Rotational momentum

If rotational momentum is constant then
k = mvr = mvv/2 \* 2πr/v \* 1/π = kinetic energy / frequency\*π
k = mvr = 2 \* kinetic energy / Hz

In physics, **rotational momentum** or **angular momentum** is the rotational equivalent of linear momentum. It is an important quantity in physics because it is a conserved quantity—the total rotational momentum of a closed system remains constant.

In three dimensions, the rotational momentum for a point particle is a pseudovector **r** × **p**, the cross product of the particle's position vector **r** (relative to some origin) and its momentum vector; the latter is **p** = *m***v** in Newtonian mechanics. This definition can be applied to each point in continua like solids or fluids, or physical fields. Unlike momentum, rotational momentum does depend on where the origin is chosen, since the particle's position is measured from it.

Just like for rotational velocity, there are two special types of rotational momentum: the spin rotational momentum and orbital rotational momentum. The spin rotational momentum of an object is defined as the rotational momentum about its centre of mass coordinate. The orbital rotational momentum of an object about a chosen origin is defined as the rotational momentum of the centre of mass about the origin. The total rotational momentum of an object is the sum of the spin and orbital angular momenta. The orbital rotational momentum vector of a point particle is always parallel and directly proportional to the orbital rotational velocity vector **ω** of the particle, where the constant of proportionality depends on both the mass of the particle and its distance from origin. The spin rotational momentum vector of a rigid body is proportional but not always parallel to the spin rotational velocity vector **Ω**, making the constant of proportionality a second-rank tensor rather than a scalar.

Rotational momentum is an extensive quantity; i.e. the total rotational momentum of any composite system is the sum of the angular momenta of its constituent parts. For a continuous rigid body, the total rotational momentum is the volume integral of rotational momentum density (i.e. rotational momentum per unit volume in the limit as volume shrinks to zero) over the entire body.

Rotational force can be defined as the rate of change of rotational momentum, analogous to force. The net *external* rotational force on any system is always equal to the *total* rotational force on the system; in other words, the sum of all internal rotational forces of any system is always 0 (this is the rotational analogue of Newton's Third Law). Therefore, for a *closed* system (where there is no net external rotational force), the *total* rotational force on the system must be 0, which means that the total rotational momentum of the system is constant. The conservation of rotational momentum helps explain many observed phenomena, for example the increase in rotational speed of a spinning figure skater as the skater's arms are contracted, the high rotational rates of neutron stars, the Coriolis effect, and the precession of gyroscopes. In general, conservation does limit the possible motion of a system, but does not uniquely determine what the exact motion is.

In quantum mechanics, rotational momentum (like other quantities) is expressed as an operator, and its one-dimensional projections have quantized eigenvalues. Rotational momentum is subject to the Heisenberg uncertainty principle, implying that at any time, only one projection (also called "component") can be measured with definite precision; the other two then remain uncertain. Because of this, the notion of a quantum particle literally "spinning" about an axis does not exist. Quantum particles *do* possess a type of non-orbital rotational momentum called "spin", but this rotational momentum does not correspond to actual physical spinning motion.

## Definition in classical mechanics

Main articles: Scalar (physics), Euclidean vector
### Orbital rotational momentum in two dimensions

Rotational momentum is a vector quantity (more precisely, a pseudovector) that represents the product of a body's rotational inertia and rotational velocity (in radians/sec) about a particular axis. However, if the particle's trajectory lies in a single plane, it is sufficient to discard the vector nature of rotational momentum, and treat it as a scalar (more precisely, a pseudoscalar). Rotational momentum can be considered a rotational analog of linear momentum. Thus, where linear momentum p is proportional to mass m and linear speed v,

# p
m
v
,

{\displaystyle p = mv,}

rotational momentum L is proportional to rotational mass I and angular speed ω measured in radians per second.

# L
I
ω
.

{\displaystyle L = I\omega.}

Unlike mass, which depends only on amount of matter, rotational mass is also dependent on the position of the axis of rotation and the shape of the matter. Unlike linear velocity, which does not depend upon the choice of origin, orbital rotational velocity is always measured with respect to a fixed origin. Therefore, strictly speaking, L should be referred to as the rotational momentum *relative to that center*.

Because

# I

r
2

m

{\displaystyle I = r^2m}

 for a single particle and

# ω

v
r

{\displaystyle \omega = \frac{v}{r} }

 for circular motion, rotational momentum can be expanded,

# L

r
2

m
⋅

v
r

,

{\displaystyle L = r^2 m \cdot \frac{v}{r},}

 and reduced to,

# L
r
m
v
,

{\displaystyle L = rmv,}

the product of the radius of rotation r and the linear momentum of the particle

# p
m
v

{\displaystyle p = mv}

, where v in this case is the equivalent linear (tangential) speed at the radius (

=
r
ω

{\displaystyle = r\omega}

).

This simple analysis can also apply to non-circular motion if only the component of the motion which is perpendicular to the radius vector is considered. In that case,

# L
r
m

v
⊥

,

{\displaystyle L = rmv\_\perp,}

where

v
⊥

=
v
sin
⁡
(
θ
)

{\displaystyle v\_\perp = v\sin(\theta)}

 is the perpendicular component of the motion. Expanding,

# L
r
m
v
sin
⁡
(
θ
)
,

{\displaystyle L = rmv\sin(\theta),}

 rearranging,

# L
r
sin
⁡
(
θ
)
m
v
,

{\displaystyle L = r\sin(\theta)mv,}

 and reducing, rotational momentum can also be expressed,

# L

r
⊥

m
v
,

{\displaystyle L = r\_\perp mv,}

where

r
⊥

=
r
sin
⁡
(
θ
)

{\displaystyle r\_\perp = r\sin(\theta)}

 is the length of the *moment arm*, a line dropped perpendicularly from the origin onto the path of the particle. It is this definition, (length of moment arm)×(linear momentum) to which the term *moment of momentum* refers.

### Scalar—rotational momentum from Lagrangian mechanics

Another approach is to define rotational momentum as the conjugate momentum (also called **canonical momentum**) of the angular coordinate

ϕ

{\displaystyle \phi}

 expressed in the Lagrangian of the mechanical system. Consider a mechanical system with a mass

m

{\displaystyle m}

 constrained to move in a circle of radius

a

{\displaystyle a}

 in the absence of any external force field. The kinetic energy of the system is

# T

1
2

m

a
2

ω
2

=

1
2

m

a
2

ϕ
˙

2

.

{\displaystyle T = \frac{1}{2}ma^2 \omega^2 = \frac{1}{2}ma^2 \dot{\phi}^2.}

And the potential energy is

# U

{\displaystyle U = 0.}

Then the Lagrangian is

L

(

ϕ
,

ϕ
˙

)

=
T
−
# U

1
2

m

a
2

ϕ
˙

2

.

{\displaystyle \mathcal{L}\left(\phi, \dot{\phi}\right) = T - U = \frac{1}{2}ma^2 \dot{\phi}^2.}

The *generalized momentum* "canonically conjugate to" the coordinate

ϕ

{\displaystyle \phi}

 is defined by

p
ϕ

=

∂

L

∂

ϕ
˙

=
m

a
2

ϕ
˙

=
I
# ω
L
.

{\displaystyle p\_\phi = \frac{\partial \mathcal{L}}{\partial \dot{\phi}} = ma^2 \dot{\phi} = I\omega = L.}

### Orbital rotational momentum in three dimensions

To completely define orbital rotational momentum in three dimensions, it is required to know the rate at which the position vector sweeps out angle, the direction perpendicular to the instantaneous plane of angular displacement, and the mass involved, as well as how this mass is distributed in space. By retaining this vector nature of rotational momentum, the general nature of the equations is also retained, and can describe any sort of three-dimensional motion about the center of rotation – circular, linear, or otherwise. In vector notation, the orbital rotational momentum of a point particle in motion about the origin can be expressed as:

L

=
I
ω
,

{\displaystyle \mathbf{L} = I\boldsymbol{\omega},}

where

# I

r
2

m

{\displaystyle I = r^2m}

 is the rotational mass for a point mass,

# ω

r

×

v

r
2

{\displaystyle \boldsymbol{\omega}=\frac{\mathbf{r}\times\mathbf{v}}{r^2}}

 is the orbital rotational velocity in radians/sec (units 1/sec) of the particle about the origin,

r

{\displaystyle \mathbf{r}}

 is the position vector of the particle relative to the origin,

# r

r

{\displaystyle r=\left\vert\mathbf{r}\right\vert}

,

v

{\displaystyle \mathbf{v} }

 is the linear velocity of the particle relative to the origin, and

m

{\displaystyle m}

 is the mass of the particle.
This can be expanded, reduced, and by the rules of vector algebra, rearranged:

L

=

(

r
2

m

)

(

r

×

v

r
2

)

=
m

(

r

×

v

)

=

r

×
m

v

=

r

×

p

,

{\displaystyle \begin{align}
 \mathbf{L} &= \left(r^2m\right)\left(\frac{\mathbf{r}\times\mathbf{v}}{r^2}\right) \\
 &= m\left(\mathbf{r}\times\mathbf{v}\right) \\
 &= \mathbf{r}\times m\mathbf{v} \\
 &= \mathbf{r}\times\mathbf{p},
\end{align}}

which is the cross product of the position vector

r

{\displaystyle \mathbf{r}}

 and the linear momentum

p

=
m

v

{\displaystyle \mathbf{p}=m\mathbf{v}}

 of the particle. By the definition of the cross product, the

L

{\displaystyle \mathbf{L}}

 vector is perpendicular to both

r

{\displaystyle \mathbf{r}}

 and

p

{\displaystyle \mathbf{p}}

. It is directed perpendicular to the plane of angular displacement, as indicated by the right-hand rule – so that the rotational velocity is seen as counter-clockwise from the head of the vector. Conversely, the

L

{\displaystyle \mathbf{L}}

 vector defines the plane in which

r

{\displaystyle \mathbf{r}}

 and

p

{\displaystyle \mathbf{p}}

 lie.

By defining a unit vector

u
^

{\displaystyle \mathbf{\hat{u}}}

 perpendicular to the plane of angular displacement, a scalar angular speed

ω

{\displaystyle \omega}

 results, where

ω

u
^

=
ω
,

{\displaystyle \omega\mathbf{\hat{u}} = \boldsymbol{\omega},}

 and

# ω

v
⊥

r

,

{\displaystyle \omega = \frac{v\_\perp}{r},}

 where

v
⊥

{\displaystyle v\_\perp}

 is the perpendicular component of the motion, as above.
The two-dimensional scalar equations of the previous section can thus be given direction:

L

=
I
ω

=
I
ω

u
^

=

(

r
2

m

)

ω

u
^

=
r
m

v
⊥

u
^

=

r
⊥

m
v

u
^

,

{\displaystyle \begin{align}
 \mathbf{L} &= I\boldsymbol{\omega}\\
 &= I\omega\mathbf{\hat{u}}\\
 &= \left(r^2m\right)\omega\mathbf{\hat{u}}\\
 &= rmv\_\perp \mathbf{\hat{u}}\\
 &= r\_\perp mv\mathbf{\hat{u}},
\end{align}}

and

L

=
r
m
v

u
^

{\displaystyle \mathbf{L} = rmv\mathbf{\hat{u}}}

 for circular motion, where all of the motion is perpendicular to the radius

r

{\displaystyle r}

.

In the spherical coordinate system the rotational momentum vector expresses as

L

=
m

r

×

v

=
m

r
2

(

θ
˙

φ
^

−

φ
˙

sin
⁡
θ

θ
^

)

.

{\displaystyle \mathbf{L} =
 m \mathbf{r} \times \mathbf{v} = m r^2 \left(\dot\theta\,\hat{\boldsymbol\varphi} - \dot\varphi \sin\theta\,\mathbf{\hat{\boldsymbol\theta}}\right).
}

## Analogy to linear momentum

Rotational momentum can be described as the rotational analog of linear momentum. Like linear momentum it involves elements of mass and displacement. Unlike linear momentum it also involves elements of position and shape.

Many problems in physics involve matter in motion about some certain point in space, be it in actual rotation about it, or simply moving past it, where it is desired to know what effect the moving matter has on the point—can it exert energy upon it or perform work about it? Energy, the ability to do work, can be stored in matter by setting it in motion—a combination of its inertia and its displacement. Inertia is measured by its mass, and displacement by its velocity. Their product,

(
amount of inertia
)
×
(
amount of displacement
)

=
amount of (inertia⋅displacement)

mass
×
velocity

=
momentum

m
×
v

=
p

{\displaystyle \begin{align}
(\text{amount of inertia}) \times (\text{amount of displacement})&=\text{amount of (inertia⋅displacement)}\\
\text{mass} \times \text{velocity} &= \text{momentum}\\
m \times v &= p\\
\end{align}
}

is the matter's momentum. Referring this momentum to a central point introduces a complication: the momentum is not applied to the point directly. For instance, a particle of matter at the outer edge of a wheel is, in effect, at the end of a lever of the same length as the wheel's radius, its momentum turning the lever about the center point. This imaginary lever is known as the *moment arm*. It has the effect of multiplying the momentum's effort in proportion to its length, an effect known as a *moment*. Hence, the particle's momentum referred to a particular point,

(
moment arm
)
×
(
amount of inertia
)
×
(
amount of displacement
)

=
moment of (inertia⋅displacement)

length
×
mass
×
velocity

=
moment of momentum

r
×
m
×
v

=
L

{\displaystyle \begin{align}
(\text{moment arm}) \times (\text{amount of inertia}) \times (\text{amount of displacement})&=\text{moment of (inertia⋅displacement)}\\
\text{length} \times \text{mass} \times \text{velocity} &= \text{moment of momentum}\\
r \times m \times v &= L\\
\end{align}
}

is the *rotational momentum*, sometimes called, as here, the *moment of momentum* of the particle versus that particular center point. The equation

# L
r
m
v

{\displaystyle L=rmv}

 combines a moment (a mass

m

{\displaystyle m}

 turning moment arm

r

{\displaystyle r}

) with a linear (straight-line equivalent) speed

v

{\displaystyle v}

. Linear speed referred to the central point is simply the product of the distance

r

{\displaystyle r}

 and the angular speed

ω

{\displaystyle \omega}

 versus the point:

# v
r
ω
,

{\displaystyle v=r\omega,}

 another moment. Hence, rotational momentum contains a double moment:

# L
r
m
r
ω
.

{\displaystyle L=rmr\omega.}

 Simplifying slightly,

# L

r
2

m
ω
,

{\displaystyle L=r^2m\omega,}

 the quantity

r
2

m

{\displaystyle r^2m}

 is the particle's rotational mass, sometimes called the second moment of mass. It is a measure of rotational inertia.

Because rotational mass is a crucial part of the spin rotational momentum, the latter necessarily includes all of the complications of the former, which is calculated by multiplying elementary bits of the mass by the squares of their distances from the center of rotation. Therefore, the total rotational mass, and the rotational momentum, is a complex function of the configuration of the matter about the center of rotation and the orientation of the rotation for the various bits.

For a rigid body, for instance a wheel or an asteroid, the orientation of rotation is simply the position of the rotation axis versus the matter of the body. It may or may not pass through the center of mass, or it may lie completely outside of the body. For the same body, rotational momentum may take a different value for every possible axis about which rotation may take place. It reaches a minimum when the axis passes through the center of mass.

For a collection of objects revolving about a center, for instance all of the bodies of the Solar System, the orientations may be somewhat organized, as is the Solar System, with most of the bodies' axes lying close to the system's axis. Their orientations may also be completely random.

In brief, the more mass and the farther it is from the center of rotation (the longer the moment arm), the greater the rotational mass, and therefore the greater the rotational momentum for a given rotational velocity. In many cases the rotational mass, and hence the rotational momentum, can be simplified by,

# I

k
2

m
,

{\displaystyle I=k^2m,}

where

k

{\displaystyle k}

 is the radius of gyration, the distance from the axis at which the entire mass

m

{\displaystyle m}

 may be considered as concentrated.
Similarly, for a point mass

m

{\displaystyle m}

 the rotational mass is defined as,

# I

r
2

m

{\displaystyle I = r^2m}

where

r

{\displaystyle r}

 is the radius of the point mass from the center of rotation,
and for any collection of particles

m
i

{\displaystyle m\_i}

 as the sum,

∑
i

I
i

=

∑
i

r
i
2

m
i

{\displaystyle \sum\_i I\_i = \sum\_i r\_i^2m\_i}

Rotational momentum's dependence on position and shape is reflected in its units versus linear momentum: kg⋅m2/s, N⋅m⋅s, or J⋅s for rotational momentum versus kg⋅m/s or N⋅s for linear momentum. When calculating rotational momentum as the product of the rotational mass times the rotational velocity, the rotational velocity must be expressed in radians per second, where the radian assumes the dimensionless value of unity. (When performing dimensional analysis, it may be productive to use orientational analysis which treats radians as a base unit, but this is outside the scope of the International system of units). Rotational momentum's units can be interpreted as rotational force⋅time or as energy⋅time per angle. An object with rotational momentum of *L* N⋅m⋅s can be reduced to zero rotation (all of the rotational energy can be transferred out of it) by an angular impulse of *L* N⋅m⋅s or equivalently, by rotational force or work of *L* N⋅m for one second, or energy of *L* J for one second.

The plane perpendicular to the axis of rotational momentum and passing through the center of mass is sometimes called the *invariable plane*, because the direction of the axis remains fixed if only the interactions of the bodies within the system, free from outside influences, are considered. One such plane is the invariable plane of the Solar System.

### Rotational momentum and rotational force

See also: Rotational force#Definition and relation to rotational momentum
Newton's second law of motion can be expressed mathematically,

F

=
m

a

,

{\displaystyle \mathbf{F}=m \mathbf{a},}

or force = mass × acceleration. The rotational equivalent for point particles may be derived as follows:

L

=
I
ω

{\displaystyle \mathbf{L} = I\boldsymbol{\omega}}

which means that the rotational force (i.e. the time derivative of the rotational momentum) is

# τ

d
I

d
t

ω
I

d
ω

d
t

.

{\displaystyle \boldsymbol{\tau} = \frac{dI}{dt}\boldsymbol{\omega} + I\frac{d\boldsymbol{\omega}}{dt}. }

Because the rotational mass is

m

r
2

{\displaystyle mr^2}

, it follows that

d
I

d
t

=
2
m
r

d
r

d
t

=
2
r

p

{\displaystyle \frac{dI}{dt} = 2mr\frac{dr}{dt} = 2rp\_{||}}

, and

d

L

d
t

=
I

d
ω

d
t

2
r

p

ω
,

{\displaystyle \frac{d\mathbf{L}}{dt} = I\frac{d\boldsymbol{\omega}}{dt} + 2rp\_{||}\boldsymbol{\omega},}

 which, reduces to

# τ
I
α
2
r

p

ω
.

{\displaystyle \boldsymbol{\tau} = I\boldsymbol{\alpha} + 2rp\_{||}\boldsymbol{\omega}.}

This is the rotational analog of Newton's Second Law. Note that the rotational force is not necessarily proportional or parallel to the rotational acceleration (as one might expect). The reason for this is that the rotational mass of a particle can change with time, something that cannot occur for ordinary mass.

## Conservation of rotational momentum

### General considerations

A rotational analog of Newton's third law of motion might be written, "In a closed system, no rotational force can be exerted on any matter without the exertion on some other matter of an equal and opposite rotational force." Hence, *rotational momentum can be exchanged between objects in a closed system, but total rotational momentum before and after an exchange remains constant (is conserved).*

Seen another way, a rotational analogue of Newton's first law of motion might be written, "A rigid body continues in a state of uniform rotation unless acted by an external influence." Thus *with no external influence to act upon it, the original rotational momentum of the system remains constant*.

The conservation of rotational momentum is used in analyzing *central force motion*. If the net force on some body is directed always toward some point, the *center*, then there is no rotational force on the body with respect to the center, as all of the force is directed along the radius vector, and none is perpendicular to the radius. Mathematically, rotational force

# τ

r

×

F

=

0

,

{\displaystyle \boldsymbol{\tau} = \mathbf{r} \times \mathbf{F} = \mathbf{0}, }

 because in this case

r

{\displaystyle \mathbf{r}}

 and

F

{\displaystyle \mathbf F }

 are parallel vectors. Therefore, the rotational momentum of the body about the center is constant. This is the case with gravitational attraction in the orbits of planets and satellites, where the gravitational force is always directed toward the primary body and orbiting bodies conserve rotational momentum by exchanging distance and velocity as they move about the primary. Central force motion is also used in the analysis of the Bohr model of the atom.

For a planet, rotational momentum is distributed between the spin of the planet and its revolution in its orbit, and these are often exchanged by various mechanisms. The conservation of rotational momentum in the Earth–Moon system results in the transfer of rotational momentum from Earth to Moon, due to tidal rotational force the Moon exerts on the Earth. This in turn results in the slowing down of the rotation rate of Earth, at about 65.7 nanoseconds per day, and in gradual increase of the radius of Moon's orbit, at about 3.82 centimeters per year.

The conservation of rotational momentum explains the rotational acceleration of an ice skater as she brings her arms and legs close to the vertical axis of rotation. By bringing part of the mass of her body closer to the axis, she decreases her body's rotational mass. Because rotational momentum is the product of rotational mass and rotational velocity, if the rotational momentum remains constant (is conserved), then the rotational velocity (rotational speed) of the skater must increase.

The same phenomenon results in extremely fast spin of compact stars (like white dwarfs, neutron stars and black holes) when they are formed out of much larger and slower rotating stars. Decrease in the size of an object *n* times results in increase of its rotational velocity by the factor of *n*2.

Conservation is not always a full explanation for the dynamics of a system but is a key constraint. For example, a spinning top is subject to gravitational rotational force making it lean over and change the rotational momentum about the nutation axis, but neglecting friction at the point of spinning contact, it has a conserved rotational momentum about its spinning axis, and another about its precession axis. Also, in any planetary system, the planets, star(s), comets, and asteroids can all move in numerous complicated ways, but only so that the rotational momentum of the system is conserved.

Noether's theorem states that every conservation law is associated with a symmetry (invariant) of the underlying physics. The symmetry associated with conservation of rotational momentum is rotational invariance. The fact that the physics of a system is unchanged if it is rotated by any angle about an axis implies that rotational momentum is conserved.

### Relation to Newton's second law of motion

While rotational momentum total conservation can be understood separately from Newton's laws of motion as stemming from Noether's theorem in systems symmetric under rotations, it can also be understood simply as an efficient method of calculation of results that can also be otherwise arrived at directly from Newton's second law, together with laws governing the forces of nature (such as Newton's third law, Maxwell's equations and Lorentz force). Indeed, given initial conditions of position and velocity for every point, and the forces at such a condition, one may use Newton's second law to calculate the second derivative of position, and solving for this gives full information on the development of the physical system with time. Note, however, that this is no longer true in quantum mechanics, due to the existence of particle spin, which is rotational momentum that cannot be described by the cumulative effect of point-like motions in space.

As an example, consider decreasing of the rotational mass, e.g. when a figure skater is pulling in her/his hands, speeding up the circular motion. In terms of rotational momentum conservation, we have, for rotational momentum *L*, rotational mass *I* and rotational velocity *ω*:

# L
d
(
I
⋅
ω
# )
d
I
⋅
ω
I
⋅
d
ω

{\displaystyle 0 = dL = d (I\cdot \omega) = dI \cdot \omega + I \cdot d\omega }

Using this, we see that the change requires an energy of:

d
# E
d

(

1
2

I
⋅

ω
2

)

=

1
2

d
I
⋅

ω
2

I
⋅
ω
⋅
d
# ω
−

1
2

d
I
⋅

ω
2

{\displaystyle dE = d \left(\frac{1}{2} I\cdot \omega^2\right) = \frac{1}{2} dI \cdot \omega^2 + I \cdot \omega \cdot d\omega = -\frac{1}{2} dI \cdot \omega^2}

so that a decrease in the rotational mass requires investing energy.

This can be compared to the work done as calculated using Newton's laws. Each point in the rotating body is accelerating, at each point of time, with radial acceleration of:

−
r
⋅

ω
2

{\displaystyle -r\cdot \omega^2}

Let us observe a point of mass *m*, whose position vector relative to the center of motion is parallel to the z-axis at a given point of time, and is at a distance *z*. The centripetal force on this point, keeping the circular motion, is:

−
m
⋅
z
⋅

ω
2

{\displaystyle -m\cdot z\cdot \omega^2}

Thus the work required for moving this point to a distance *dz* farther from the center of motion is:

d
# W
−
m
⋅
z
⋅

ω
2

⋅
d
# z
−
m
⋅

ω
2

⋅
d

(

1
2

z
2

)

{\displaystyle dW = -m\cdot z\cdot \omega^2\cdot dz = -m\cdot \omega^2\cdot d\left(\frac{1}{2} z^2\right)}

For a non-pointlike body one must integrate over this, with *m* replaced by the mass density per unit *z*. This gives:

d
# W
−

1
2

d
I
⋅

ω
2

{\displaystyle dW = - \frac{1}{2}dI \cdot \omega^2}

which is exactly the energy required for keeping the rotational momentum conserved.

Note, that the above calculation can also be performed per mass, using kinematics only. Thus the phenomena of figure skater accelerating tangential velocity while pulling her/his hands in, can be understood as follows in layman's language: The skater's palms are not moving in a straight line, so they are constantly accelerating inwards, but do not gain additional speed because the accelerating is always done when their motion inwards is zero. However, this is different when pulling the palms closer to the body: The acceleration due to rotation now increases the speed; but because of the rotation, the increase in speed does not translate to a significant speed inwards, but to an increase of the rotation speed.

### In Lagrangian formalism

In Lagrangian mechanics, rotational momentum for rotation around a given axis, is the conjugate momentum of the generalized coordinate of the angle around the same axis. For example,

L
z

{\displaystyle L\_z}

, the rotational momentum around the z axis, is:

L
z

=

∂

L

∂

θ
˙

z

{\displaystyle L\_z = \frac{\partial \cal{L}}{\partial \dot\theta\_z}}

where

L

{\displaystyle \cal{L}}

 is the Lagrangian and

θ
z

{\displaystyle \theta\_z}

 is the angle around the z axis.

Note that

θ
˙

z

{\displaystyle \dot\theta\_z}

, the time derivative of the angle, is the rotational velocity

ω
z

{\displaystyle \omega\_z}

. Ordinarily, the Lagrangian depends on the rotational velocity through the kinetic energy: The latter can be written by separating the velocity to its radial and tangential part, with the tangential part at the x-y plane, around the z-axis, being equal to:

∑
i

1
2

m
i

v
T

i
2

=

∑
i

1
2

m
i

(

x
i
2

y
i
2

)

ω
z

i

2

{\displaystyle \sum\_i \frac{1}{2}m\_i {v\_T}\_i^2 = \sum\_i \frac{1}{2}m\_i (x\_i^2+y\_i^2) {{\omega\_z}\_i}^2}

where the subscript i stands for the i-th body, and *m*, *v**T* and *ω**z* stand for mass, tangential velocity around the z-axis and rotational velocity around that axis, respectively.

For a body that is not point-like, with density *ρ*, we have instead:

1
2

∫
ρ
(
x
,
y
,
z
)
(

x
i
2

y
i
2

)

ω
z

i

2

=

1
2

I
z

i

ω
z

i

2

{\displaystyle \frac{1}{2}\int \rho(x,y,z)(x\_i^2+y\_i^2) {{\omega\_z}\_i}^2 = \frac{1}{2} {I\_z}\_i {{\omega\_z}\_i}^2}

where *I*z is the rotational mass around the z-axis.

Thus, assuming the potential energy does not depend on *ω**z* (this assumption may fail for electromagnetic systems), we have the rotational momentum of the i-th object:

L
z

i

=

∂

L

∂

ω
z

i

=

∂

E
k

∂

ω
z

i

=

I
z

i

⋅

ω
z

i

{\displaystyle \begin{align}
 {L\_z}\_i &= \frac{\partial \cal{L}}{\partial {{\omega\_z}\_i}} = \frac{\partial E\_k}{\partial {{\omega\_z}\_i}} \\
 &= {I\_z}\_i \cdot {\omega\_z}\_i
\end{align}}

We have thus far rotated each object by a separate angle; we may also define an overall angle *θ*z by which we rotate the whole system, thus rotating also each object around the z-axis, and have the overall rotational momentum:

L
z

=

∑
i

I
z

i

⋅

ω
z

i

{\displaystyle L\_z = \sum\_i {I\_z}\_i \cdot {\omega\_z}\_i}

From Euler-Lagrange equations it then follows that:

# 0

∂

L

∂

θ
z

i

−

d

d
t

(

∂

L

∂

θ
˙

z

i

)

=

∂

L

∂

θ
z

i

−

d

L
z

i

d
t

{\displaystyle 0 = \frac{\partial \cal{L}}{\partial {{\theta\_z}\_i}} - \frac{d}{dt}\left(\frac{\partial \cal{L}}{\partial {{\dot\theta\_z}\_i}}\right) = \frac{\partial \cal{L}}{\partial {{\theta\_z}\_i}} - \frac{d{L\_z}\_i}{dt}}

Since the lagrangian is dependent upon the angles of the object only through the potential, we have:

d

L
z

i

d
t

=

∂

L

∂

θ
z

i

=
−

∂
V

∂

θ
z

i

{\displaystyle \frac{d{L\_z}\_i}{dt} = \frac{\partial \cal{L}}{\partial {{\theta\_z}\_i}} = -\frac{\partial V}{\partial {{\theta\_z}\_i}}}

which is the rotational force on the i-th object.

Suppose the system is invariant to rotations, so that the potential is independent of an overall rotation by the angle *θ*z (thus it may depend on the angles of objects only through their differences, in the form

V
(

θ
z

i

,

θ
z

j

# )
V
(

θ
z

i

−

θ
z

j

)

{\displaystyle V({\theta\_z}\_i, {\theta\_z}\_j) = V({\theta\_z}\_i - {\theta\_z}\_j)}

). We therefore get for the total rotational momentum:

d

L
z

d
t

=
−

∂
V

∂

θ
z

=
0

{\displaystyle \frac{d L\_z}{dt} = -\frac{\partial V}{\partial {\theta\_z}} = 0 }

And thus the rotational momentum around the z-axis is conserved.

This analysis can be repeated separately for each axis, giving conversation of the rotational momentum vector. However, the angles around the three axes cannot be treated simultaneously as generalized coordinates, since they are not independent; in particular, two angles per point suffice to determine its position. While it is true that in the case of a rigid body, fully describing it requires, in addition to three translational degrees of freedom, also specification of three rotational degrees of freedom; however these cannot be defined as rotations around the Cartesian axes (see Euler angles). This caveat is reflected in quantum mechanics in the non-trivial commutation relations of the different components of the rotational momentum operator.

### In Hamiltonian formalism

Equivalently, in Hamiltonian mechanics the Hamiltonian can be described as a function of the rotational momentum. As before, the part of the kinetic energy related to rotation around the z-axis for the i-th object is:

1
2

I
z

i

ω
z

i

2

=

L
z

i

2

2

I
z

i

{\displaystyle \frac{1}{2} {I\_z}\_i {{\omega\_z}\_i}^2 = \frac{{{L\_z}\_i}^2}{2{I\_z}\_i}}

which is analogous to the energy dependence upon momentum along the z-axis,

p
z

i

2

2
m

i

{\displaystyle \frac{{{p\_z}\_i}^2}{{2m}\_i}}

.

Hamilton's equations relate the angle around the z-axis to its conjugate momentum, the rotational momentum around the same axis:

d

θ
z

i

d
t

=

∂

H

∂

L
z

i

=

L
z

i

I
z

i

d

L
z

i

d
t

=
−

∂

H

∂

θ
z

i

=
−

∂
V

∂

θ
z

i

{\displaystyle \begin{align}
 \frac{d{\theta\_z}\_i}{dt} &= \frac{\partial \cal{H}}{\partial {L\_z}\_i} = \frac{{L\_z}\_i}{{I\_z}\_i} \\
 \frac{d{L\_z}\_i}{dt} &= -\frac{\partial \cal{H}}{\partial {\theta\_z}\_i} = -\frac{\partial V}{\partial {\theta\_z}\_i}
\end{align}}

The first equation gives

L
z

i

=

I
z

i

⋅

θ
˙

z

i

=

I
z

i

⋅

ω
z

i

{\displaystyle {L\_z}\_i = {I\_z}\_i \cdot {{\dot\theta\_z}\_i} = {I\_z}\_i \cdot {\omega\_z}\_i}

And so we get the same results as in the Lagrangian formalism.

Note, that for combining all axes together, we write the kinetic energy as:

E
k

=

1
2

∑
i

p

i

2

2

m
i

=

∑
i

(

p
r

i

2

2

m
i

1
2

L

i

T

I
i

−
1

L

i

)

{\displaystyle
 E\_k = \frac{1}{2}\sum\_i \frac{|\bf{p}\_i|^2}{2m\_i}
 = \sum\_i \left(\frac{{{p\_r}\_i}^2}{2m\_i} + \frac{1}{2}{\bf{L}\_i}^\text{T}{I\_i}^{-1} \bf{L}\_i\right)
}

where *p*r is the momentum in the radial direction, and the rotational mass is a 3-dimensional matrix; bold letters stand for 3-dimensional vectors.

For point-like bodies we have:

E
k

=

∑
i

(

p
r

i

2

2

m
i

L

i

2

2

m
i

r
i

2

)

{\displaystyle E\_k = \sum\_i \left(\frac{{{p\_r}\_i}^2}{2m\_i} + \frac{|{\bf{L}\_i}|^2}{2m\_i {r\_i}^2}\right)}

This form of the kinetic energy part of the Hamiltonian is useful in analyzing central potential problems, and is easily transformed to a quantum mechanical work frame (e.g. in the hydrogen atom problem).

## Rotational momentum in orbital mechanics

Main articles: Specific relative rotational momentum
While in classical mechanics the language of rotational momentum can be replaced by Newton's laws of motion, it is particularly useful for motion in central potential such as planetary motion in the solar system. Thus, the orbit of a planet in the solar system is defined by its energy, rotational momentum and angles of the orbit major axis relative to a coordinate frame.

In astrodynamics and celestial mechanics, a *massless* (or *per unit mass*) rotational momentum is defined

h

=

r

×

v

,

{\displaystyle \mathbf{h} = \mathbf{r} \times \mathbf{v}, }

called *specific rotational momentum*. Note that

L

=
m

h

.

{\displaystyle \mathbf{L} = m\mathbf{h}.}

 Mass is often unimportant in orbital mechanics calculations, because motion is defined by gravity. The primary body of the system is often so much larger than any bodies in motion about it that the smaller bodies have a negligible gravitational effect on it; it is, in effect, stationary. All bodies are apparently attracted by its gravity in the same way, regardless of mass, and therefore all move approximately the same way under the same conditions.

## Solid bodies

Rotational momentum is also an extremely useful concept for describing rotating rigid bodies such as a gyroscope or a rocky planet.
For a continuous mass distribution with density function *ρ*(**r**), a differential volume element *dV* with position vector **r** within the mass has a mass element *dm* = *ρ*(**r**)*dV*. Therefore, the infinitesimal rotational momentum of this element is:

d

L

=

r

×
d
m

v

=

r

×
ρ
(

r

)
d
V

v

=
d
V

r

×
ρ
(

r

)

v

{\displaystyle d\mathbf{L} = \mathbf{r}\times dm \mathbf{v} = \mathbf{r}\times \rho(\mathbf{r}) dV \mathbf{v} = dV \mathbf{r}\times \rho(\mathbf{r}) \mathbf{v}}

and integrating this differential over the volume of the entire mass gives its total rotational momentum:

L

=

∫
V

d
V

r

×
ρ
(

r

)

v

{\displaystyle \mathbf{L}=\int\_V dV \mathbf{r}\times \rho(\mathbf{r}) \mathbf{v}}

In the derivation which follows, integrals similar to this can replace the sums for the case of continuous mass.

### Collection of particles

For a collection of particles in motion about an arbitrary origin, it is informative to develop the equation of rotational momentum by resolving their motion into components about their own center of mass and about the origin. Given,

m
i

{\displaystyle m\_i}

 is the mass of particle

i

{\displaystyle i}

,

R

i

{\displaystyle \mathbf{R}\_i}

 is the position vector of particle

i

{\displaystyle i}

 vs the origin,

V

i

{\displaystyle \mathbf V\_i}

 is the velocity of particle

i

{\displaystyle i}

 vs the origin,

R

{\displaystyle \mathbf{R}}

 is the position vector of the center of mass vs the origin,

V

{\displaystyle \mathbf V }

 is the velocity of the center of mass vs the origin,

r

i

{\displaystyle \mathbf r\_i}

 is the position vector of particle

i

{\displaystyle i}

 vs the center of mass,

v

i

{\displaystyle \mathbf v\_i}

 is the velocity of particle

i

{\displaystyle i}

 vs the center of mass,
The total mass of the particles is simply their sum,

# M

∑
i

m
i

.

{\displaystyle M=\sum\_i m\_i.}

The position vector of the center of mass is defined by,

M

R

=

∑
i

m
i

R

i

.

{\displaystyle M\mathbf{R}=\sum\_i m\_i \mathbf{R}\_i.}

By inspection,

R

i

=

R

r

i

{\displaystyle \mathbf{R}\_i = \mathbf{R} + \mathbf{r}\_i}

 and

V

i

=

V

v

i

.

{\displaystyle \mathbf{V}\_i = \mathbf{V} + \mathbf{v}\_i.}

The total rotational momentum of the collection of particles is the sum of the rotational momentum of each particle,

L

=

∑
i

(

R

i

×

m
i

V

i

)

{\displaystyle \mathbf{L} = \sum\_i \left( \mathbf{R}\_i \times m\_i \mathbf{V}\_i \right)}

     (1)

Expanding

R

i

{\displaystyle \mathbf{R}\_i}

,

L

=

∑
i

[

(

R

r

i

)

×

m
i

V

i

]

=

∑
i

[

R

×

m
i

V

i

r

i

×

m
i

V

i

]

{\displaystyle \begin{align}
 \mathbf{L} &= \sum\_i \left[\left(\mathbf{R} + \mathbf{r}\_i\right) \times m\_i\mathbf{V}\_i \right] \\
 &= \sum\_i \left[ \mathbf{R} \times m\_i\mathbf{V}\_i + \mathbf{r}\_i \times m\_i\mathbf{V}\_i \right]
\end{align}}

Expanding

V

i

{\displaystyle \mathbf V\_i}

,

L

=

∑
i

[

R

×

m
i

(

V

v

i

)

r

i

×

m
i

(

V

v

i

)

]

=

∑
i

[

R

×

m
i

V

R

×

m
i

v

i

r

i

×

m
i

V

r

i

×

m
i

v

i

]

=

∑
i

R

×

m
i

V

∑
i

R

×

m
i

v

i

∑
i

r

i

×

m
i

V

∑
i

r

i

×

m
i

v

i

{\displaystyle \begin{align}
 \mathbf{L} &= \sum\_i \left[ \mathbf{R} \times m\_i\left(\mathbf{V} + \mathbf{v}\_i\right) + \mathbf{r}\_i \times m\_i(\mathbf{V} + \mathbf{v}\_i) \right ]\\
 &= \sum\_i \left[ \mathbf{R} \times m\_i\mathbf{V} + \mathbf{R} \times m\_i\mathbf{v}\_i + \mathbf{r}\_i \times m\_i\mathbf{V} + \mathbf{r}\_i \times m\_i\mathbf{v}\_i \right]\\
 &= \sum\_i \mathbf{R} \times m\_i\mathbf{V} + \sum\_i \mathbf{R} \times m\_i\mathbf{v}\_i + \sum\_i \mathbf{r}\_i \times m\_i\mathbf{V} + \sum\_i \mathbf{r}\_i \times m\_i\mathbf{v}\_i
\end{align}}

It can be shown that (see sidebar),

∑
i

m
i

r

i

=

0

{\displaystyle \sum\_i m\_i\mathbf{r}\_i = \mathbf{0}}

r

i

=

R

i

−

R

m
i

r

i

=

m
i

(

R

i

−

R

)

∑
i

m
i

r

i

=

∑
i

m
i

(

R

i

−

R

)

=

∑
i

(

m
i

R

i

−

m
i

R

)

=

∑
i

m
i

R

i

−

∑
i

m
i

R

=

∑
i

m
i

R

i

−

(

∑
i

m
i

)

R

=

∑
i

m
i

R

i

−
M

R

{\displaystyle \begin{align}
 \mathbf{r}\_i &= \mathbf{R}\_i - \mathbf{R} \\
 m\_i\mathbf{r}\_i &= m\_i\left(\mathbf{R}\_i - \mathbf{R}\right) \\
 \sum\_i m\_i\mathbf{r}\_i &= \sum\_i m\_i\left(\mathbf{R}\_i - \mathbf{R}\right)\\
 &= \sum\_i (m\_i\mathbf{R}\_i - m\_i\mathbf{R})\\
 &= \sum\_i m\_i\mathbf{R}\_i - \sum\_i m\_i\mathbf{R}\\
 &= \sum\_i m\_i\mathbf{R}\_i - \left( \sum\_i m\_i \right) \mathbf{R}\\
 &= \sum\_i m\_i\mathbf{R}\_i - M\mathbf{R}
\end{align}}

which, by the definition of the center of mass, is

0

,

{\displaystyle \mathbf{0},}

 and similarly for

∑
i

m
i

v

i

.

{\displaystyle \sum\_i m\_i\mathbf{v}\_i.}

∑
i

m
i

r

i

=

0

{\displaystyle \sum\_i m\_i\mathbf{r}\_i = \mathbf{0}}

 and

∑
i

m
i

v

i

=

0

,

{\displaystyle \sum\_i m\_i\mathbf{v}\_i = \mathbf{0},}

therefore the second and third terms vanish,

L

=

∑
i

R

×

m
i

V

∑
i

r

i

×

m
i

v

i

.

{\displaystyle \mathbf{L} = \sum\_i \mathbf{R} \times m\_i\mathbf{V} + \sum\_i \mathbf{r}\_i \times m\_i\mathbf{v}\_i .}

The first term can be rearranged,

∑
i

R

×

m
i

V

=

R

×

∑
i

m
i

V

=

R

×
M

V

,

{\displaystyle \sum\_i \mathbf{R} \times m\_i\mathbf{V} = \mathbf{R} \times \sum\_i m\_i\mathbf{V} = \mathbf{R} \times M\mathbf{V},}

and total rotational momentum for the collection of particles is finally,

L

=

R

×
M

V

∑
i

r

i

×

m
i

v

i

{\displaystyle \mathbf{L} = \mathbf{R} \times M\mathbf{V} + \sum\_i \mathbf{r}\_i \times m\_i\mathbf{v}\_i }

     (2)

The first term is the rotational momentum of the center of mass relative to the origin. Similar to **Single particle**, below, it is the rotational momentum of one particle of mass *M* at the center of mass moving with velocity **V**. The second term is the rotational momentum of the particles moving relative to the center of mass, similar to **Fixed center of mass**, below. The result is general—the motion of the particles is not restricted to rotation or revolution about the origin or center of mass. The particles need not be individual masses, but can be elements of a continuous distribution, such as a solid body.

Rearranging equation (**2**) by vector identities, multiplying both terms by "one", and grouping appropriately,

L

=
M
(

R

×

V

)

∑
i

[

m
i

(

r

i

×

v

i

)

]

,

=

R
2

R
2

M

(

R

×

V

)

∑
i

[

r
i
2

r
i
2

m
i

(

r

i

×

v

i

)

]

,

=

R
2

M

(

R

×

V

R
2

)

∑
i

[

r
i
2

m
i

(

r

i

×

v

i

r
i
2

)

]

,

{\displaystyle \begin{align}
 \mathbf{L} &= M(\mathbf{R} \times \mathbf{V}) + \sum\_i \left[m\_i\left(\mathbf{r}\_i \times \mathbf{v}\_i\right)\right], \\
 &= \frac{R^2}{R^2}M\left(\mathbf{R} \times \mathbf{V}\right) + \sum\_i \left[ \frac{r\_i^2}{r\_i^2}m\_i\left(\mathbf{r}\_i \times \mathbf{v}\_i\right)\right] , \\
 &= R^2M \left( \frac{\mathbf{R} \times \mathbf{V}}{R^2} \right) + \sum\_i \left[ r\_i^2 m\_i \left( \frac{\mathbf{r}\_i \times \mathbf{v}\_i}{r\_i^2} \right) \right] , \\
\end{align}}

gives the total rotational momentum of the system of particles in terms of rotational mass

I

{\displaystyle I}

 and rotational velocity

ω

{\displaystyle \boldsymbol{\omega}}

,

L

=

I
R

ω
R

∑
i

I
i

ω
i

.

{\displaystyle \mathbf{L} = I\_R\boldsymbol{\omega}\_R + \sum\_i I\_i\boldsymbol{\omega}\_i.}

     (3)

#### Single particle case

In the case of a single particle moving about the arbitrary origin,

r

i

=

v

i

=

0

,

r

=

R

,

v

=

V

,

m

=
M
,

{\displaystyle \begin{align}
 \mathbf{r}\_i &= \mathbf{v}\_i = \mathbf{0}, \\
 \mathbf{r} &= \mathbf{R}, \\
 \mathbf{v} &= \mathbf{V}, \\
 m &= M,
\end{align}}

∑
i

r

i

×

m
i

v

i

=

0

,

{\displaystyle \sum\_i \mathbf{r}\_i \times m\_i\mathbf{v}\_i = \mathbf{0},}

∑
i

I
i

ω
i

=

0

,

{\displaystyle \sum\_i I\_i\boldsymbol{\omega}\_i = \mathbf{0},}

 and equations (**2**) and (**3**) for total rotational momentum reduce to,

L

=

R

×
m

V

=

I
R

ω
R

.

{\displaystyle \mathbf{L} = \mathbf{R} \times m\mathbf{V} = I\_R\boldsymbol{\omega}\_R.}

#### Case of a fixed center of mass

For the case of the center of mass fixed in space with respect to the origin,

V

=

0

,

{\displaystyle \mathbf{V} = \mathbf{0},}

R

×
M

V

=

0

,

{\displaystyle \mathbf{R} \times M\mathbf{V} = \mathbf{0},}

I
R

ω
R

=

0

,

{\displaystyle I\_R\boldsymbol{\omega}\_R = \mathbf{0},}

 and equations (**2**) and (**3**) for total rotational momentum reduce to,

L

=

∑
i

r

i

×

m
i

v

i

=

∑
i

I
i

ω
i

.

{\displaystyle \mathbf{L} = \sum\_i \mathbf{r}\_i \times m\_i\mathbf{v}\_i = \sum\_i I\_i\boldsymbol{\omega}\_i.}

## Rotational momentum in general relativity

In modern (20th century) theoretical physics, rotational momentum (not including any intrinsic rotational momentum – see below) is described using a different formalism, instead of a classical pseudovector. In this formalism, rotational momentum is the 2-form Noether charge associated with rotational invariance. As a result, rotational momentum is not conserved for general curved spacetimes, unless it happens to be asymptotically rotationally invariant.

In classical mechanics, the rotational momentum of a particle can be reinterpreted as a plane element:

L

=

r

∧

p

,

{\displaystyle \mathbf{L} = \mathbf{r} \wedge \mathbf{p} \,,}

in which the exterior product ∧ replaces the cross product × (these products have similar characteristics but are nonequivalent). This has the advantage of a clearer geometric interpretation as a plane element, defined from the **x** and **p** vectors, and the expression is true in any number of dimensions (two or higher). In Cartesian coordinates:

L

=

(

x

p
y

−
y

p
x

)

e

x

∧

e

y

(

y

p
z

−
z

p
y

)

e

y

∧

e

z

(

z

p
x

−
x

p
z

)

e

z

∧

e

x

=

L

x
y

e

x

∧

e

y

L

y
z

e

y

∧

e

z

L

z
x

e

z

∧

e

x

,

{\displaystyle \begin{align}
 \mathbf{L} &= \left(xp\_y - yp\_x\right)\mathbf{e}\_x \wedge \mathbf{e}\_y + \left(yp\_z - zp\_y\right)\mathbf{e}\_y \wedge \mathbf{e}\_z + \left(zp\_x - xp\_z\right)\mathbf{e}\_z \wedge \mathbf{e}\_x\\
 &= L\_{xy}\mathbf{e}\_x \wedge \mathbf{e}\_y + L\_{yz}\mathbf{e}\_y \wedge \mathbf{e}\_z + L\_{zx}\mathbf{e}\_z \wedge \mathbf{e}\_x \,,
\end{align}}

or more compactly in index notation:

L

i
j

=

x
i

p
j

−

x
j

p
i

.

{\displaystyle L\_{ij} = x\_i p\_j - x\_j p\_i\,.}

The rotational velocity can also be defined as an antisymmetric second order tensor, with components *ωij*. The relation between the two antisymmetric tensors is given by the rotational mass which must now be a fourth order tensor:

L

i
j

=

I

i
j
k
ℓ

ω

k
ℓ

.

{\displaystyle L\_{ij} = I\_{ijk\ell} \omega\_{k\ell} \,. }

Again, this equation in **L** and **ω** as tensors is true in any number of dimensions. This equation also appears in the geometric algebra formalism, in which **L** and **ω** are bivectors, and the rotational mass is a mapping between them.

In relativistic mechanics, the relativistic rotational momentum of a particle is expressed as an antisymmetric tensor of second order:

M

α
β

=

X
α

P
β

−

X
β

P
α

{\displaystyle M\_{\alpha\beta} = X\_\alpha P\_\beta - X\_\beta P\_\alpha}

in the language of four-vectors, namely the four position *X* and the four momentum *P*, and absorbs the above **L** together with the motion of the centre of mass of the particle.

In each of the above cases, for a system of particles, the total rotational momentum is just the sum of the individual particle angular momenta, and the centre of mass is for the system.

## Rotational momentum in quantum mechanics

Main articles: Angular momentum operator
Rotational momentum in quantum mechanics differs in many profound respects from rotational momentum in classical mechanics. In relativistic quantum mechanics, it differs even more, in which the above relativistic definition becomes a tensorial operator.

### Spin, orbital, and total rotational momentum

Main articles: Spin (physics)

The classical definition of rotational momentum as

L

=

r

×

p

{\displaystyle \mathbf{L}=\mathbf{r}\times\mathbf{p}}

 can be carried over to quantum mechanics, by reinterpreting **r** as the quantum position operator and **p** as the quantum momentum operator. **L** is then an operator, specifically called the *orbital rotational momentum operator*. The components of the rotational momentum operator satisfy the commutation relations of the Lie algebra so(3). Indeed, these operators are precisely the infinitesimal action of the rotation group on the quantum Hilbert space. (See also the discussion below of the rotational momentum operators as the generators of rotations.)

However, in quantum physics, there is another type of rotational momentum, called *spin rotational momentum*, represented by the spin operator **S**. Almost all elementary particles have nonvanishing spin. Spin is often depicted as a particle literally spinning around an axis, but this is a misleading and inaccurate picture: spin is an intrinsic property of a particle, unrelated to any sort of motion in space and fundamentally different from orbital rotational momentum. All elementary particles have a characteristic spin (possibly zero), for example electrons have "spin 1/2" (this actually means "spin ħ/2"), photons have "spin 1" (this actually means "spin ħ"), and pi-mesons have spin 0.

Finally, there is total rotational momentum **J**, which combines both the spin and orbital rotational momentum of all particles and fields. (For one particle, **J** = **L** + **S**.) Conservation of rotational momentum applies to **J**, but not to **L** or **S**; for example, the spin–orbit interaction allows rotational momentum to transfer back and forth between **L** and **S**, with the total remaining constant. Electrons and photons need not have integer-based values for total rotational momentum, but can also have fractional values.

In molecules the total rotational momentum **F** is the sum of the rovibronic (orbital) rotational momentum **N**, the electron spin rotational momentum **S**, and the nuclear spin rotational momentum **I**. For electronic singlet states the rovibronic rotational momentum is denoted **J** rather than **N**. As explained by Van Vleck, the components of the molecular rovibronic rotational momentum referred to molecule-fixed axes have different commutation relations from those for the components about space-fixed axes.

### Quantization

In quantum mechanics, rotational momentum is quantized – that is, it cannot vary continuously, but only in "quantum leaps" between certain allowed values. For any system, the following restrictions on measurement results apply, where

ℏ

{\displaystyle \hbar}

 is the reduced Planck constant and

n
^

{\displaystyle \hat n}

 is any Euclidean vector such as x, y, or z:

L

n
^

{\displaystyle L\_\hat{n}}

…
,
−
2
ℏ
,
−
ℏ
,
0
,
ℏ
,
2
ℏ
,
…

{\displaystyle \ldots, -2\hbar, -\hbar, 0, \hbar, 2\hbar, \ldots}

S

n
^

{\displaystyle S\_\hat{n}}

 or

J

n
^

{\displaystyle J\_\hat{n}}

…
,
−

3
2

ℏ
,
−
ℏ
,
−

1
2

ℏ
,
0
,

1
2

ℏ
,
ℏ
,

3
2

ℏ
,
…

{\displaystyle \ldots, -\frac{3}{2}\hbar, -\hbar, -\frac{1}{2}\hbar, 0, \frac{1}{2}\hbar, \hbar, \frac{3}{2}\hbar, \ldots}

L
2

=

L
x
2

L
y
2

L
z
2

{\displaystyle \begin{align}
 &L^2 \\
 ={} &L\_x^2 + L\_y^2 + L\_z^2
\end{align}}

[

ℏ
2

n
(
n
1
)

]

{\displaystyle \left[\hbar^2 n(n + 1)\right]}

, where

# n
0
,
1
,
2
,
…

{\displaystyle n = 0, 1, 2, \ldots}

S
2

{\displaystyle S^2}

 or

J
2

{\displaystyle J^2}

[

ℏ
2

n
(
n
1
)

]

{\displaystyle \left[\hbar^2 n(n + 1)\right]}

, where

# n
0
,

1
2

,
1
,

3
2

,
…

{\displaystyle n = 0, \frac{1}{2}, 1, \frac{3}{2}, \ldots}

(There are additional restrictions as well, see rotational momentum operator for details.)

The reduced Planck constant

ℏ

{\displaystyle \hbar}

 is tiny by everyday standards, about 10−34 J s, and therefore this quantization does not noticeably affect the rotational momentum of macroscopic objects. However, it is very important in the microscopic world. For example, the structure of electron shells and subshells in chemistry is significantly affected by the quantization of rotational momentum.

Quantization of rotational momentum was first postulated by Niels Bohr in his Bohr model of the atom and was later predicted by Erwin Schrödinger in his Schrödinger equation.

### Uncertainty

In the definition

L

=

r

×

p

{\displaystyle \mathbf{L}=\mathbf{r}\times\mathbf{p}}

, six operators are involved: The position operators

r
x

{\displaystyle r\_x}

,

r
y

{\displaystyle r\_y}

,

r
z

{\displaystyle r\_z}

, and the momentum operators

p
x

{\displaystyle p\_x}

,

p
y

{\displaystyle p\_y}

,

p
z

{\displaystyle p\_z}

. However, the Heisenberg uncertainty principle tells us that it is not possible for all six of these quantities to be known simultaneously with arbitrary precision. Therefore, there are limits to what can be known or measured about a particle's rotational momentum. It turns out that the best that one can do is to simultaneously measure both the rotational momentum vector's magnitude and its component along one axis.

The uncertainty is closely related to the fact that different components of an rotational momentum operator do not commute, for example

L
x

L
y

≠

L
y

L
x

{\displaystyle L\_xL\_y \neq L\_yL\_x}

. (For the precise commutation relations, see rotational momentum operator.)

### Total rotational momentum as generator of rotations

As mentioned above, orbital rotational momentum **L** is defined as in classical mechanics:

L

=

r

×

p

{\displaystyle \mathbf{L}=\mathbf{r}\times\mathbf{p}}

, but *total* rotational momentum **J** is defined in a different, more basic way: **J** is defined as the "generator of rotations". More specifically, **J** is defined so that the operator

R
(

n
^

,
ϕ
)
≡
exp
⁡

(

−

i
ℏ

ϕ

J

⋅

n

^

)

{\displaystyle R(\hat{n},\phi) \equiv \exp\left(-\frac{i}{\hbar}\phi\, \mathbf{J}\cdot \hat{\mathbf{n}}\right)}

is the rotation operator that takes any system and rotates it by angle

ϕ

{\displaystyle \phi}

 about the axis

n

^

{\displaystyle \hat{\mathbf{n}}}

. (The "exp" in the formula refers to operator exponential) To put this the other way around, whatever our quantum Hilbert space is, we expect that the rotation group SO(3) will act on it. There is then an associated action of the Lie algebra so(3) of SO(3); the operators describing the action of so(3) on our Hilbert space are the (total) rotational momentum operators.

The relationship between the rotational momentum operator and the rotation operators is the same as the relationship between Lie algebras and Lie groups in mathematics. The close relationship between rotational momentum and rotations is reflected in Noether's theorem that proves that rotational momentum is conserved whenever the laws of physics are rotationally invariant.

## Rotational momentum in electrodynamics

See also: Momentum#Particle in field
When describing the motion of a charged particle in an electromagnetic field, the canonical momentum **P** (derived from the Lagrangian for this system) is not gauge invariant. As a consequence, the canonical rotational momentum **L** = **r** × **P** is not gauge invariant either. Instead, the momentum that is physical, the so-called *kinetic momentum* (used throughout this article), is (in SI units)

p

=
m

v

=

P

−
e

A

{\displaystyle \mathbf{p} = m\mathbf{v} = \mathbf{P} - e \mathbf{A} }

where *e* is the electric charge of the particle and **A** the magnetic vector potential of the electromagnetic field. The gauge-invariant rotational momentum, that is *kinetic rotational momentum*, is given by

K

=

r

×
(

P

−
e

A

)

{\displaystyle \mathbf{K}= \mathbf{r} \times ( \mathbf{P} - e\mathbf{A} )}

The interplay with quantum mechanics is discussed further in the article on canonical commutation relations.

## Rotational momentum in optics

In *classical Maxwell electrodynamics* the Poynting vector
is a linear momentum density of electromagnetic field.

S

(

r

,
t
# )

ϵ
0

c
2

E

(

r

,
t
)
×

B

(

r

,
t
)
.

{\displaystyle \mathbf{S}(\mathbf{r}, t) = \epsilon\_0 c^2 \mathbf{E}(\mathbf{r}, t) \times \mathbf{B}(\mathbf{r}, t).
}

The rotational momentum density vector

L

(

r

,
t
)

{\displaystyle \mathbf{L}(\mathbf{r}, t)}

 is given by a vector product
as in classical mechanics:

L

(

r

,
t
# )

r

×

S

(

r

,
t
)
.

{\displaystyle \mathbf{L}(\mathbf{r}, t) = \mathbf{r} \times \mathbf{S}(\mathbf{r}, t).
}

The above identities are valid *locally*, i.e. in each space point

r

{\displaystyle \mathbf{r}}

 in a given moment

t

{\displaystyle t}

.

## History

Newton, in the *Principia*, hinted at rotational momentum in his examples of the First Law of Motion,

*A top, whose parts by their cohesion are perpetually drawn aside from rectilinear motions, does not cease its rotation, otherwise than as it is retarded by the air. The greater bodies of the planets and comets, meeting with less resistance in more free spaces, preserve their motions both progressive and circular for a much longer time.*
He did not further investigate rotational momentum directly in the *Principia*,

*From such kind of reflexions also sometimes arise the circular motions of bodies about their own centres. But these are cases which I do not consider in what follows; and it would be too tedious to demonstrate every particular that relates to this subject.*
However, his geometric proof of the law of areas is an outstanding example of Newton's genius, and indirectly proves rotational momentum conservation in the case of a central force.

### The Law of Areas

Main articles: Classical central-force problem, Areal velocity
#### Newton's derivation

As a planet orbits the Sun, the line between the Sun and the planet sweeps out equal areas in equal intervals of time. This had been known since Kepler expounded his second law of planetary motion. Newton derived a unique geometric proof, and went on to show that the attractive force of the Sun's gravity was the cause of all of Kepler's laws.

During the first interval of time, an object is in motion from point **A** to point **B**. Undisturbed, it would continue to point **c** during the second interval. When the object arrives at **B**, it receives an impulse directed toward point **S**. The impulse gives it a small added velocity toward **S**, such that if this were its only velocity, it would move from **B** to **V** during the second interval. By the rules of velocity composition, these two velocities add, and point **C** is found by construction of parallelogram **BcCV**. Thus the object's path is deflected by the impulse so that it arrives at point **C** at the end of the second interval. Because the triangles **SBc** and **SBC** have the same base **SB** and the same height **Bc** or **VC**, they have the same area. By symmetry, triangle **SBc** also has the same area as triangle **SAB**, therefore the object has swept out equal areas **SAB** and **SBC** in equal times.

At point **C**, the object receives another impulse toward **S**, again deflecting its path during the third interval from **d** to **D**. Thus it continues to **E** and beyond, the triangles **SAB**, **SBc**, **SBC**, **SCd**, **SCD**, **SDe**, **SDE** all having the same area. Allowing the time intervals to become ever smaller, the path **ABCDE** approaches indefinitely close to a continuous curve.

Note that because this derivation is geometric, and no specific force is applied, it proves a more general law than Kepler's second law of planetary motion. It shows that the Law of Areas applies to any central force, attractive or repulsive, continuous or non-continuous, or zero.

#### Conservation of rotational momentum in the Law of Areas

The proportionality of rotational momentum to the area swept out by a moving object can be understood by realizing that the bases of the triangles, that is, the lines from **S** to the object, are equivalent to the radius r, and that the heights of the triangles are proportional to the perpendicular component of velocity v⊥. Hence, if the area swept per unit time is constant, then by the triangular area formula 12(base)(height), the product (base)(height) and therefore the product rv⊥ are constant: if r and the base length are decreased, v⊥ and height must increase proportionally. Mass is constant, therefore rotational momentum rmv⊥ is conserved by this exchange of distance and velocity.

In the case of triangle **SBC**, area is equal to 12(**SB**)(**VC**). Wherever **C** is eventually located due to the impulse applied at **B**, the product (**SB**)(**VC**), and therefore rmv⊥ remain constant. Similarly so for each of the triangles.

### After Newton

Leonhard Euler, Daniel Bernoulli, and Patrick d'Arcy all understood rotational momentum in terms of conservation of areal velocity, a result of their analysis of Kepler's second law of planetary motion. It is unlikely that they realized the implications for ordinary rotating matter.

In 1736 Euler, like Newton, touched on some of the equations of rotational momentum in his *Mechanica* without further developing them.

Bernoulli wrote in a 1744 letter of a "moment of rotational motion", possibly the first conception of rotational momentum as we now understand it.

In 1799, Pierre-Simon Laplace first realized that a fixed plane was associated with rotation—his *invariable plane*.

Louis Poinsot in 1803 began representing rotations as a line segment perpendicular to the rotation, and elaborated on the "conservation of moments".

In 1852 Léon Foucault used a gyroscope in an experiment to display the Earth's rotation.

William J. M. Rankine's 1858 *Manual of Applied Mechanics* defined rotational momentum in the modern sense for the first time:

*...a line whose length is proportional to the magnitude of the rotational momentum, and whose direction is perpendicular to the plane of motion of the body and of the fixed point, and such, that when the motion of the body is viewed from the extremity of the line, the radius-vector of the body seems to have right-handed rotation.*
In an 1872 edition of the same book, Rankine stated that "The term *rotational momentum* was introduced by Mr. Hayward," probably referring to R.B. Hayward's article *On a Direct Method of estimating Velocities, Accelerations, and all similar Quantities with respect to Axes moveable in any manner in Space with Applications,* which was introduced in 1856, and published in 1864. Rankine was mistaken, as numerous publications feature the term starting in the late 18th to early 19th centuries. However, Hayward's article apparently was the first use of the term and the concept seen by much of the English-speaking world. Before this, rotational momentum was typically referred to as "momentum of rotation" in English.

## See also

* Absolute angular momentum
* angular momentum coupling
* angular momentum of light
* angular momentum diagrams (quantum mechanics)
* Clebsch–Gordan coefficients
* Linear-rotational analogs
* Orders of magnitude (angular momentum)
* Pauli–Lubanski pseudovector
* Relative angular momentum
* Relativistic angular momentum
* Rigid rotor
* Rotational energy
* Specific relative angular momentum
* Yrast