Lecture on rigid dynamics Sri.S. N. Mishra
Objective
• Moment of Inertia of a body
• Parallel Axis Theorem
• Radius of Gyration
• Moment of Inertia of Composite Bodies
Moment and Angular Acceleration
• When M 0, rigid body experiences
angular acceleration
• Relation between M and a is analogous to
relation between F and a
aIma M,F
Moment of Inertia
Mass = Resistance
Moment of Inertia
• This mass analog is called the
moment of inertia, I, of the object
– r = moment arm
– SI units are kg m2
m
dmrI 2
dzdydxrI 2
dVrI
dVdm
2
:density volume theis where, Using
RodThin
12
1 2MLI
L
end)at (axis RodThin
3
1 2MLI
L
Disk Solid
2
1 2MRI
R
Cylinder Hollow
)(2
1 2
2
2
1 RRMI
R2
R2
Cylinder Hollow dThin Walle
2MRI
R
a
b
center)(through Plater Rectangula
)(12
1 22 baMI
a
b
edge)(about Plater RectangulaThin
3
1 2MaI
Sphere Solid
5
2 2MRI
R
Sphere Hollow dThin Walle
3
2 2MRI
R
Moments of inertia for some common geometric solids
Parallel Axis Theorem
• The moment of inertia about any axis parallel to
and at distance d away from the axis that
passes through the centre of mass is:
• Where
– IG= moment of inertia for mass centre G
– m = mass of the body
– d = perpendicular distance between the parallel axes.
2mdII GO
Radius of Gyration
Frequently tabulated data related to moments of inertia will be
presented in terms of radius of gyration.
m
IkormkI 2
Moment of Inertia of Composite bodies
1. Divide the composite area into simple body.
2. Compute the moment of inertia of each simple body about its
centroidal axis from table.
3. Transfer each centroidal moment of inertia to a parallel reference axis
4. The sum of the moments of inertia for each simple body about the
parallel reference axis is the moment of inertia of the composite
body.
5. Any cutout area has must be assigned a negative moment; all others
are considered positive.
Moment of inertia of a hollow cylinder
• Moment of Inertia of a
solid cylinder
• A hollow cylinder
I = 1/2 mR2
= -m1
R1 R2
m2
I = 1/2 m1R12 - 1/2 m2R2
2 = 1/2 M (R12 - R2
2 )
M
Example 17-3
2
2
1rmIG kgmm
m
kgVm ddd 71.15)]01.0()25.0([8000 2
3
2
22
22
kg.m473.1
)25.0)(71.15()25.0)(71.15(2
1
2
1)(
mkgmkg
dmrmI dddOd
kgmmm
kgVm hhh 93.3)]01.0()125.0([8000 2
3
2
22
22
kg.m276.0
)25.0)(93.3()125.0)(93.3(2
1
2
1)(
mkgmkg
dmrmI hhhOh
2.2.1276.0473.1
)()(
mkg
III OhOdO
2
22
22
kg.m276.0
)25.0)(93.3()125.0)(93.3(2
1
2
1)(
mkgmkg
dmrmI hhhOh
kgmmm
kgVm ddd 71.15)]01.0()25.0([8000 2
3
kgmmm
kgVm hhh 93.3)]01.0()125.0([8000 2
3
2
2
3ddzz rmI )
2
1( 22
dmrm hhh
2)25.0)(71.15(
2
3zzI ))25.0)(93.3()125.0)(93.3(
2
1( 22 mkgmkg
Example 17-4
222 slug.ft414.0)ft2)(ft/s2.32
Ib10(
3
1
3
1)( mlI OOA
2
2222
slug.ft346.1
)2)(2.32
10()2)(
2.32
10(
12
1
12
1)(
mdmlI OBC
2slug.ft76.1346.1414.0 OI
m
myy
~
ft5.1)2.32/10()2.32/10(
)2.32/10(2)2.32/10(1~
m
myy 2
2
2
slug.ft362.0
)5.1)(2.32
20(76.1
G
G
GO
I
I
mdII