Moment of inertia
Moment of inertia is a property of rotating bodies that defines its resistance to a change in angular velocity about an axis of rotation. It is how rotation of a body is affected by Newton's law of inertia, which states "Every body perseveres in its state of being at rest or of moving uniformly straight forward except insofar as it is compelled to change its state by forces impressed."  In this context, inertia refers to resistance to change.
Moment of inertia applies to an extended body in which the mass is constrained to rotate around an axis. It arises as a combination of mass and geometry in the study of the movement of continuous bodies, or assemblies of particles, known as rigid body dynamics.
It is the moment of inertia of the pole carried by a tight-rope walker that resists rotation and helps the walker maintain balance. In the same way the long axis of a dragster resists turning forces which helps keep it moving in a straight line.
A body rotating around an axis has the property that a torque must be applied to change its angular velocity. The amount of torque needed is proportional to the size of the change in angular velocity, better known as angular acceleration. The constant of proportionality is a parameter of the body that combines its mass and its shape, known as the moment of inertia. In classical mechanics, moment of inertia may also be called mass moment of inertia, rotational inertia, polar moment of inertia, or the angular mass (SI units kg·m2, US units lbm ft2).
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. The term moment of inertia was introduced by Leonhard Euler in his book Theoria motus corporum solidorum seu rigidorum in 1765.
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.
Moment of inertia also appears in momentum, kinetic energy, and in Newtons 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.
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.
Moment of inertia of a pendulum
The mass of a simple pendulum supported by a light string accelerates due to the force of gravity. Its movement is constrained to a circle around the pivot point, so it can be considered to be a compound pendulum that consists of a single mass point.
The ratio of the torque due to gravity about the pivot and the resistance of mass to angular acceleration, the moment of inertia of the particle is found to be the product the mass of the particle with the square of its distance to the pivot. The moments of inertia of an assembly of particles, or continuous body, that rotates around the pivot is the sum the moment of inertia of each of the particles.:395-396:51-53
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. Associated with this torque is an angular acceleration of the string and mass around this axis. Let be the component of force in the direction of the acceleration of the pendulum mass and the length of the string, so the acceleration of the mass is , then , becomes,
where is a unit vector perpendicular to the plane of the pendulum. The quantity I = mr2 is the moment of inertia of this single mass around the pivot point.
The quantity I = mr2 also appears in the angular momentum of a simple pendulum, which is calculated from the velocity v = rω of the pendulum mass around the pivot. This is given by
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 = mr2 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 mr2 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, and its moment of inertia is the sum of the moments of inertia of all of the components in this assembly about the pivot point. 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.: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,:1296-1297
Center of oscillation
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,
The seconds pendulum, which provides the "tick" and "tock" of a grandfather clock, takes one second to swing from side-to-side. 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.
Measuring moment of inertia
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. The period of oscillation of the trifilar pendulum yields the moment of inertia of the system.
Calculating moment of inertia
The moment of inertia of a body is calculated by summing mr2 for every particle in the body about a given rotation 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.
Consider the kinetic energy of an assembly of N masses mi that lie at the distances ri from the pivot point P, which is the sum of the kinetic energy of the individual masses,:516-517:1084-1085 :1296-1300
This shows that the moment of inertia of the body is the sum of each of the mr2 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.
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 cross-section are often confused. A beam along the z-axis has stresses in the cross-section in the x-y plane that are calculated using the second moment of this area around either the x-axis or y-axis depending on the load. The moment of inertia of mass distributed along a body with the shape of this cross-section is the second moment of this area about the z-axis weighted by its density. The second moment of area around an axis perpendicular to the area is called the polar moment of the area, and is the sum of the second moments about the x and y axes.
Example calculation of moment of inertia
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.
- The moment of inertia of a thin rod with constant cross-section s and density ρ and with length l about a perpendicular axis through its center of mass is determined by integration.:1301 Align the x-axis with the rod and locate the origin its center of mass at the center of the rod, then
where m = ρsℓ is the mass of the rod.
- The moment of an inertia of a thin disc of constant thickness s, radius R, and density ρ about an axis through its center and perpendicular to its face (parallel to its axis of rotational symmetry) is determined by integration.:1301 Align the z-axis with the axis of the disc and define a volume element as dV = sr drdθ, then
where m = πR2ρs is its mass.
- The moment of inertia of the compound pendulum is now obtained by adding the moment of inertia of the rod and the disc around the pivot point P as,
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:1301
then the radius r of the disc at the cross-section z along the z-axis is
Therefore, the moment of inertia of the ball is the sum of the moment of inertias of the discs along the z-axis,
where m = (4/3)πR3ρ is the mass of the ball.
Moment of inertia in planar movement of a rigid body
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.
If a system of n particles, Pi, 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 vi
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 ei from the reference point R to a point ri , and the unit vector ti = kxei 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.
Angular momentum in planar movement
Use the center of mass C as the reference point so
and define the moment of inertia relative to the center of mass IC as
then the equation for angular momentum simplifies to:1028
The moment of inertia IC 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.
Kinetic energy in planar movement
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 IC so the kinetic energy is given by:1084
The moment of inertia IC is the polar moment of inertia of the body.
Newton's laws for planar movement
where ri denotes the trajectory of each particle.
The kinematics of a rigid body yields the formula for the acceleration of the particle Pi 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 ei from the reference point R to a point ri and the unit vectors ti = kxei , so
This yields the resultant torque on the system as
where eixei = 0, and eixti = k is the unit vector perpendicular to the plane for all of the particles Pi .
The parameter IC is the polar moment of inertia of the moving body.
The inertia matrix for spatial movement of a rigid 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 three dimensional space. This inertia matrix appears in the calculation of the angular momentum, kinetic energy and resultant torque of the rigid system of particles.
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 Pi, i = 1,..., n be located at the coordinates ri with velocities vi 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 VR is the velocity of R.
where the terms containing VR sum to zero by definition of the center of mass.
In order to define the inertia matrix, introduce the skew-symmetric matrix [B] constructed from a vector b that performs the cross product operation, such that
This matrix [B] has the components of b = (bx, by,bz) as its elements, in the form
Now construct the skew-symmetric matrix [Δri]= [ri-C] obtained from the relative position vector Δri=ri - C, and use this skew-symmetric matrix to define,
where [IC ] 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 Pi, i = 1,...,n be located at the coordinates ri with velocities vi, then the kinetic energy is
where Δri= ri-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 skew-symmetric matrix [Δri] so the kinetic energy becomes
Thus, the kinetic energy of the rigid system of particles is given by
where [IC] is the inertia matrix relative to the center of mass and M is the total mass.
where ai is the acceleration of the particle Pi. The kinematics of a rigid body yields the formula for the acceleration of the particle Pi 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 skew-symmetric matrix [Δri]=[ri-C] to represent the cross product (ri - 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 [IC] is the inertia matrix relative to the center of mass.
Parallel axis theorem
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.
Consider the inertia matrix [IR] 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 [IC] 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 skew-symmetric 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 mr2 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 skew-symmetry property of [r].
The inertia matrix and the scalar moment of inertia around an arbitrary axis
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.
Let a rigid assembly of rigid system of N particles, Pi, i = 1,...,N, have coordinates ri. 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 Pi given by
where [I] is the identity matrix and [S ST] 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 skew-symmetric 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 [Δri] is the skew symmetric matrix obtained from the vector Δr=ri-R.
Thus, the moment of inertia around the line L through R in the direction S is obtained from the calculation
where [IR] 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 tensor
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. The inertia tensor is constructed from the nine component tensors, (the symbol is the tensor product)
where ei, 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 Pk, k = 1,...,N each of mass mk with position coordinates rk=(xk, yk, zk), 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 Iij= Iji.
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 I12 is obtained by the computation
and can be interpreted as the moment of inertia around the x-axis when the object rotates around the y-axis.
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 Ixx and Ixy, for the components of the inertia tensor.
Identities for a skew-symmetric matrix
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 ST] is the outer product matrix formed from the unit vector S along the line L. Recall that skew-symmetric matrix [S] is constructed so that [S]y=S x y. The matrix [I-SST] in this equation subtracts the component of Δr=r-R 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 RT] 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 inertia matrix in different reference frames
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 body-fixed reference frame. 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 body-fixed frame are constant.
Body frame inertia matrix
Let the body frame inertia matrix relative to the center of mass be denoted [ICB], 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 [ICB] 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
The columns of the rotation matrix [Q] define the directions of the principal axes of the body, and the constants I1, I2 and I3 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.
For bodies with constant density an axis of rotational symmetry is a principal axis.
The moment of inertia matrix in body-frame coordinates is a quadratic form that defines a surface in the body called Poinsot's ellipsoid. Let [Λ] be the inertia matrix relative to the center of mass aligned with the principal axes, then the surface
defines an ellipsoid in the body frame. Write this equation in the form,
to see that the semi-principal diameters of this ellipsoid are given by
Let a point x on this ellipsoid be defined in terms of its magnitude and direction, x=|x|n, where n is a unit vector. Then the relationship presented above, between the inertia matrix and the scalar moment of inertia In around an axis in the direction n, yields
Thus, the magnitude of a point x in the direction n on the inertia ellipsoid is
- Central moment
- Instant center of rotation
- List of moment of inertia tensors
- Rotational energy
- Stretch rule
- Tire balance
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