When we talk about the moment of inertia of a disk we can say that it is quite similar to that for a solid cylinder with any given measure of length. However, for a disk, we have to take it as a special character. Generally, it is used as a base for building the moment of inertia expression for different other shapes, such as a cylinder or a sphere. Meanwhile, we can also find the moment of inertia of a circular disc with respect to different situations. They are as follows; 1. Solid disk Here the axis of rotation is the central axis of the disk. It is expressed as; 2. Axis at Rim In this case, the axis of rotation of a solid disc is at the rim. It is given as; 3. Disc With a Hole Here the axis will be at the centre. It is expressed as; In order to explain how to calculate the moment of inertia of a disk, we will take the example of a uniform thin disk which is rotating about an axis through its centre. In the figure, we can see a uniform thin disk with radius r rotating about a Z-axis passing through the centre. As we have a thin disk, the mass is distributed all over the x and y plane. Then, we move on to establishing the relation for surface mass density (σ) where it is defined as or said to be the mass per unit surface area. Since the disk is uniform, therefore, the surface mass density will also be constant where; σ= m / A or σA=m so dm=σ(dA) Now it is time for the simplification of the area where it can be assumed the area to be made of a collection of rings that are mostly thin in nature. The thin rings are said to be the mass increment (dm) of radius r which are at equal distance from the axis. The small area (dA) of every ring is further expressed by the length (2πr) times the small width of the rings (dr.) It is given as; A = πr2, dA = d(πr2) = πdr2 = 2rdr Now, we add all the rings from a radius range of 0 to R to get the full area of the disk. The radius range that is given is the value that is used in the integration of dr. If we put all these together then we get; I = O∫R r2σ(πr)dr I = 2 π σO∫R r3dr I = 2 πσ r4 / 4 |oR I = 2 πσ (R4 / 4 – 0) I = 2 π (m / 4 )(R4 / 4) I = 2 π (m / π r2 )(R4 / 4) I = ½ mR2 This expression clearly proves all that we have discussed so far. ⇒ Check Other Object’s Moment of Inertia:Parallel Axis theorem
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Moment of inertia, denoted by I, measures the extent to which an object resists rotational acceleration about a particular axis, and is the rotational analogue to mass (which determines an object's resistance to linear acceleration). Mass moments of inertia have units of dimension ML2([mass] × [length]2). It should not be confused with the second moment of area, which is used in beam calculations. The mass moment of inertia is often also known as the rotational inertia, and sometimes as the angular mass.
For simple objects with geometric symmetry, one can often determine the moment of inertia in an exact closed-form expression. Typically this occurs when the mass density is constant, but in some cases the density can vary throughout the object as well. In general, it may not be straightforward to symbolically express the moment of inertia of shapes with more complicated mass distributions and lacking symmetry. When calculating moments of inertia, it is useful to remember that it is an additive function and exploit the parallel axis and perpendicular axis theorems.
This article mainly considers symmetric mass distributions, with constant density throughout the object, and the axis of rotation is taken to be through the center of mass unless otherwise specified.
Following are scalar moments of inertia. In general, the moment of inertia is a tensor, see below.
A point mass does not have a moment of inertia around its own axis, but using the parallel axis theorem a moment of inertia around a distant axis of rotation is achieved.
This expression assumes that the rod is an infinitely thin (but rigid) wire. This is a special case of the thin rectangular plate with axis of rotation at the center of the plate, with w = L and h = 0.
This expression assumes that the rod is an infinitely thin (but rigid) wire. This is also a special case of the thin rectangular plate with axis of rotation at the end of the plate, with h = L and w = 0.
This is a special case of a torus for a = 0 (see below), as well as of a thick-walled cylindrical tube with open ends, with r1 = r2 and h = 0.
This is a special case of the solid cylinder, with h = 0. That
I
x
=
I
y
=
I
z
2
{\displaystyle I_{x}=I_{y}={\frac {I_{z}}{2}}\,}
is a consequence of the perpendicular axis theorem.
I
x
=
I
y
=
1
4
m
(
r
1
2
+
r
2
2
)
{\displaystyle I_{x}=I_{y}={\frac {1}{4}}m(r_{1}^{2}+r_{2}^{2})}
I
x
=
I
y
=
1
4
π
ρ
A
(
r
2
4
−
r
1
4
)
{\displaystyle I_{x}=I_{y}={\frac {1}{4}}\pi \rho _{A}(r_{2}^{4}-r_{1}^{4})}
This expression assumes that the shell thickness is negligible. It is a special case of the thick-walled cylindrical tube for r1 = r2.
Also, a point mass m at the end of a rod of length r has this same moment of inertia and the value r is called the radius of gyration.
This is a special case of the thick-walled cylindrical tube, with r1 = 0.
I
z
=
1
2
m
(
r
2
2
+
r
1
2
)
=
m
r
2
2
(
1
−
t
+
t
2
2
)
{\displaystyle I_{z}={\frac {1}{2}}m\left(r_{2}^{2}+r_{1}^{2}\right)=mr_{2}^{2}\left(1-t+{\frac {t^{2}}{2}}\right)}
[1][2]
I
x
=
I
y
=
π
ρ
h
12
(
3
(
r
2
4
−
r
1
4
)
+
h
2
(
r
2
2
−
r
1
2
)
)
{\displaystyle I_{x}=I_{y}={\frac {\pi \rho h}{12}}\left(3(r_{2}^{4}-r_{1}^{4})+h^{2}(r_{2}^{2}-r_{1}^{2})\right)}
I
h
o
l
l
o
w
=
1
12
m
s
2
{\displaystyle I_{\mathrm {hollow} }={\frac {1}{12}}ms^{2}\,\!}
[3]
I
x
,
s
o
l
i
d
=
I
y
,
s
o
l
i
d
=
I
z
,
s
o
l
i
d
=
39
ϕ
+
28
150
m
s
2
{\displaystyle I_{x,\mathrm {solid} }=I_{y,\mathrm {solid} }=I_{z,\mathrm {solid} }={\frac {39\phi +28}{150}}ms^{2}\,\!}
(where
ϕ
=
1
+
5
2
{\displaystyle \phi ={\frac {1+{\sqrt {5}}}{2}}}
) [3]
I
x
,
s
o
l
i
d
=
I
y
,
s
o
l
i
d
=
I
z
,
s
o
l
i
d
=
ϕ
2
10
m
s
2
{\displaystyle I_{x,\mathrm {solid} }=I_{y,\mathrm {solid} }=I_{z,\mathrm {solid} }={\frac {\phi ^{2}}{10}}ms^{2}\,\!}
[3] When the cavity radius r1 = 0, the object is a solid ball (above).
When r1 = r2,
r
2
5
−
r
1
5
r
2
3
−
r
1
3
=
5
3
r
2
2
{\displaystyle {\frac {r_{2}^{5}-r_{1}^{5}}{r_{2}^{3}-r_{1}^{3}}}={\frac {5}{3}}r_{2}^{2}}
, and the object is a hollow sphere.
About an axis passing through the tip: (Axis of rotation along a side of the plate)
For a similarly oriented cube with sides of length
s
{\displaystyle s}
,
I
C
M
=
1
6
m
s
2
{\displaystyle I_{\mathrm {CM} }={\frac {1}{6}}ms^{2}\,\!}
For a cube with sides
s
{\displaystyle s}
,
I
=
1
6
m
s
2
{\displaystyle I={\frac {1}{6}}ms^{2}\,\!}
.
For a cube with sides
s
{\displaystyle s}
,
I
=
1
6
m
s
2
{\displaystyle I={\frac {1}{6}}ms^{2}\,\!}
.
This list of moment of inertia tensors is given for principal axes of each object.
To obtain the scalar moments of inertia I above, the tensor moment of inertia I is projected along some axis defined by a unit vector n according to the formula:
where the dots indicate tensor contraction and the Einstein summation convention is used. In the above table, n would be the unit Cartesian basis ex, ey, ez to obtain Ix, Iy, Iz respectively.
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