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Moment of inertia is the mass property of a rigid body that defines the torque needed for a desired angular acceleration about an axis of rotation. Moment of inertia depends on the shape of the body and may be different around different axes of rotation. A larger moment of inertia around a given axis requires more torque to increase the rotation, or to stop the rotation, of a body about that axis. Moment of inertia depends on the amount and distribution of its mass, and can be found through the sum of moments of inertia of the masses making up the whole object, under the same conditions. For example, if , then . In classical mechanics, moment of inertia may also be called mass moment of inertia, rotational inertia, polar moment of inertia, or the angular mass.
For planar movement of a body, the trajectories of all of its points lie in parallel planes, and the rotation occurs only about an axis perpendicular to this plane. In this case, the body has a single moment of inertia, which is measured around this axis.
For spatial movement of a body, the moment of inertia is defined by its symmetric 3x3 inertia matrix. The inertia matrix is often described as a symmetric rank two tensor, having six independent components. The inertia matrix includes offdiagonal terms called products of inertia that couple torque around one axis to acceleration about another axis. Each body has a set of mutually perpendicular axes, called principal axes, for which the offdiagonal terms of the inertia matrix are zero, and a torque around a principal axis only affects the acceleration about that axis.
When a body is rotating around an axis, a torque must be applied to change its angular momentum. The amount of torque needed for any given change in angular momentum is proportional to the size of that change. Moment of inertia has units of kg·m^{2} in SI units and lb_{m} ft^{2} in US units.
In 1673 Christiaan Huygens introduced this parameter in his study of the oscillation of a body hanging from a pivot, known as a compound pendulum.^{[1]} The term moment of inertia was introduced by Leonhard Euler in his book Theoria motus corporum solidorum seu rigidorum in 1765,^{[1]}^{[2]} and it is incorporated into Euler's second law.
The natural frequency of oscillation of a compound pendulum is obtained from the ratio of the torque imposed by gravity on the mass of the pendulum to the resistance to acceleration defined by the moment of inertia. Comparison of this natural frequency to that of a simple pendulum consisting of a single point of mass provides a mathematical formulation for moment of inertia of an extended body.^{[3]}^{[4]}
Moment of inertia also appears in momentum, kinetic energy, and in Newton's laws of motion for a rigid body as a physical parameter that combines its shape and mass. There is an interesting difference in the way moment of inertia appears in planar and spatial movement. Planar movement has a single scalar that defines the moment of inertia, while for spatial movement the same calculations yield a 3x3 matrix of moments of inertia, called the inertia matrix or inertia tensor.^{[5]}^{[6]}
The moment of inertia of a rotating flywheel is used in a machine to resist variations in applied torque in order to smooth its rotational output. The moment of inertia of an airplane about its longitudinal, horizontal and vertical axes determines how steering forces on the control surfaces of its wings, elevators and tail affect the plane in roll, pitch and yaw.
A simple pendulum is a point mass suspended by a string so that its movement is constrained to a circle around a pivot point. The mass of a simple pendulum supported by a light string accelerates due to the force of gravity.
The moment of inertia of the pendulum about the pivot point is its resistance to movement due to the torque due to gravity. Mathematically, it is the ratio of the torque due to gravity about the pivot of a pendulum to its angular acceleration about that pivot point. For a simple pendulum this is found to be the product of the mass of the particle with the square of its distance to the pivot. This is shown as follows:
The force of gravity on the mass of a simple pendulum generates a torque around the axis perpendicular to the plane of the pendulum movement. Here F is the tangential component of the net force on the mass. Associated with this torque is an angular acceleration of the string and mass around this axis. Since the mass is constrained to a circle the tangential acceleration of the mass is . Since the torque equation becomes:
where e is a unit vector perpendicular to the plane of the pendulum. (The second to the last step occurs because of the BACCAB rule using the fact that is always perpendicular to r.) The quantity I = mr^{2} is the moment of inertia of this single mass around the pivot point.
The quantity I = mr^{2} also appears in the angular momentum of a simple pendulum, which is calculated from the velocity v = ω×r of the pendulum mass around the pivot, where ω is the angular velocity of the mass about the pivot point. This angular momentum is given by
using math similar to that used to derive the previous equation.
Similarly, the kinetic energy of the pendulum mass is defined by the velocity of the pendulum around the pivot to yield
This shows that the quantity I = mr^{2} is how mass combines with the shape of a body to define rotational inertia. The moment of inertia of an arbitrarily shaped body is the sum of the values mr^{2} for all of the elements of mass in the body.
A compound pendulum is a body formed from an assembly of particles or continuous shapes that rotates rigidly around a pivot. Its moments of inertia is the sum the moments of inertia of each of the particles that is composed of.^{[7]}^{[8]}^{:395–396}^{[9]}^{:51–53} The natural frequency () of a compound pendulum depends on its moment of inertia, ,
where is the mass of the object, is local acceleration of gravity, and is the distance from the pivot point to the centre of mass of the object. Measuring this frequency of oscillation over small angular displacements provides an effective way of measuring moment of inertia of a body.^{[10]}^{:516–517}
Thus, to determine the moment of inertia of the body, simply suspend it from a convenient pivot point so that it swings freely in a plane perpendicular to the direction of the desired moment of inertia, then measure its natural frequency or period of oscillation (), to obtain
where is the period (duration) of oscillation (usually averaged over multiple periods).
The moment of inertia of the body about its center of mass, , is then calculated using the parallel axis theorem to be
where is the mass of the body and is the distance from the pivot point to the center of mass .
Moment of inertia of a body is often defined in terms of its radius of gyration, which is the radius of a ring of equal mass around the center of mass of a body that has the same moment of inertia. The radius of gyration is calculated from the body's moment of inertia and mass as the length,^{[11]}^{:1296–1297}
A simple pendulum that has the same natural frequency as a compound pendulum defines the length from the pivot to a point called the center of oscillation of the compound pendulum. This point also corresponds to the center of percussion. The length is determined from the formula,
or
The seconds pendulum, which provides the "tick" and "tock" of a grandfather clock, takes one second to swing from sidetoside. This is a period of two seconds, or a natural frequency of π radians/second for the pendulum. In this case, the length is given by,
Notice that the centre of oscillation of the seconds pendulum must be adjusted to accommodate use in locations with different values for the local acceleration of gravity. Kater's pendulum is an example of a compound pendulum that is used to measure gravity called a gravimeter.
The moment of inertia of complex systems such as a vehicle or airplane around its vertical axis can be measured by suspending the system from three points to form a trifilar pendulum. A trifilar pendulum is a platform supported by three wires designed to oscillate in torsion around its vertical centroidal axis.^{[12]} The period of oscillation of the trifilar pendulum yields the moment of inertia of the system.^{[13]}
The moment of inertia, I, is defined as the ratio of an applied torque to the angular acceleration along a principal axis of the object, where then τ, and I are scalars, that is
An equivalent definition of I uses the angular momentum L as follows,
where is the angular velocity of the object.
Apply this definition to a simple pendulum to see that the moment of inertia of the mass m about the pivot point at a distance r is
This generalizes to define the moment of inertia of a body about an axis S as the sum of all elemental point masses dm each multiplied by the square of its perpendicular distance r to the axis.
The moment of inertia about an axis of a body is calculated by summing mr^{2} for every particle in the body, where r is the perpendicular distance to the specified axis. In order to see how moment of inertia arises in the study of the movement of an extended body, it is convenient to consider a rigid assembly of point masses. (This equation can be used for axes that are not principal axes provided that it is understood that this does not fully describe the moment of inertia.^{[14]})
Consider the kinetic energy of an assembly of N masses m_{i} that lie at the distances r_{i} from the pivot point P, which is the nearest point on the axis of rotation. It is the sum of the kinetic energy of the individual masses,^{[10]}^{:516–517}^{[11]}^{:1084–1085} ^{[11]}^{:1296–1300}
This shows that the moment of inertia of the body is the sum of each of the mr^{2} terms, that is
Thus, moment of inertia is a physical property that combines the mass and distribution of the particles around the rotation axis. Notice that rotation about different axes of the same body yield different moments of inertia.
The moment of inertia of a continuous body rotating about a specified axis is calculated in the same way, with the summation replaced by the integral,
Again r is the radius vector to a point in the body from the specified axis through the pivot P, and ρ(r) is the mass density at each point r. The integration is evaluated over the volume V of the body. The moment of inertia of a flat surface is similar with the mass density being replaced by its areal mass density with the integral evaluated over its area.
Note on second moment of area: The moment of inertia of a body moving in a plane and the second moment of area of a beam's crosssection are often confused. The moment of inertia of body with the shape of the crosssection is the second moment of this area about the zaxis perpendicular to the crosssection, weighted by its density. This is also called the polar moment of the area, and is the sum of the second moments about the x and y axes.^{[15]} The stresses in a beam are calculated using the second moment of the crosssectional area around either the xaxis or yaxis depending on the load.
The moment of inertia of a compound pendulum constructed from a thin disc mounted at the end of a thin rod that oscillates around a pivot at the other end of the rod, begins with the calculation of the moment of inertia of the thin rod and thin disc about their respective centers of mass.^{[11]}
where m = ρsℓ is the mass of the rod.
where m = πR^{2}ρs is its mass.
where L is the length of the pendulum. Notice that the parallel axis theorem is used to shift the moment of inertia from the center of mass to the pivot point of the pendulum.
A list of moments of inertia formulas for standard body shapes provides a way to obtain the moment of inertial of a complex body as an assembly of simpler shaped bodies. The parallel axis theorem is used to shift the reference point of the individual bodies to the reference point of the assembly.
As one more example, consider the moment of inertia of a solid sphere of constant density about an axis through its center of mass. This is determined by summing the moment of inertias of the thin discs that form the sphere. If the surface of the ball is defined by the equation^{[11]}^{:1301}
then the radius r of the disc at the crosssection z along the zaxis is
Therefore, the moment of inertia of the ball is the sum of the moment of inertias of the discs along the zaxis,
where m = (4/3)πR^{3}ρ is the mass of the ball.
If a mechanical system is constrained to move parallel to a fixed plane, then the rotation of a body in the system occurs around an axis k perpendicular to this plane. In this case, the moment of inertia of the mass in this system is a scalar known as the polar moment of inertia. The definition of the polar moment of inertia can be obtained by considering momentum, kinetic energy and Newton's laws for the planar movement of a rigid system of particles.^{[7]}^{[10]}^{[16]}^{[17]}
If a system of n particles, P_{i}, i = 1,...,n, are assembled into a rigid body, then the momentum of the system can be written in terms of positions relative to a reference point R, and absolute velocities v_{i}
where ω is the angular velocity of the system and V is the velocity of R.
For planar movement the angular velocity vector is directed along the unit vector k which is perpendicular to the plane of movement. Introduce the unit vectors e_{i} from the reference point R to a point r_{i} , and the unit vector t_{i} = k × e_{i} so
This defines the relative position vector and the velocity vector for the rigid system of the particles moving in a plane.
Note on the cross product: When a body moves parallel to a ground plane, the trajectories of all the points in the body lie in planes parallel to this ground plane. This means that any rotation that the body undergoes must be around an axis perpendicular to this plane. Planar movement is often presented as projected onto this ground plane so that the axis of rotation appears as a point. In this case, the angular velocity and angular acceleration of the body are scalars and the fact that they are vectors along the rotation axis is ignored. This is usually preferred for introductions to the topic. But in the case of moment of inertia, the combination of mass and geometry benefits from the geometric properties of the cross product. For this reason, in this section on planar movement the angular velocity and accelerations of the body are vectors perpendicular to the ground plane, and the cross product operations are the same as used for the study of spatial rigid body movement.
The angular momentum vector for the planar movement of a rigid system of particles is given by^{[7]}^{[10]}
Use the center of mass C as the reference point so
and define the moment of inertia relative to the center of mass I_{C} as
then the equation for angular momentum simplifies to^{[11]}^{:1028}
The moment of inertia I_{C} about an axis perpendicular to the movement of the rigid system and through the center of mass is known as the polar moment of inertia.
For a given amount of angular momentum, a decrease in the moment of inertia results in an increase in the angular velocity. Figure skaters can change their moment of inertia by pulling in their arms. Thus, the angular velocity achieved by a skater with outstretched arms results in a greater angular velocity when the arms are pulled in, because of the reduced moment of inertia.
The kinetic energy of a rigid system of particles moving in the plane is given by^{[7]}^{[10]}
This equation expands to yield three terms
Let the reference point be the center of mass C of the system so the second term becomes zero, and introduce the moment of inertia I_{C} so the kinetic energy is given by^{[11]}^{:1084}
The moment of inertia I_{C} is the polar moment of inertia of the body.
Newton's laws for a rigid system of N particles, P_{i}, i = 1,..., N, can be written in terms of a resultant force and torque at a reference point R, to yield^{[7]}^{[10]}
where r_{i} denotes the trajectory of each particle.
The kinematics of a rigid body yields the formula for the acceleration of the particle P_{i} in terms of the position R and acceleration A of the reference particle as well as the angular velocity vector ω and angular acceleration vector α of the rigid system of particles as,
For systems that are constrained to planar movement, the angular velocity and angular acceleration vectors are directed along k perpendicular to the plane of movement, which simplifies this acceleration equation. In this case, the acceleration vectors can be simplified by introducing the unit vectors e_{i} from the reference point R to a point r_{i} and the unit vectors t_{i} = k × e_{i} , so
This yields the resultant torque on the system as
where e_{i} × e_{i} = 0, and e_{i} × t_{i} = k is the unit vector perpendicular to the plane for all of the particles P_{i} .
Use the center of mass C as the reference point and define the moment of inertia relative to the center of mass I_{C} , then the equation for the resultant torque simplifies to^{[11]}^{:1029}
The parameter I_{C} is the polar moment of inertia of the moving body.
The scalar moments of inertia appear as elements in a matrix when a system of particles is assembled into a rigid body that moves in threedimensional space. This inertia matrix appears in the calculation of the angular momentum, kinetic energy and resultant torque of the rigid system of particles.^{[3]}^{[4]}^{[5]}^{[6]}^{[18]}
An important application of the inertia matrix and Newton's laws of motion is the analysis of a spinning top. This is discussed in the article on gyroscopic precession. A more detailed presentation can be found in the article on Euler's equations of motion.
Let the system of particles P_{i}, i = 1,..., n be located at the coordinates r_{i} with velocities v_{i} relative to a fixed reference frame. For a (possibly moving) reference point R, the relative positions are
and the (absolute) velocities are
where ω is the angular velocity of the system, and V_{R} is the velocity of R.
If the reference point R in the assembly, or body, is chosen as the center of mass C, then its angular momentum takes the form,^{[3]}^{[6]}
where the terms containing V_{R} sum to zero by definition of the center of mass.
In order to define the inertia matrix, introduce the skewsymmetric matrix [B] constructed from a vector b that performs the cross product operation, such that
This matrix [B] has the components of b = (b_{x}, b_{y},b_{z}) as its elements, in the form
Now construct the skewsymmetric matrix [Δr_{i}]= [r_{i}C] obtained from the relative position vector Δr_{i}=r_{i}  C, and use this skewsymmetric matrix to define,
where [I_{C} ] defined by
is the inertia matrix of the rigid system of particles measured relative to the center of mass C.
The kinetic energy of a rigid system of particles can be formulated in terms of the center of mass and a matrix of mass moments of inertia of the system. Let the system of particles P_{i}, i = 1,...,n be located at the coordinates r_{i} with velocities v_{i}, then the kinetic energy is^{[3]}^{[6]}
where Δr_{i}= r_{i}C is the position vector of a particle relative to the center of mass.
This equation expands to yield three terms
The second term in this equation is zero because C is the center of mass. Introduce the skewsymmetric matrix [Δr_{i}] so the kinetic energy becomes
Thus, the kinetic energy of the rigid system of particles is given by
where [I_{C}] is the inertia matrix relative to the center of mass and M is the total mass.
The inertia matrix appears in the application of Newton's second law to a rigid assembly of particles. The resultant torque on this system is,^{[3]}^{[6]}
where a_{i} is the acceleration of the particle P_{i}. The kinematics of a rigid body yields the formula for the acceleration of the particle P_{i} in terms of the position R and acceleration A of the reference point, as well as the angular velocity vector ω and angular acceleration vector α of the rigid system as,
Use the center of mass C as the reference point, and introduce the skewsymmetric matrix [Δr_{i}]=[r_{i}C] to represent the cross product (r_{i}  C)x, in order to obtain
This calculation uses the identity
obtained from the Jacobi identity for the triple cross product.
Thus, the resultant torque on the rigid system of particles is given by
where [I_{C}] is the inertia matrix relative to the center of mass.
The inertia matrix of a body depends on the choice of the reference point. There is a useful relationship between the inertia matrix relative to the center of mass C and the inertia matrix relative to another point R. This relationship is called the parallel axis theorem.^{[3]}^{[6]}
Consider the inertia matrix [I_{R}] obtained for a rigid system of particles measured relative to a reference point R, given by
Let C be the center of mass of the rigid system, then
where d is the vector from the center of mass C to the reference point R. Use this equation to compute the inertia matrix,
Expand this equation to obtain
The first term is the inertia matrix [I_{C}] relative to the center of mass. The second and third terms are zero by definition of the center of mass C. And the last term is the total mass of the system multiplied by the square of the skewsymmetric matrix [d] constructed from d.
The result is the parallel axis theorem,
where d is the vector from the center of mass C to the reference point R.
Note on the minus sign: By using the skew symmetric matrix of position vectors relative to the reference point, the inertia matrix of each particle has the form m[r]^{2}, which is similar to the mr^{2} that appears in planar movement. However, to make this to work out correctly a minus sign is needed. This minus sign can be absorbed into the term m[r]^{T}[r], if desired, by using the skewsymmetry property of [r].
The relationship between the inertia matrix of a rigid body and the scalar moment of inertia of the same body about a specified axis is important and rarely presented in detail. The following calculation expands the derivation presented by Kane and Levinson.^{[6]}
Let a rigid assembly of rigid system of N particles, P_{i}, i = 1,...,N, have coordinates r_{i}. Choose R as a reference point and compute the moment of inertia around an axis L defined by the unit vector S through the reference point R. The moment of inertia of the system around this line L=R+tS is computed by determining the perpendicular vector from this axis to the particle P_{i} given by
where [I] is the identity matrix and [S S^{T}] is the outer product matrix formed from the unit vector S along the line L.
In order to relate this scalar moment of inertia to the inertia matrix of the body, introduce the skewsymmetric matrix [S] such that [S]y=S x y, then we have the identity
which relies on the fact that S is a unit vector.
The magnitude squared of the perpendicular vector is
The simplification of this equation uses the identity
where the dot and the cross products have been interchanged. Expand the cross products to compute
where [Δr_{i}] is the skew symmetric matrix obtained from the vector Δr=r_{i}R.
Thus, the moment of inertia around the line L through R in the direction S is obtained from the calculation
or
where [I_{R}] is the moment of inertia matrix of the system relative to the reference point R.
This shows that the inertia matrix can be used to calculate the moment of inertia of a body around any specified rotation axis in the body.
The inertia matrix is often described as the inertia tensor, which consists of the same moments of inertia and products of inertia about the three coordinate axes.^{[6]}^{[16]} The inertia tensor is constructed from the nine component tensors, (the symbol is the tensor product)
where e_{i}, i=1,2,3 are the three orthogonal unit vectors defining the inertial frame in which the body moves. Using this basis the inertia tensor is given by
This tensor is of degree two because the component tensors are each constructed from two basis vectors. In this form the inertia tensor is also called the inertia binor.
For a rigid system of particles P_{k}, k = 1,...,N each of mass m_{k} with position coordinates r_{k}=(x_{k}, y_{k}, z_{k}), the inertia tensor is given by
where E is the identity tensor
The inertia tensor for a continuous body is given by
where r defines the coordinates of a point in the body and ρ(r) is the mass density at that point. The integral is taken over the volume V of the body. The inertia tensor is symmetric because I_{ij}= I_{ji}.
Alternatively it can also be written in terms of the hat operator as:
The inertia tensor can be used in the same way as the inertia matrix to compute the scalar moment of inertia about an arbitrary axis in the direction n,
where the dot product is taken with the corresponding elements in the component tensors. A product of inertia term such as I_{12} is obtained by the computation
and can be interpreted as the moment of inertia around the xaxis when the object rotates around the yaxis.
The components of tensors of degree two can be assembled into a matrix. For the inertia tensor this matrix is given by,
It is common in rigid body mechanics to use notation that explicitly identifies the x, y, and z axes, such as I_{xx} and I_{xy}, for the components of the inertia tensor.
In order to compute moment of inertia of a mass around an axis, the perpendicular vector from the mass to the axis is needed. If the axis L is defined by the unit vector S through the reference point R, then the perpendicular vector from the line L to the point r is given by
where [I] is the identity matrix and [S S^{T}] is the outer product matrix formed from the unit vector S along the line L. Recall that skewsymmetric matrix [S] is constructed so that [S]y=S x y. The matrix [ISS^{T}] in this equation subtracts the component of Δr=rR that is parallel to S.
The previous sections show that in computing the moment of inertia matrix this operator yields a similar operator using the components of the vector Δr that is
It is helpful to keep the following identities in mind In order to compare the equations that define the inertia tensor and the inertia matrix.
Let [R] be the skew symmetric matrix associated with the position vector R=(x, y, z), then the product in the inertia matrix becomes
This can be viewed as another way of computing the perpendicular distance from an axis to a point, because the matrix formed by the outer product [R R^{T}] yields the identify
where [I] is the 3x3 identity matrix.
Also notice, that
where tr denotes the sum of the diagonal elements of the outer product matrix, known as its trace.
The use of the inertia matrix in Newton's second law assumes its components are computed relative to axes parallel to the inertial frame and not relative to a bodyfixed reference frame.^{[6]}^{[16]} This means that as the body moves the components of the inertia matrix change with time. In contrast, the components of the inertia matrix measured in a bodyfixed frame are constant.
Let the body frame inertia matrix relative to the center of mass be denoted [I_{C}^{B}], and define the orientation of the body frame relative to the inertial frame by the rotation matrix [A], such that,
where vectors y in the body fixed coordinate frame have coordinates x in the inertial frame. Then, the inertia matrix of the body measured in the inertial frame is given by
Notice that [A] changes as the body moves, while [I_{C}^{B}] remains constant.
Measured in the body frame the inertia matrix is a constant real symmetric matrix. A real symmetric matrix has the eigendecomposition into the product of a rotation matrix [Q] and a diagonal matrix [Λ], given by
where
The columns of the rotation matrix [Q] define the directions of the principal axes of the body, and the constants I_{1}, I_{2} and I_{3} are called the principal moments of inertia. This result was first shown by J. J. Sylvester (1852), and is a form of Sylvester's law of inertia.^{[19]}^{[20]}
For bodies with constant density an axis of rotational symmetry is a principal axis.
The moment of inertia matrix in bodyframe coordinates is a quadratic form that defines a surface in the body called Poinsot's ellipsoid.^{[21]} Let [Λ] be the inertia matrix relative to the center of mass aligned with the principal axes, then the surface
or
defines an ellipsoid in the body frame. Write this equation in the form,
to see that the semiprincipal diameters of this ellipsoid are given by
Let a point x on this ellipsoid be defined in terms of its magnitude and direction, x=xn, where n is a unit vector. Then the relationship presented above, between the inertia matrix and the scalar moment of inertia I_{n} around an axis in the direction n, yields
Thus, the magnitude of a point x in the direction n on the inertia ellipsoid is
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