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Wikipedia: moment of inertia

This article is about the moment of inertia of a rotating object. For the moment of inertia dealing with bending of a plane, see second moment of area.

Moment of inertia, also called mass moment of inertia and, sometimes, the angular mass, (SI units kg m², Former British units slug ft²), is the rotational analog of mass. That is, it is the inertia of a rigid rotating body with respect to its rotation. The moment of inertia plays much the same role in rotational dynamics as mass does in basic dynamics, determining the relationship between angular momentum and angular velocity, torque and angular acceleration, and several other quantities. While a simple scalar treatment of the moment of inertia suffices for many situations, a more advanced tensor treatment allows the analysis of such complicated systems as spinning tops and gyroscope motion.

The symbols $I$ and sometimes $J$ are usually used to refer to the moment of inertia.

Overview

The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. For example, consider two discs (A and B) of the same mass. Disc A has a larger radius than disc B. Assuming that there is uniform thickness and mass distribution, it requires more effort to accelerate disc A (change its angular velocity) because its mass is distributed further from its axis of rotation: mass that is further out from that axis must, for a given angular velocity, move more quickly than mass closer in. In this case, disc A has a larger moment of inertia than disc B.

Divers minimizing their moments of inertia in order to increase their rates of rotation.

The moment of inertia of an object can change if its shape changes. A figure skater who begins a spin with arms outstretched provides a striking example. By pulling in her arms, she reduces her moment of inertia, causing her to spin faster (by the conservation of angular momentum).

The moment of inertia has two forms, a scalar form $I$ (used when the axis of rotation is known) and a more general tensor form that does not require knowing the axis of rotation. The scalar moment of inertia $I$ (often called simply the "moment of inertia") allows a succinct analysis of many simple problems in rotational dynamics, such as objects rolling down inclines and the behavior of pulleys. For instance, while a block of any shape will slide frictionlessly down a decline at the same rate, rolling objects may descend at different rates, depending on their moments of inertia. A hoop will descend more slowly than a solid disk of equal diameter because more of its mass is located far from the axis of rotation, and thus needs to move faster if the hoop rolls at the same angular velocity. However, for (more complicated) problems in which the axis of rotation can change, the scalar treatment is inadequate, and the tensor treatment must be used (although shortcuts are possible in special situations). Examples requiring such a treatment include gyroscopes, tops, and even satellites, all objects whose alignment can change.

The moment of inertia can also be called the mass moment of inertia (especially by mechanical engineers) to avoid confusion with the second moment of area, which is sometimes called the moment of inertia (especially by structural engineers) and denoted by the same symbol $I$ . The easiest way to differentiate these quantities is through their units. In addition, the moment of inertia should not be confused with the polar moment of inertia, which is a measure of an object's ability to resist torsion (twisting).

Scalar moment of inertia

Definition

The (scalar) moment of inertia of a point mass rotating about a known axis is defined by

Failed to parse (unknown function\stackrel): I \ \stackrel{\mathrm{def}}{=}\ m r^2\,\!

where

m is the mass,

and r is the (perpendicular) distance of the point mass to the axis of rotation.

The moment of inertia is additive. Thus, for a rigid body consisting of $N$ point masses $m i$ with distances $r i$ to the rotation axis, the total moment of inertia equals the sum of the point-mass moments of inertia:

Failed to parse (unknown function\stackrel): I \ \stackrel{\mathrm{def}}{=}\ \sum_{i=1}^{N} {m_{i} r_{i}^2}\,\!

For a solid body described by a continuous mass density function ρ(x,y,z), the moment of inertia about a known axis can be calculated by integrating the square of the distance (weighted by the mass density) from a point in the body to the rotation axis :

Failed to parse (unknown function\stackrel): I \ \stackrel{\mathrm{def}}{=}\ \iiint_V r^2 \,\rho(x,y,z)\,dx\,dy\,dz \!

where

V is the volume occupied by the object. (While the triple integral may be taken over all space, only the region where ρ(x,y,z) ≠ 0 will contribute).

ρ is the spatial density function of the object, and

x, y, z are cartesian coordinates of a point inside the body.

Diagram for the calculation of a disk's moment of inertia. Here k is 1/2 and r is the radius used in determining the moment.

The moment of inertia for many non-point objects can also be found or approximated as the product of three terms:

$I \approx k\cdot M\cdot {R}^2 \,\!$

where

k is the inertial constant,

M is the mass, and

R is the radius of the object from the center of mass (in some cases, the length of the object is used instead.)

Inertial constants are used to account for the differences in the placement of the mass from the center of rotation. Examples include:

k = 1, thin ring or thin-walled cylinder around its center,
k = 2/5, solid sphere around its center
k = 1/2, solid cylinder or disk around its center.

For more examples, see the List of moments of inertia.

Parallel axis theorem

Main article: Parallel axis theorem

Once the moment of inertia has been calculated for rotations about the center of mass of a rigid body, one can conveniently recalculate the moment of inertia for all parallel rotation axes as well, without having to resort to the formal definition. If the axis of rotation is displaced by a distance $R$ from the center of mass axis of rotation (e.g. spinning a disc about a point on its periphery, rather than through its center,) the new moment of inertia equals:

$I_{\mathrm{displaced}} = I_{\mathrm{center}} + M R^{2} \,\!$

This theorem is also known as the parallel axes rule and is a special case of Steiner's parallel-axis theorem.

Equations involving the moment of inertia

The rotational kinetic energy of a system can be expressed in terms of its moment of inertia. For a system with $N$ point masses $m i$ moving with speeds $v i$ , the rotational kinetic energy $T$ equals

$T = \sum_{i=1}^{N} \frac{1}{2} m_{i} v_{i}^{2}\,\! = \sum_{i=1}^{N} \frac{1}{2} m_{i} (\omega r_{i})^{2} = \frac{1}{2} \sum_{i=1}^{N} m_{i} r_{i}^{2} \omega^{2} = \frac{1}{2} I \omega^{2}$

where $ω$ is the common angular velocity (in radians per second). The final formula $T=\frac{1}{2} I \omega^{2}\,\!$ also holds for a continuous distribution of mass with a generalisation of the above derivation from a discrete summation to an integration.

In the special case where the angular momentum vector is parallel to the angular velocity vector, one can relate them by the equation

$L = I\omega \;$

where L is the angular momentum and $ω$ is the angular velocity. However, this equation does not hold in many cases of interest, such as the torque-free precession of a rotating object, although its more general tensor form is always correct.

When the moment of inertia is constant, one can also relate the torque on an object and its angular acceleration in a similar equation:

$N = I\alpha \!$

where N is the torque and $α$ is the angular acceleration.

Moment of inertia tensor

For the same object, different axes of rotation will have different moments of inertia about those axes. In general, the moments of inertia are not equal unless the object is symmetric about all axes. The moment of inertia tensor is a convenient way to summarize all moments of inertia of an object with one quantity. It may be calculated with respect to any point in space, although for practical purposes the center of mass is most commonly used.

Definition

For a rigid object of $N$ point masses $m k$ , the moment of inertia tensor is given by

$\mathbf{I} = \begin{bmatrix} I_{xx} & I_{xy} & I_{xz} \\ I_{yx} & I_{yy} & I_{yz} \\ I_{zx} & I_{zy} & I_{zz} \end{bmatrix}$ .

Its components are defined as

Failed to parse (unknown function\stackrel): I_{ij} \ \stackrel{\mathrm{def}}{=}\ \sum_{k=1}^{N} m_{k} (r_k^{2}\delta_{ij} - r_{ki}r_{kj})\,\!

where

i, j equal 1, 2, or 3 for x, y, and z, respectively,

r_k is the distance of mass k from the point about which the tensor is calculated, and

$δ i j$ is the Kronecker delta.

The diagonal elements are more succinctly written as

Failed to parse (unknown function\stackrel): I_{xx} \ \stackrel{\mathrm{def}}{=}\ \sum_{k=1}^{N} m_{k} (y_{k}^{2}+z_{k}^{2}),\,\!

Failed to parse (unknown function\stackrel): I_{yy} \ \stackrel{\mathrm{def}}{=}\ \sum_{k=1}^{N} m_{k} (x_{k}^{2}+z_{k}^{2}),\,\!

Failed to parse (unknown function\stackrel): I_{zz} \ \stackrel{\mathrm{def}}{=}\ \sum_{k=1}^{N} m_{k} (x_{k}^{2}+y_{k}^{2}),\,\!

while the off-diagonal elements, also called the products of inertia, are

Failed to parse (unknown function\stackrel): I_{xy} = I_{yx} \ \stackrel{\mathrm{def}}{=}\ -\sum_{k=1}^{N} m_{k} x_{k} y_{k},\,\!

Failed to parse (unknown function\stackrel): I_{xz} = I_{zx} \ \stackrel{\mathrm{def}}{=}\ -\sum_{k=1}^{N} m_{k} x_{k} z_{k},\,\!

and

Failed to parse (unknown function\stackrel): I_{yz} = I_{zy} \ \stackrel{\mathrm{def}}{=}\ -\sum_{k=1}^{N} m_{k} y_{k} z_{k},\,\!

Here $I x x$ denotes the moment of inertia around the $x$ -axis when the objects are rotated around the x-axis, $I x y$ denotes the moment of inertia around the $y$ -axis when the objects are rotated around the $x$ -axis, and so on.

These quantities can be generalized to an object with continuous density in a similar fashion to the scalar moment of inertia. One then has

$\mathbf{I}=\iiint_V \rho(x,y,z)\left( r^2 \mathbf{E}_{3} - \mathbf{r}\otimes \mathbf{r}\right)\, dxdydz,$

where $\otimes$ is the outer product, E₃ is the 3 × 3 identity matrix, and V is a region of space completely containing the object.

Derivation of the tensor components

It is not very obvious how best to derive the formula given above. One succinct and nice derivation is the given by Landau and Lifshitz. To state it in words, the formula can be derived by writing out an expression for total kinetic energy by identifying the velocity of every point as a sum of center-of-mass translational velocity + rotational velocity about the center of mass. By gathering the terms for the rotational velocity term, the moment of inertia tensor can be found.

Reduction to scalar

For any axis $\hat{\mathrm{n}}$ , represented as a column vector with elements n_i, the scalar form I can be calculated from the tensor form I as

$I = \mathbf{\hat{n}^{T}} \mathbf{I}\, \mathbf{\hat{n}} = \sum_{j=1}^{3} \sum_{k=1}^{3} n_{j} I_{jk} n_{k} .$

The range of both summations correspond to the three Cartesian coordinates.

The following equivalent expression avoids the use of transposed vectors which are not always supported in maths libraries:

$I = \left(\mathbf{{I}^{T}} \mathbf{\hat{n}}\right) \cdot \mathbf{\hat{n}}.$

However it should be noted that although this equation is mathematically equivalent to the equation above for any matrix, inertia tensors are symmetrical. This means that it can be further simplified to:

$I = \left(\mathbf{{I}} \mathbf{\hat{n}}\right) \cdot \mathbf{\hat{n}}.$

Principal moments of inertia

Since the moment of inertia tensor is real and symmetric, it is possible to find a Cartesian coordinate system in which it is diagonal, having the form

$\mathbf{I} = \begin{bmatrix} I_{1} & 0 & 0 \\ 0 & I_{2} & 0 \\ 0 & 0 & I_{3} \end{bmatrix}$

where the coordinate axes are called the principal axes and the constants $I 1$ , $I 2$ and $I 3$ are called the principal moments of inertia. The unit vectors along the principal axes are usually denoted as $(\mathbf{e}_{1}, \mathbf{e}_{2}, \mathbf{e}_{3})$ .

When all principal moments of inertia are distinct, the principal axes are uniquely specified. If two principal moments are the same, the rigid body is called a symmetrical top and there is no unique choice for the two corresponding principal axes. If all three principal moments are the same, the rigid body is called a spherical top (although it need not be spherical) and any axis can be considered a principal axis, meaning that the moment of inertia is the same about any axis.

The principal axes are often aligned with the object's symmetry axes. If a rigid body has an axis of symmetry of order $m$ , i.e., is symmetrical under rotations of 360°/m about a given axis, the symmetry axis is a principal axis. When $m > 2$ , the rigid body is a symmetrical top. If a rigid body has at least two symmetry axes that are not parallel or perpendicular to each other, it is a spherical top, e.g., a cube or any other Platonic solid. A practical example of this mathematical phenomenon is the routine automotive task of balancing a tire, which basically means adjusting the distribution of mass of a car wheel such that its principal axis of inertia is aligned with the axle so the wheel does not wobble.

Parallel axes theorem

Once the moment of inertia tensor has been calculated for rotations about the center of mass of the rigid body, there is a useful labor-saving method to compute the tensor for rotations offset from the center of mass.

If the axis of rotation is displaced by a vector R from the center of mass, the new moment of inertia tensor equals

$\mathbf{I}^{\mathrm{displaced}} = \mathbf{I}^{\mathrm{center}} + M \left[ \left(\mathbf{R} \cdot \mathbf{R}\right) \mathbf{E}_{3} - \mathbf{R} \otimes \mathbf{R} \right]$

where $M$ is the total mass of the rigid body, E₃ is the 3 × 3 identity matrix, and $\otimes$ is the outer product.

Other mechanical quantities

Using the tensor I, the kinetic energy can be written as a double inner product

$T = \frac{1}{2} \boldsymbol\omega \cdot \mathbf{I} \cdot \boldsymbol\omega = \frac{1}{2} I_{1} \omega_{1}^{2} + \frac{1}{2} I_{2} \omega_{2}^{2} + \frac{1}{2} I_{3} \omega_{3}^{2}$

and the angular momentum can be written as a single inner product

$\mathbf{L} = \mathbf{I} \cdot \boldsymbol\omega = \omega_{1} I_{1} \mathbf{e}_{1} + \omega_{2} I_{2} \mathbf{e}_{2} + \omega_{3} I_{3} \mathbf{e}_{3}$

Taken together, one can express the rotational kinetic energy in terms of the angular momentum $(L 1, L 2, L 3)$ in the principal axis frame as

$T = \frac{L_{1}^{2}}{2I_{1}} + \frac{L_{2}^{2}}{2I_{2}} + \frac{L_{3}^{2}}{2I_{3}}.\,\!$

The rotational kinetic energy and the angular momentum are constants of the motion (conserved quantities) in the absence of an overall torque. The angular velocity ω is not constant; even without a torque, the endpoint of this vector may move in a plane (see Poinsot's construction).

See the article on the rigid rotor for more ways of expressing the kinetic energy of a rigid body.

References

Goldstein H. (1980) Classical Mechanics, 2nd. ed., Addison-Wesley. ISBN 0-201-02918-9

Landau LD and Lifshitz EM. (1976) Mechanics, 3rd. ed., Pergamon Press. ISBN 0-08-021022-8 (hardcover) and ISBN 0-08-029141-4 (softcover).

Marion JB and Thornton ST. (1995) Classical Dynamics of Systems and Particles, 4th. ed., Thomson. ISBN 0-03-097302-3

Symon KR. (1971) Mechanics, 3rd. ed., Addison-Wesley. ISBN 0-201-07392-7

Tenenbaum, RA. (2004) Fundamentals of Applied Dynamics, Springer. ISBN 0-387-00887-X

External links

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moment of inertia

Overview

Scalar moment of inertia

Definition

Parallel axis theorem

Equations involving the moment of inertia

Moment of inertia tensor

Definition

Derivation of the tensor components

Reduction to scalar

Principal moments of inertia

Parallel axes theorem

Other mechanical quantities

See also

References

External links

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