paritoshchahal(991345820) mathproject[final]
DESCRIPTION
Explanation of moment of inertia through integralsTRANSCRIPT
Sheridan Institute of Technology and Advanced Learning
Faculty of Applied Science and Technology
School of Mechanical and Electrical Engineering & Technology
_____________________________________________
MATH22558 Integral Calculus - 1155_56353
Project Report
Instructor: Patrick Keenan
Class #: 56353
Paritosh Chahal (991345820)
Date of Submission: 11th August 2015
1
Table of Contents
Objective: ...................................................................................................................................................... 2
Introduction: ................................................................................................................................................. 2
Techniques of finding moment of inertia: .................................................................................................... 3
Formula of Moment of inertia of an object with constant Mass βmβ: ..................................................... 3
1. Moment of inertia for a piece of mass βdmβ about center of mass: ................................................ 5
2. Moment of inertia of an arbitrary plane area about the x-axis: ....................................................... 6
3. Moment of inertia of an arbitrary plane area about the y-axis: ..................................................... 11
4. Moment of inertia of a volume about the x-axis: ........................................................................... 11
5. Moment of inertia of a volume about the y-axis: ........................................................................... 11
Proof of Moment of Inertia of shapes: ...................................................................................................... 11
1. Cylinder: .......................................................................................................................................... 11
2. Rectangles ....................................................................................................................................... 13
3. Moment of Inertia of rectangle about x-axis: ................................................................................. 14
4. Cylinder about some axis: ............................................................................................................... 15
5. Moment of inertia of area bounded by 2 curves: about y-axis: ..................................................... 16
6. Moment of inertia of area bounded by 2 curves: about x-axis: ..................................................... 17
Appendix: .................................................................................................................................................... 18
References: ................................................................................................................................................. 21
2
Objective: The purpose of this report is to prove the moment of inertia formula of any given object through
integration.
Introduction: As learned in Applied Mechanics 1, moment of inertia is defined as the measure of a body to
resist rotation [in other words, moment of inertia can be described as the βmassβ in rotational
dynamics formulas (fig1)].
Fig 1: Inertia of an object has the same job as βmassβ of an object has in linear dynamics
Moment of Inertia is an important subject in many different fields of mechanical engineering.
For instance, moment of inertia is widely used in measuring the strength of different type of
beams, rods, and building structures. To elaborate, as learned in the course Mechanics of
Materials, βArea Moment of Inertiaβ (I) is used to find the maximum bending stress1, maximum
horizontal shearing stress2, shear flow
3 along a shear plane, and maximum deflection
5 of
cantilever beams (fig 2) and simply supported beam (fig 3).
3
Fig 2: Cantilever Beam
Fig 3: Simply supported Beam
In addition, other than βArea Moment of Inertiaβ (I), the βPolar Moment of Inertiaβ (J) is used to
measure find the maximum torsion5 a rod (solid of hollow) can resist.
Other than the βstructural (beam is technically considered as a structure) mechanics fieldβ, the
moment of inertia also has many uses in the dynamics field. TO elaborate, moment of inertia can
be used to find how fast an object can accelerate while rotating with reference to its center, how
much time does a crane (or pulley system) takes to lift an object with mass βxβ, and energy
transfer at different instances when a wheel is rolled down a slope (which help in understanding
the designs of transportation systems such as cars, buses, trucks etc.).
Techniques of finding moment of inertia: Even though moment of inertia has same definition for every rigid object in the universe;
different techniques are used to find moment of inertia depending on the shape and type of
object.
Formula of Moment of inertia of an object with constant Mass βmβ:
The basic formula of Moment of Inertia is as given:
πΌ = π β π 2
Where:
M= mass of the object
R= Distance from the center
4
Looking at the formula above, one would say that the above formula is a quadratic function (as
learned in MATH 17688- Mathematics 1) where inertia is doubled as the length of the constant
mass βMβ from the center of rotation is increased by 1 unit.
Same formula (stated above) can be used for finding the moment of inertia of 2D with constant
area:
πΌ = βπ΄ β π 2
Where:
A= Area of the shape (constant)
R= Distance from the center of rotation
Using the above two formulas, we will derive formulas for the following in the next few
sections:
1. Moment of inertia for a strip about center of mass
2. Moment of inertia of an arbitrary plane area about the x-axis
3. Moment of inertia of an arbitrary plane area about the y-axis
4. Moment of inertia of a volume of revolution about the x-axis.
5. Moment of inertia of a volume of revolution about the y-axis.
5
1. Moment of inertia for a piece of mass βdmβ about center of mass: Suppose we have a cylinder with radius βRβ, mass βMβ and height βhβ (fig 4):
Fig 4: Cylinder with height βhβ and radius βRβ
Suppose we want to find the moment of Inertia of a thin strip of material with mass dm (fig 5
shows the circular cross section of the cylinder) with reference to the center axis (red)
NOTE: As learned in Math
Fig 5: The cross-section of cylinder in figure 4
Moment of inertia for the single strip is:
πΌ = π₯2(ππ) [1]
Above equation can be to any arbitrary shape.
R
h
x
6
In terms of second moment of inertia:
Let the βdmβ piece in figure 5 has area βdAβ:
πΌ = π₯2(ππ΄) [2]
2. Moment of inertia of an arbitrary plane area about the x-axis:
Fig 6: Random arbitrary shape
To find the moment of inertia of the arbitrary shape (with area βAβ) above, one has to
take the sum of all small βdAβ from equation [2]. So the moment of inertia of an arbitrary
shape (with reference to center of mass of the shape is:
(π2)βππ΄ = β« π2 β ππ΄π΄
0 [3]
Now to find the moment of inertia of the same arbitrary shape above with respect to the
x-axis (fig 6); one may need to use the parallel axis theorem.
Fig 7: Parallel axis theorem
7
The moment of inertia is the least when the reference (rotation) axis is at the center of gravity of
any arbitrary shape; as the reference axis increased further away from the center of mass, the
moment of inertia of the arbitrary shape increases:
πΌ = πΌππ +ππ2(Parallel axis theorem formula) [4]
πΌ = πΌππ + π΄π2 (Second Moment of Inertia) [5]
1. Say we have an arbitrary shape (of area βAβ), and we have to find the moment of inertia
of this shape with respect to origin βOβ (fig 7)
Fig 7
NOTE: The rotation axis is βcoming out of the paperβ
i. Moment of inertia of the shape with respect to center of gravity:
πΌ = βπ2(ππ΄) [6]
ii. Changing the reference point from βCGβ to βOβ (fig 8), the moment of inertia will
be:
πΌ = (π + π )2πππ΄ [7] (where R is given distance between βOβ and βCGβ)
NOTE: (π + π )2 = (π π₯ + ππ₯)2 + (π π¦ + ππ¦)
2
y
x
O
T
8
Fig 8
So the equation 7 changes to πΌ = β« [(π π₯ + ππ₯)2 + (π π¦ + ππ¦)
2] π ππ΄
iii. Using the binomial expansion on βupdatedβ equation 8:
πΌ = β« [((π π₯2) + 2(π π₯ )(ππ₯) + (ππ₯
2)) + ((π π¦)2 + 2(π π¦)(ππ¦) + (ππ¦)
2) ]π ππ΄
πΌ = β« [(π π₯2) + (π π¦)
2 + (ππ¦)2+ (ππ₯
2) + 2(π π₯ )(ππ₯) + 2(π π¦)(ππ¦)]π ππ΄ [9]
iv. We can divide equation 9 into 4 different notation to make the proof a bit more
simpler:
πΌ = [β«(π π₯2) + (π π¦)
2 + β«(ππ¦)2+ (ππ₯
2) + β«2(π π₯ )(ππ₯) + β« 2(π π¦)(ππ¦)] β ππ΄ [10]
v. In equation 10 the first notation has a constant value which sums up to βRβ:
β«[(π π₯2) + (π π¦)
2] ππ΄ = π 2β«ππ΄ = π 2(π΄)
vi. Second notation is quite similar to equation 6:
β«[(ππ¦)2+ (ππ₯
2)] ππ΄ = π2(π΄) = πΌπΆπΊ
R
T
Tx
Ty
Ry
Rx
9
vii. For final two notation, one may observe that they have two constants respectively
that can be taken out:
β«[2(π π₯ )(ππ₯)] ππ΄ = 2π π₯ β«(ππ₯)ππ΄ [10]
β«[2(π π¦ )(ππ¦)] ππ΄ = 2π π¦ β«(ππ¦)ππ΄[11]
The above two equation is very similar to the center of mass equation:
πΆπΊπ₯ =β«π₯β² β ππ΄
π΄
πΆπΊπ¦ =β«π¦β² β ππ΄
π΄
Where βxββ and βyββ is the effective distance between the center of mass (i.e.
gravity) point and origin of the x-y axes (fig 9).
Fig 9
Since the βxβ and βyβ axis (which helped us find the distance dA) from fig 8 is within the center
of gravity (fig 10), the sum of all the βTyβ and βTxβ values will sum up to zero*.
*NOTE: According to vector rules, the values of βTyβ and βTxβ on one side are positive; and
negative on other side.
10
Fig 10
The above statement proves the equation 10 and 11 will sum up to 0.
β«[2(π π₯ )(ππ₯)] ππ΄ = 2π π₯ β«(ππ₯)ππ΄ = 0
β«[2(π π¦ )(ππ¦)] ππ΄ = 2π π¦ β«(ππ¦)ππ΄ = 0
viii. Summing them all up gives us the parallel axis theorem formula:
πΌπ₯ = πΌπΆπΊ + (π 2(π΄))
2. Therefore, to find the moment of inertia about the βx-axisβ of the arbitrary shape from
figure 8:
πΌπ¦ = πΌπΆπΊ + (π π¦2(π΄))[12] (where Ry is a given distance between the axis and point βCGβ
of body).
Y
X
11
3. Moment of inertia of an arbitrary plane area about the y-axis: To find the moment of inertia about the y-axis; the parallel axis theorem is used:
πΌπ¦ = πΌπΆπΊ + (π π₯2(π΄))[13] (Where Rx is a given distance between the axis and point βCGβ
of body)
4. Moment of inertia of a volume about the x-axis: The moment of inertia of a solid 3D model (with a definite volume) [fig 4] have similar
techniques as finding second moment of inertia of 2D arbitrary shapes.
The moment of inertia of 3D model with reference from center of axis (fig 4) is:
πΌ = β« π₯2 β ππ
NOTE: ππ = ππ£ π π [Volume X density]
πΌππ = π β« π₯2 β ππ£ [14]
To find the moment of inertia of a volume with respect to x-axis; we use parallel axis theorem:
πΌπ₯ = πΌππ + (π π¦2(π))[15] (where Ry is a given distance)
5. Moment of inertia of a volume about the y-axis: To find the moment of inertia of a volume with respect to y-axis; we use parallel axis theorem:
πΌπ¦ = πΌππ + (π π₯2(π))[16] (Where Rx is a given distance)
Proof of Moment of Inertia of shapes:
1. Cylinder: Letβs use the above information gathered to make the equations above to find the moment of
inertia of a cylinder about its center axis from fig 4:
12
From equation 14 we know:
πΌππ = π β« π₯2 β ππ£
Fig 11
Sum of all the small all the thin cylindrical shell with thickness βdrβ (fig11) can help us to find
the moment of inertia of the solid cylinder
ππ£ = (π ππππππ2 β π π ππππππ
2 )π(β)
ππ£ = (π ππππππ β π π ππππππ)(π ππππππ + π π ππππππ)π(β)
ππ£ = (ππ)(π ππππππ + π π ππππππ)π(β)
ππ£ = (2)(π ππππππ + π π ππππππ)
2π(β)(ππ)
NOTE: (π ππππππ+π π ππππππ)
2 is the average distance βRβ from center of the circle
ππ£ = (2)π π(β)(ππ)
πΌππ = πβ«π 2(2π )π(ππ)(β)
πΌππ = 2 π(π)(β)β«π 3(ππ)
dr
13
πΌππ = 2 π(π)(β)β«π 3(ππ)
πΌππ = 2 π(π)(β) (π 4
4)
πΌππ = (π 2) π(π)(β) (
π 2
2)
πΌππ =1
2(π)(π 2)[17]
2. Rectangles
Fig 12
Now say we have to find the moment of inertia of the rectangle above with fixed dimensions of
βL * Wβ with respect to the axis passing through the center of gravity of the shape.
Fig 13
πΌππ = β«π 2(ππ΄)
πΌππ = β« π 2(π β ππ¦)πΏ/2
βπΏ/2 [NOTE: Let R = some distance βyβ)
L
W
14
πΌππ = π β« π¦2(ππ¦)
πΏ/2
βπΏ/2
πΌππ = π [1
3(πΏ
2)3
β {β1
3(πΏ
2)3
}]
πΌππ = π [2
24(πΏ3)]
πΌππ = [πΏ3π
12] [18]
3. Moment of Inertia of rectangle about x-axis:
Fig 14
πΌπ = β«π 2(ππ΄)
πΌπ₯ = β« π 2(π β ππ¦)πΏ/2
βπΏ/2 [NOTE: Let R = some distance βyβ)
πΌπ₯ = πβ«π¦2(ππ¦)
πΏ
0
πΌπ₯ = π [1
3(πΏ
1)3
β {β1
3(0)3}]
πΌπ₯ = [πΏ3π
3] [19]
15
Using Parallel Axis Theorem:
πΌπ₯ = πΌπΆπΊ + (π π¦2(π΄))
πΌπ₯ = [πΏ3π
12] + ((
πΏ
2)2(πΏ β π))[NOTE:π π¦
2 = (πΏ
2)2]
[πΏ3π
12] +
(
(πΏ2
4) (πΏ βπ)
)
[πΏ3π
12] + (
3πΏ3π
12)
4πΏ3π
12
πΌπ₯ = [πΏ3π
3] [19]
4. Cylinder about some axis:
Fig 15
Using Parallel Axis Theorem:
πΌπ¦ = πΌππ + (π 2 (π))[16] (Where R is a given distance between y-axis and CG point of
cylinder)
πΌπ¦ = (1
2ππ 2) + (π 2 (π))
πΌπ¦ = (3
2ππ 2)[21]
R
Some axis
16
5. Moment of inertia of area bounded by 2 curves: about y-axis: Area bounded by curves y = x
2 and y = x [fig 16]:
Fig 16: Area bounded by the two functions
Since we are not given the centroid of the βareaβ, we can find it using integration (and
afterwards use parallel axis theorem), or derive an equation to find moment of inertia of areas
about x-axis.
I = r2
(A) = β(r2)(dA); to make the area if fig 16 follow this equation, the following should be
done:
Fig 17
y
x
x
dA
17
NOTE: In fig 17, dA = (dx)(x-x2), and x = R
πΌ = β« (π₯)2(π₯ β π₯2)ππ₯1
0
πΌ = β« π₯3 β π₯4ππ₯1
0
πΌ = (1
4π₯4) β (
1
5π₯5) + π
Use definite integral in the next few steps to find the answer (do it yourself, for practice).
6. Moment of inertia of area bounded by 2 curves: about x-axis: Same function as before:
NOTE: dA (βxβ in terms of y)= [(y)0.5
-(y)]dy, and R = βyβ
πΌ = β« (π¦)2(π¦0.5 β π¦)ππ¦1
0
πΌ = β« (π¦1 β π¦2)ππ¦1
0
πΌ = (1
2π¦2) β (
1
3π¦3) + πΆ
Use definite integral in the next few steps to find the answer (do it yourself, for practice).
y
dA
18
Appendix: 1. Maximum ending stress:
19
2. Horizontal shearing stress:
20
3. Shear Flow:
21
References: http://calculus.nipissingu.ca/tutorials/area_volume.html (Volumes by Cylindrical shells)
http://ummalqura-phy.com/HYPER1/icyl.html
http://www.intmath.com/applications-integration/4-volume-solid-revolution.php
https://www.youtube.com/watch?v=6I7iKz72HW4
http://www.intmath.com/applications-integration/6-moments-inertia.php
http://www.ce.siue.edu/examples/Worked_examples_Internet_text-only/Data_files-Worked_Exs-
Word_&_pdf/Moment_Inertia_Integration.pdf
http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html
http://www.strucalc.com/engineering-resources/normal-stress-bending-stress-shear-stress/
http://timbertoolbox.com/beamdesign.htm
http://www.slideshare.net/msheer/lesson-06-shearing-stresses (slide 8)
http://engineering-references.sbainvent.com/strength_of_materials/shear-flow.php
Morrow, H., & Kokernak, R. (2014). Statics and strength of materials (7th ed., pp. 305-336). Upper
Saddle River, NJ: Prentice Hall.
Walker, Keith M. "Equilibrium." Applied Mechanics for Engineering Technology. Upper Saddle River, NJ:
Pearson/Prentice Hall, 2004. 283-300. Print.