððð¥ð¥ =
ðð3
3 ï¿œ sinðððð ð¥ð¥ ï¿œ1 â cos2
ðððð ð¥ð¥ï¿œ ððð¥ð¥
ðð
0
Let cos(ðð ððâ )ð¥ð¥ = t. Differentiating on both sides, we have
âï¿œðððð sin
ðððð ð¥ð¥ï¿œ
ððð¥ð¥ðððð = 1 â sin ï¿œ
ðððð ð¥ð¥ï¿œ ððð¥ð¥ = â
ðððððððð
The bounds of the integral change from x = 0 to t = 1 and from x = a to t = - 1. Backsubstituting in the previous integral gives
ðŒðŒð¥ð¥ =ðð3
3 ï¿œ (1 â ðð2) ï¿œâðððð ððððï¿œ
â1
1= â
ðð4
3ððï¿œ(1 â ðð2)ðððð
â1
1
⎠ðŒðŒð¥ð¥ = âðð4
3ðð ï¿œðð âðð3
3 ᅵᅵð¡ð¡=1
ð¡ð¡=â1
= âðð4
3ðð ï¿œâ23 â
23ᅵ =
4ðð4
9ðð
4. False. Proceeding similarly with the moment of inertia about the y-axis, the following integral is proposed,
ðŒðŒðŠðŠ = ï¿œð¥ð¥2ðððð = ððï¿œ ð¥ð¥2 sinðððð ð¥ð¥ ððð¥ð¥
ðð
0
⎠ðŒðŒðŠðŠ = ðð ï¿œ2 ï¿œððððï¿œð¥ð¥ sin ï¿œððððï¿œð¥ð¥ + ï¿œ2â ï¿œððððï¿œ
2ð¥ð¥2ï¿œ cos ï¿œððððï¿œð¥ð¥
ï¿œððððï¿œ3 ᅵᅵ
ð¥ð¥=0
ð¥ð¥=ðð
Accordingly,
ðŒðŒðŠðŠ = ððᅵᅵ2ï¿œððððï¿œ ðð sin ï¿œððððï¿œðð + ï¿œ2â ï¿œððððï¿œ
2ð¥ð¥2ï¿œ cos ï¿œððððï¿œ ðð
ï¿œððððï¿œ3 ï¿œ
â ï¿œ2 ï¿œððððï¿œ0 sin ï¿œððððï¿œ0 + ï¿œ2 â ï¿œððððï¿œ
2ð¥ð¥2ï¿œ cos ï¿œððððï¿œ0
ï¿œððððï¿œ3 ᅵᅵ
⎠ðŒðŒðŠðŠ =ðð4[(ðð2 â 2) â 2]
ðð3 =ðð4(ðð2 â 4)
ðð3
P.3 â Solution
1. True. A circular cross-section such as the one considered here begs the use of polar coordinates. The area of the differential area element shown below is ðððð =(ðððððð)ðððð.
In polar coordinates, the y-coordinate is transformed as ðŠðŠ = ðð sinðð. We can then set up the integral as follows,
ðŒðŒð¥ð¥ = ï¿œðŠðŠ2ðððð
ðŽðŽ= ï¿œ ï¿œ ðð2 sin2 ðð ðððððððððð
ðð0
0
ðð 2â
âðð 2â
⎠ðŒðŒð¥ð¥ = ï¿œ ï¿œ ðð3 sin2 ðð ðððððððððð0
0
ðð 2â
âðð 2â= ï¿œ ï¿œ ðð3 sin2 ðð ðððððððð
ðð0
0
ðð 2â
âðð 2â
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⎠ðŒðŒð¥ð¥ =ðð04
4 ï¿œ sin2 ðð ðððððððððð 2â
âðð 2â
At this point, note that sin2 2ðð â¡ (1 2â )(1â cos 2ðð). Accordingly,
ðŒðŒð¥ð¥ =ðð04
4 ï¿œ sin2 ðð ðððððððððð 2â
âðð 2â=ðð04
8 ï¿œðð âsin 2ðð
2 ᅵᅵâðð 2â
ðð 2â
=ðððð04
8
2. False. In polar coordinates, the x-coordinate is transformed as ð¥ð¥ = ðð cosðð. Substituting this quantity into the equation for the moment of inertia and integrating, it follows that
ðŒðŒðŠðŠ = ï¿œð¥ð¥2ðððð
ðŽðŽ= ï¿œ ï¿œ ðð2 cos2 ðð ðððððððððð
ðð0
0
ðð 2â
âðð 2â
⎠ðŒðŒðŠðŠ = ï¿œ ï¿œ ðð3 cos2 ðð ðððððððððð0
0
ðð 2â
âðð 2â= ï¿œ ï¿œ ðð3 cos2 ðð ðððððððð
ðð0
0
ðð 2â
âðð 2â
⎠ðŒðŒðŠðŠ =ðð04
4 ï¿œ cos2 ðð ðððððððððð 2â
âðð 2â
However, cos2 ðð â¡ (1 2â )(cos 2ðð + 1). Thus,
ðŒðŒðŠðŠ =ðð04
8 ï¿œ (cos 2ðð + 1)ðððððð 2â
âðð 2â=ðð04
8 ï¿œðð âsin 2ðð
2 ᅵᅵâðð 2â
ðð 2â
=ðððð04
8
3. True. Consider the following illustration of the shaded area.
The elemental contribution of the strip to the overall moment of inertia is
ðððŒðŒð¥ð¥ = ðððŒðŒï¿œÌ ï¿œð¥ + ðððððŠðŠï¿œ
which, upon substitution of the pertaining variables, becomes
ðððŒðŒð¥ð¥ =ððð¥ð¥ à ðŠðŠ3
12 + ðŠðŠ à ððð¥ð¥ ÃðŠðŠ2
2 =ðŠðŠ3ððð¥ð¥
3
The overall moment of inertia follows by integrating the relation above, that is,
ðŒðŒð¥ð¥ =13ï¿œ ðŠðŠ3ððð¥ð¥
1
0=
13ï¿œ ï¿œððð¥ð¥2ï¿œ
3ððð¥ð¥
1
0=
13ï¿œ ðð3ð¥ð¥2ððð¥ð¥
1
0
There is no simple antiderivative available to evaluate the integral above. Thus, a numerical procedure is in order. Let us apply Simpsonâs rule,
ï¿œ ðð(ð¥ð¥)ððð¥ð¥ð¥ð¥ðð
ð¥ð¥0ââ3
[ðŠðŠ0 + 4(ðŠðŠ1 + ðŠðŠ3 + â¯+ ðŠðŠððâ1) + 2(ðŠðŠ2 + ðŠðŠ4 + â¯+ ðŠðŠððâ2) + ðŠðŠðð]
The bounds of the integral are x = 0 and x = 1. Let the number of intervals be equal to 6, so that the width h of each area element is written as
â =upper limit â lower limit
6 =1 â 0
6 â 0.167
Then, the following table is prepared.
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We are now ready to evaluate the integral via Simpsonâs rule, giving
â« ðð3ð¥ð¥2ððð¥ð¥ð¥ð¥ððð¥ð¥0
â (1 6â )3
[1.000 + 4(1.087 + 2.123 + 8.098) + 2(1.397 + 3.814) +20.086] = 4.263
Moment of inertia Ix is then
ðŒðŒð¥ð¥ = 13 â« ðð3ð¥ð¥2ððð¥ð¥1
0 = 13
à 4.263 = 1.421 m4
which is greater than 1 m4, thus implying that statement 3 is true.
4. False. As before, consider an elemental area of thickness dx, as shown.
The area of the elemental strip is dA = ydx. Then, the moment of inertia of the area about the y-axis is
ðŒðŒðŠðŠ = ï¿œð¥ð¥2ðððð
ðŽðŽ= ï¿œ ð¥ð¥2ðŠðŠððð¥ð¥
1
0
Substituting y = ððð¥ð¥2, we have
ðŒðŒðŠðŠ = ï¿œ ð¥ð¥2ððð¥ð¥2ððð¥ð¥1
0
This integral, much like the previous one, does not lend itself to simple analytical methods. We shall evaluate it numerically using Simpsonâs rule. Let the number of intervals be equal to 6, so that the width h of each area element becomes h = 1/6 â 0.167. The following table is prepared.
We now have enough information to evaluate the integral; that is,
â« ð¥ð¥2ððð¥ð¥2ððð¥ð¥ð¥ð¥ððð¥ð¥0
â (1 6â )3
[0 + 4(0.029 + 0.323 + 1.400) + 2(0.125 + 0.697) +2.718] = 0.632 m4
which is less than 1 m4, thus implying that statement 4 is false.
n x n y n
0 0 1.0001 0.167 1.0872 0.334 1.3973 0.501 2.1234 0.668 3.8145 0.835 8.0986 1 20.086
n x n y n
0 0 0.0001 0.167 0.0292 0.334 0.1253 0.501 0.3234 0.668 0.6975 0.835 1.4006 1 2.718
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P.4 â Solution
Part A: The problem involves a straightforward application of the integral formula for moment of inertia, followed by use of the definition of radius of gyration with respect to the y-axis,
ð ð ðŠðŠ = ï¿œðŒðŒðŠðŠðð
where Iy is the moment of inertia about the y-axis and A is the area of the surface. Before applying this formula, however, we require the points at which the curve intercepts the x-axis, namely,
ðŠðŠ = â14ð¥ð¥2 + 4ð¥ð¥ â 7 = 0 â ð¥ð¥ = 2 and 14
That is, the parabola in question intercepts the x-axis at x = 2 and x = 14. We can now determine the moment of inertia Iy,
ðŒðŒðŠðŠ = ï¿œ ï¿œ ð¥ð¥2ðððŠðŠððð¥ð¥â14ð¥ð¥
2+4ð¥ð¥â7
0
14
2= 5126 [u. l. ]4
The area A of the surface is, in turn,
ðð = ï¿œ ï¿œâ14 ð¥ð¥
2 + 4ð¥ð¥ â 7ï¿œððð¥ð¥14
2= 72 [u. a. ]
Finally, the radius of gyration Ry is such that
ð ð ðŠðŠ = ï¿œ512672 = 8.44 [u. l. ]
â The correct answer is C.
Part B: First, we need to locate the points at which the curve intersects the horizontal line,
5 = â14ð¥ð¥
2 + 4ð¥ð¥ â 7 â â14ð¥ð¥
2 + 4ð¥ð¥ â 12 = 0
ð¥ð¥ = 4 and 12
That is, the curve intersects the line at x = 4 and x = 12. We can now produce the integral for the moment of inertia,
ðŒðŒðŠðŠ = ï¿œ ï¿œ ð¥ð¥2ðððŠðŠððð¥ð¥â14ð¥ð¥
2+4ð¥ð¥â7
5
12
4= 1433.6 [u. l]4
We also require area A1 = A â A2, as illustrated below.
The total area A is such that
ðð = ï¿œ ï¿œâ14 ð¥ð¥
2 + 4ð¥ð¥ â 7ï¿œððð¥ð¥12
4= 61.33 [u. a. ]
In addition, A2 = (12 â 4)Ã5 = 40 u.a., so that A1 = A â A2 = 61.33 â 40 = 21.33 u.a. Finally, the radius of gyration Ry is such that
ð ð ðŠðŠ = ï¿œðŒðŒðŠðŠðð1
= ᅵ143421.33
= 8.20 [u. l. ]
â The correct answer is C.
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P.5 â Solution
The section can be represented as the sum of a solid rectangular area and a negative rectangular area, as shown.
Since the y-axis passes through the centroid of both rectangular segments, we have
ðŒðŒð¥ð¥ = ðŒðŒ1 â ðŒðŒ2
Here, I1 is such that
ðŒðŒ1 =12
(6.5)(14)3 = 1486.3 in.4
and I2 equals
ðŒðŒ2 =1
12(6)(13)3 = 1098.5 in.4
Finally, the required moment of inertia is
ðŒðŒð¥ð¥ = ðŒðŒ1 â ðŒðŒ2 = 388 in.4
â The correct answer is B.
P.6 â Solution
The area is divided into 3 rectangles, as shown below.
The total moment of inertia is the sum of the contributions of each part of the section,
ðŒðŒð¥ð¥ = ðŒðŒ1 + ðŒðŒ2 + ðŒðŒ3
I1 is determined as
ðŒðŒ1 =(600)(200)3
12 = 400 (106) mm4
I2 can be obtained with the parallel-axis theorem,
ðŒðŒ2 =ððâ3
12 + ðððŠðŠ2ðð =(200)(600)3
12 + 5002(200)(600) = 33,600 (106) mm4
The same applies to I3, which is calculated as
ðŒðŒ3 =ððâ3
12 + ðððŠðŠ2ðð =(800)(200)2
12 + 9002(200)(800) = 129,600 (106) mm4
Finally, moment of inertia Ix equals
ðŒðŒð¥ð¥ = ðŒðŒ1 + ðŒðŒ2 + ðŒðŒ3 = (400 + 33,600 + 129,600)(106) mm4
= 163,600 (106) mm4 = 163.6 (109) mm4
â The correct answer is B.
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P.7 â Solution
The area is subdivided into a rectangle (section 1), a semicircle (2), and a void circle (3), as shown.
Using tabulated results, the moment of inertia of part 1 about the x-axis is
ðŒðŒ1 =(120)(80)3
12 = 5.12 (106) mm4
Moment of inertia of part 2 about the x-axis is
ðŒðŒ2 =ððð ð 4
8 =ðð(40)4
8 = 1.01 (106) mm4
Finally, we have the moment of inertia of part 3, which, being a void area, must be subtracted from the total moment of inertia,
ðŒðŒ3 = âððð ð 4
4 = âðð(40)4
4 = â0.126 (106) mm4
The total moment of inertia, Ix, is given by
ðŒðŒð¥ð¥ = ðŒðŒ1 + ðŒðŒ2 â ðŒðŒ3 = (5.12 + 1.01â 0.126) à 106 = 6.0 (106) mm4
â The correct answer is A.
P.8 â Solution
We divide the composite area into a triangle (section 1), a rectangle (2), a half-circle (3), and a circular cutout (4); see below.
The contribution to the moment of inertia due to triangle 1 is
ðŒðŒ1 =14 à 12 à 83 = 1536 in.4
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The contribution owing to rectangle 2, in turn, follows from the parallel-axis theorem,
ðŒðŒ2 =1
12 Ã 12 Ã 83 + 122 Ã 8 Ã 12 = 14,336 in.4
The contribution relative to half-circle 3 is
ðŒðŒ3 = ï¿œðð8 â
89ððï¿œ (6)4 + ï¿œ16 +
4 à 63ðð ï¿œ
2
Ã12ðð(6)2 = 19,593 in.4
Finally, the contribution from circular cutout 4 is
ðŒðŒ4 = â14 à ðð(2)4 + (16)2 à ðð à 22 = â3230 in.4
which, being a hole, is associated with a negative sign. The final moment of inertia is
ðŒðŒðŠðŠ = ðŒðŒ1 + ðŒðŒ2 + ðŒðŒ3 + ðŒðŒ4 = 1536 + 14,336 + 19,593â 3230 = 32,235 in.4
â The correct answer is D.
P.9 â Solution
1. False. Since the rectangular beam cross-sectional area is symmetrical about the x- and y-axes, the product of inertia equals zero, Ixy = 0. As for the moments of inertia relative to the x- and y-axes, we have
ðŒðŒð¥ð¥ =(3)(6)3
12 = 54 in.4 ; ðŒðŒðŠðŠ =(6)(3)3
12 = 13.5 in.4
We can then apply the following equations for the moments of inertia with respect to the u and v axes.
ðŒðŒð¢ð¢ =ðŒðŒð¥ð¥ + ðŒðŒðŠðŠ
2 +ðŒðŒð¥ð¥ â ðŒðŒðŠðŠ
2 cos 2ðð â ðŒðŒð¥ð¥ðŠðŠ sin 2ðð
ðŒðŒð£ð£ =ðŒðŒð¥ð¥ + ðŒðŒðŠðŠ
2 âðŒðŒð¥ð¥ â ðŒðŒðŠðŠ
2 cos 2ðð + ðŒðŒð¥ð¥ðŠðŠ sin 2ðð
Substituting Ix = 54 in.4, Iy = 13.5 in4, Ixy = 0, and ðð = 30o, it follows that
ðŒðŒð¢ð¢ =54 + 13.5
2 +54 â 13.5
2 cos 2(30)â 0 sin 2(30) = 43.9 in.4
2. True. Indeed,
ðŒðŒð£ð£ =54 + 13.5
2 â54 â 13.5
2 cos 2(30) + 0 sin 2(30) = 23.6 in.4
3. True. Indeed,
ðŒðŒð¢ð¢ð£ð£ =ðŒðŒð¥ð¥âðŒðŒðŠðŠ2
sin 2ðð + ðŒðŒð¥ð¥ðŠðŠ cos 2ðð = 54â13.52
sin 2(30) + 0 cos 2(30) = 17.5 in4
P.10 â Solution
The area is subdivided into two rectangles, as illustrated below.
The coordinates of the centroid are determined as
ð¥ð¥ =âð¥ð¥ï¿œððâðð =
0.25(0.5)(6)ᅵᅵᅵᅵᅵᅵᅵᅵᅵArea 1
+ 3.25(5.5)(0.5)ᅵᅵᅵᅵᅵᅵᅵᅵᅵArea 2
0.5(6) + 5.5(0.5) = 1.68 in
ðŠðŠ =âðŠðŠï¿œððâðð =
0.25(0.5)(5.5) + 3(6)(0.5)0.5(5.5) + 6(0.5) = 1.68 in.
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Hence, the location of the centroid C is ð¶ð¶(ï¿œÌ ï¿œð¥,ðŠðŠï¿œ) = (1.68, 1.68). We are now ready to determine the moments of inertia relative to the axes with origin at the centroid. For Ix, we have
ðŒðŒð¥ð¥ = ðŒðŒ1 + ðŒðŒ2
where I1 pertains to cross-sectional area 1 and I2 pertains to cross-sectional area 2; that is,
ðŒðŒ1 = ï¿œ(0.5)(6)3
12 + 0.5(6)(3â 1.68)2ï¿œ = 14.23 in.4
ðŒðŒ2 = ï¿œ(5.5)(0.5)3
12 + 5.5(0.5)(1.68â 0.25)2ï¿œ = 5.68 in.4
so that ðŒðŒð¥ð¥ = 14.23 + 5.68 = 19.91 in.4 Similarly, for Iy, we have
ðŒðŒðŠðŠ = ðŒðŒ1 + ðŒðŒ2
where I1 and I2 are given by
ðŒðŒ1 = ï¿œ6(0.5)3
12 + 0.5(6)(1.68â 0.25)2ï¿œ = 6.20 in.4
ðŒðŒ2 = ï¿œ0.5(5.5)3
12 + 0.5(5.5)(1.57)2ᅵ = 13.71 in.4
Therefore, ðŒðŒðŠðŠ = 6.20 + 13.71 = 19.91 in.4 We also require the product of inertia Ixy,
ðŒðŒð¥ð¥ðŠðŠ = 6(0.5)(â1.435)(1.32) + 5.5(0.5)(1.57)(â1.435) = â11.92 in.4
Finally, we establish the orientation of the principal axes,
tan 2ðððð = â2ðŒðŒð¥ð¥ðŠðŠ
ï¿œðŒðŒð¥ð¥ â ðŒðŒðŠðŠï¿œ= â
2(â11.92)(19.91â 19.91)ᅵᅵᅵᅵᅵᅵᅵᅵᅵᅵᅵ
=0
â â
Clearly, then, 2ðððð = arctan(â) â 90o andâ 90o, which implies that
2ðððð = 90o â ðððð = 45o andâ 45o
â The correct answer is C.
Answer Summary
Problem 1 1A A 1B B 1C B
Problem 2 T/F Problem 3 T/F
Problem 4 4A C 4B C
Problem 5 B Problem 6 B Problem 7 A Problem 8 D Problem 9 T/F
Problem 10 C
References HIBBELER. R. (2010). Engineering Mechanics: Statics. 12th edition. Upper Saddle
River: Pearson. HIBBELER, R. (2013). Engineering Mechanics: Statics. 13th edition. Upper Saddle
River: Pearson. BEDFORD, A. and FOWLER, W. (2008). Engineering Mechanics: Statics. 5th
edition. Upper Saddle River: Pearson.
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