Angular velocity

Classical mechanics
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In physics, the angular velocity is defined as the rate of change of angular displacement and is a vector quantity (more precisely, a pseudovector) which specifies the angular speed (rotational speed) of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, degrees per hour, etc. Angular velocity is usually represented by the symbol omega (ω, rarely Ω).

The direction of the angular velocity vector is perpendicular to the plane of rotation, in a direction which is usually specified by the right-hand rule.^[1]

Angular velocity of a particle

Particle in two dimensions

The angular velocity of the particle at P with respect to the origin O is determined by the perpendicular component of the velocity vector v.

The angular velocity describes the speed of rotation and the orientation of the instantaneous axis about which the rotation occurs. The direction of the angular velocity pseudovector will be along the axis of rotation; in this case (counter-clockwise rotation) the vector points up.

The angular velocity of a particle is measured around or relative to a point, called the origin. As shown in the diagram (with angles ɸ and θ in radians), if a line is drawn from the origin (O) to the particle (P), then the velocity (v) of the particle has a component along the radius (radial component, v_‖) and a component perpendicular to the radius (cross-radial component, v_⊥). If there is no radial component, then the particle moves in a circle. On the other hand, if there is no cross-radial component, then the particle moves along a straight line from the origin.

A radial motion produces no change in the direction of the particle relative to the origin, so for purposes of finding the angular velocity the radial component can be ignored. Therefore, the rotation is completely produced by the perpendicular motion around the origin, and the angular velocity is completely determined by this component.

In two dimensions the angular velocity ω is given by

\omega = \frac{d\phi}{dt}

This is related to the cross-radial (tangential) velocity by:^[1]

\mathrm{v}_\perp=r\,\frac{d\phi}{dt}

An explicit formula for v_⊥ in terms of v and θ is:

\mathrm{v}_\perp=|\mathrm{\mathbf{v}}|\,\sin(\theta)

Combining the above equations gives a formula for ω:

\omega=\frac{|\mathrm{\mathbf{v}}|\sin(\theta)}{|\mathrm{\mathbf{r}}|}

In two dimensions the angular velocity is a single number that has no direction, but it does have a sense or orientation. In two dimensions the angular velocity is a pseudoscalar, a quantity that changes its sign under a parity inversion (for example if one of the axes is inverted or if they are swapped). The positive direction of rotation is taken, by convention, to be in the direction towards the y axis from the x axis. If parity is inverted, but the sense of a rotation does not, then the sign of the angular velocity changes.

There are three types of angular velocity involved in the movement on an ellipse corresponding to the three anomalies (true, eccentric and mean).

Particle in three dimensions

In three dimensions, the angular velocity becomes a bit more complicated. The angular velocity in this case is generally thought of as a vector, or more precisely, a pseudovector. It now has not only a magnitude, but a direction as well. The magnitude is the angular speed, and the direction describes the axis of rotation. The right-hand rule indicates the positive direction of the angular velocity pseudovector.

Let $\bold u$ be a unitary vector along the instantaneous rotation axis, so that from the top of the vector the rotation is counter-clock-wise. The angular velocity vector $\vec \omega$ is then defined as:

\boldsymbol\omega = \frac{d\phi}{dt}\bold u

Just as in the two dimensional case, a particle will have a component of its velocity along the radius from the origin to the particle, and another component perpendicular to that radius. The combination of the origin point and the perpendicular component of the velocity defines a plane of rotation in which the behavior of the particle (for that instant) appears just as it does in the two dimensional case. The axis of rotation is then a line normal to this plane, and this axis defined the direction of the angular velocity pseudovector, while the magnitude is the same as the pseudoscalar value found in the 2-dimensional case. Using the unit vector $\bold u$ defined before, the angular velocity vector may be written in a manner similar to that for two dimensions:

\boldsymbol\omega=\frac{|\bold v|\sin(\theta)}{|\bold r|}\bold u

which, by the definition of the cross product, can be written:

\boldsymbol\omega=\frac{\bold r\times\bold v}{|\bold r|^2}

Addition of angular velocity vectors

If a point rotates with $\omega_2$ in a frame $F_2$ which rotates itself with angular speed $\omega_1$ with respect to an external frame $F_1$ , we can define the addition of $\omega_1 + \omega_2$ as the angular velocity vector of the point with respect to $F_1$ .

With this operation defined like this, angular velocity, which is a pseudovector, becomes also a real vector because it has two operations:

An internal operation (addition) which is associative, commutative, distributive and with zero and unity elements
An external operation (external product), with the normal properties for an external product.

This is the definition of a vector space. The only property that presents difficulties to prove is the commutativity of the addition. This can be proven from the fact that the velocity tensor W (see below) is skew-symmetric. Therefore $R=e^{Wt}$ is a rotation matrix and in a time dt is an infinitesimal rotation matrix. Therefore it can be expanded as $R = I + W\cdot dt + {1 \over 2} (W \cdot dt)^2 + ...$

The composition of rotations is not commutative, but when they are infinitesimal rotations the first order approximation of the previous series can be taken and $(I+W_1\cdot dt)(I+W_2 \cdot dt)=(I+W_2 \cdot dt)(I+W_1\cdot dt)$ and therefore $\omega_1 + \omega_2 = \omega_2 + \omega_1$

Rotating frames

Given a rotating frame composed by three unitary vectors, all the three must have the same angular speed in any instant. In such a frame each vector is a particular case of the previous case (moving particle), in which the module of the vector is constant.

Though it just a particular case of a moving particle, this is a very important one for its relationship with the rigid body study, and special tools have been developed for this case. There are two possible ways to describe the angular velocity of a rotating frame: the angular velocity vector and the angular velocity tensor. Both entities are related and they can be calculated from each other.

Angular velocity vector for a frame

It is defined as the angular velocity of each of the vectors of the frame, in a consistent way with the general definition.

It is known by the Euler's rotation theorem that for a rotating frame there exists an instantaneous axis of rotation in any instant. In the case of a frame, the angular velocity vector is over the instantaneous axis of rotation.

Any transversal section of a plane perpendicular to this axis has to behave as a two dimensional rotation. Thus, the magnitude of the angular velocity vector at a given time t is consistent with the two dimensions case.

Angular velocity is a vector defining an addition operation. Components can be calculated from the derivatives of the parameters defining the moving frame (Euler angles or rotation matrices)

Addition of angular velocity vectors in frames

Schematic construction for addition of angular velocity vectors for rotating frames

As in the general case, the addition operation for angular velocity vectors can be defined using movement composition. In the case of rotating frames, the movement composition is simpler than the general case because the final matrix is always a product of rotation matrices.

As in the general case, addition is commutative $\omega_1 + \omega_2 = \omega_2 + \omega_1$

Components from the vectors of the frame

Substituting in the expression

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

any vector e of the frame we obtain $\boldsymbol\omega=\frac{\boldsymbol e\times\dot{\boldsymbol e}}{|{\mathbf e}|^2}$ , and therefore $\boldsymbol\omega = \mathbf{e}_1\times\dot{\mathbf{e}}_1=\mathbf{e}_2\times\dot{\mathbf{e}}_2=\mathbf{e}_3\times\dot{\mathbf{e}}_3.$

As the columns of the matrix of the frame are the components of its vectors, this allows also to calculate $\omega$ from the matrix of the frame and its derivative.

Components from Euler angles

Diagram showing Euler frame in green

The components of the angular velocity pseudovector were first calculated by Leonhard Euler using his Euler angles and an intermediate frame made out of the intermediate frames of the construction:

One axis of the reference frame (the precession axis)
The line of nodes of the moving frame respect the reference frame (nutation axis)
One axis of the moving frame (the intrinsic rotation axis)

Euler proved that the projections of the angular velocity pseudovector over these three axes was the derivative of its associated angle (which is equivalent to decompose the instant rotation in three instantaneous Euler rotations). Therefore:^[2]

\boldsymbol\omega = \dot\alpha\bold u_1+\dot\beta\bold u_2+\dot\gamma \bold u_3

This basis is not orthonormal and it is difficult to use, but now the velocity vector can be changed to the fixed frame or to the moving frame with just a change of bases. For example, changing to the mobile frame:

\boldsymbol\omega = (\dot\alpha\sin\beta\sin\gamma+\dot\beta\cos\gamma)\bold I+(\dot\alpha\sin\beta\cos\gamma-\dot\beta\sin\gamma)\bold J +(\dot\alpha\cos\beta+\dot\gamma)\bold K

where $\bold I,\bold J,\bold K$ are unit vectors for the frame fixed in the moving body. This example has been made using the Z-X-Z convention for Euler angles.^[3]

Components from infinitesimal rotation matrices

The components of the angular velocity vector can be calculated from infinitesimal rotations (if available) as follows:

As any rotation matrix has a single real eigenvalue, which is +1, this eigenvalue shows the rotation axis.
Its module can be deduced from the value of the infinitesimal rotation.

Angular velocity tensor

Properties of angular velocity tensors

In general, the angular velocity in an n-dimensional space is the time derivative of the angular displacement tensor which is a second rank skew-symmetric tensor.

This tensor W will have n(n-1)/2 independent components and this number is the dimension of the Lie algebra of the Lie group of rotations of an n-dimensional inner product space.^[4]

Exponential of W

In three dimensions angular velocity can be represented by a pseudovector because second rank tensors are dual to pseudovectors in three dimensions.

As $\frac {dA(t)} {dt} = W\cdot A(t)$ . This can be read as a differential equation that defines A(t) knowing W(t).

\frac {dA(t)} {A} = W \cdot {dt}

And if the angular speed is constant then $W$ is also constant and the equation can be integrated. The result is:

A(t) = e^{W \cdot t}

which shows a connection with the Lie group of rotations.

W is skew-symmetric

It is possible to prove that angular velocity tensor are skew symmetric matrices which means that a $W = \frac {dR(t)}{dt}\cdot {R^T}$ satisfies $W^T= -W$ .

To prove it we start taking the time derivative of $\mathcal{R}\mathcal{R}^T$ being R(t) a rotation matrix:

\mathcal{I}=\mathcal{R}\mathcal{R}^T

because R(t) is a rotation matrix

0=\frac{d\mathcal{R}}{dt}\mathcal{R}^T+\mathcal{R}\frac{d\mathcal{R}^T}{dt}

Applying the formula (AB)^T = B^TA^T:

0 = \frac{d\mathcal{R}}{dt}\mathcal{R}^T+\left(\frac{d\mathcal{R}}{dt}\mathcal{R}^T\right)^T = W + W^T

Thus, W is the negative of its transpose, which implies it is a skew symmetric matrix.

Duality with respect to the velocity vector

The tensor is a matrix with this structure:

W(t) = \begin{pmatrix} 0 & -\omega_z(t) & \omega_y(t) \\ \omega_z(t) & 0 & -\omega_x(t) \\ -\omega_y(t) & \omega_x(t) & 0 \\ \end{pmatrix}

As it is a skew symmetric matrix it has a Hodge dual vector which is precisely the previous angular velocity vector $\vec \omega$ :

\boldsymbol\omega=[\omega_x,\omega_y,\omega_z]

Coordinate-free description

At any instant, $t$ , the angular velocity tensor represents a linear map between the position vectors $\mathbf{r}(t)$ and their velocity vectors $\mathbf{v}(t)$ of a rigid body rotating around the origin:

\mathbf{v} = W\mathbf{r}

where we omitted the $t$ parameter, and regard $\mathbf{v}$ and $\mathbf{r}$ as elements of the same 3-dimensional Euclidean vector space $V$ .

The relation between this linear map and the angular velocity pseudovector $\omega$ is the following.

Because of W is the derivative of an orthogonal transformation, the

B(\mathbf{r},\mathbf{s}) = (W\mathbf{r}) \cdot \mathbf{s}

bilinear form is skew-symmetric. (Here $\cdot$ stands for the scalar product). So we can apply the fact of exterior algebra that there is a unique linear form $L$ on $\Lambda^2 V$ that

L(\mathbf{r}\wedge \mathbf{s}) = B(\mathbf{r},\mathbf{s})

where $\mathbf{r}\wedge \mathbf{s} \in \Lambda^2 V$ is the wedge product of $\mathbf{r}$ and $\mathbf{s}$ .

Taking the dual vector L* of L we get

(W\mathbf{r})\cdot \mathbf{s} = L^* \cdot (\mathbf{r}\wedge \mathbf{s})

Introducing $\omega := *L^*$ , as the Hodge dual of L*, and apply further Hodge dual identities we arrive at

(W\mathbf{r}) \cdot \mathbf{s} = * ( *L^* \wedge \mathbf{r} \wedge \mathbf{s}) = * (\omega \wedge \mathbf{r} \wedge \mathbf{s}) = *(\omega \wedge \mathbf{r}) \cdot \mathbf{s} = (\omega \times \mathbf{r}) \cdot \mathbf{s}

where

\omega \times \mathbf{r} := *(\omega \wedge \mathbf{r})

by definition.

Because $\mathbf{s}$ is an arbitrary vector, from nondegeneracy of scalar product follows

W\mathbf{r} = \omega \times \mathbf{r}

Angular velocity as a vector field

For angular velocity tensor maps velocities to positions, it is a vector field. In particular, this vector field is a Killing vector field belonging to an element of the Lie algebra so(3) of the 3-dimensional rotation group SO(3). This element of so(3) can also be regarded as the angular velocity vector.

Rigid body considerations

Consistency

We have supposed that the rigid body rotates around an arbitrary point. We should prove that the angular velocity previously defined is independent from the choice of origin, which means that the angular velocity is an intrinsic property of the spinning rigid body.

Proving the independence of angular velocity from choice of origin

See the graph to the right: The origin of lab frame is O, while O₁ and O₂ are two fixed points on the rigid body, whose velocity is $\mathbf{v}_1$ and $\mathbf{v}_2$ respectively. Suppose the angular velocity with respect to O₁ and O₂ is $\boldsymbol{\omega}_1$ and $\boldsymbol{\omega}_2$ respectively. Since point P and O₂ have only one velocity,

\mathbf{v}_1 + \boldsymbol{\omega}_1\times\mathbf{r}_1 = \mathbf{v}_2 + \boldsymbol{\omega}_2\times\mathbf{r}_2

\mathbf{v}_2 = \mathbf{v}_1 + \boldsymbol{\omega}_1\times\mathbf{r} = \mathbf{v}_1 + \boldsymbol{\omega}_1\times (\mathbf{r}_1 - \mathbf{r}_2)

The above two yields that

(\boldsymbol{\omega}_1-\boldsymbol{\omega}_2) \times \mathbf{r}_2=0

Since the point P (and thus $\mathbf{r}_2$ ) is arbitrary, it follows that

\boldsymbol{\omega}_1 = \boldsymbol{\omega}_2

If the reference point is the instantaneous axis of rotation the expression of velocity of a point in the rigid body will have just the angular velocity term. This is because the velocity of instantaneous axis of rotation is zero. An example of instantaneous axis of rotation is the hinge of a door. Another example is the point of contact of a purely rolling spherical (or, more generally, convex) rigid body.

Angular velocity symbol

When preparing electronic documents, some document editing software will attempt to use the Symbol typeface to render the ω character. Where the font is not supported, a w is displayed instead ("v=rw" instead of "v=rω", for instance). As w represents weight, not angular velocity, this can lead to confusion.

References

↑ 1.0 1.1 Hibbeler, Russell C. (2009). Engineering Mechanics. Upper Saddle River, New Jersey: Pearson Prentice Hall. pp. 314, 153. ISBN 978-0-13-607791-6. (EM1)
↑ K.S.HEDRIH: Leonhard Euler (1707–1783) and rigid body dynamics
↑ "online tool to calculate angular speed vectors". Aeroengineering.info. Retrieved 2013-01-23.
↑ Rotations and Angular Momentum on the Classical Mechanics page of the website of John Baez, especially Questions 1 and 2.

Symon, Keith (1971). Mechanics. Addison-Wesley, Reading, MA. ISBN 0-201-07392-7.
Landau, L.D.; Lifshitz, E.M. (1997). Mechanics. Butterworth-Heinemann. ISBN 0-7506-2896-0.

External links

Look up angular velocity in Wiktionary, the free dictionary.

Wikimedia Commons has media related to Angular velocity.

A college text-book of physics By Arthur Lalanne Kimball (Angular Velocity of a particle)
Pickering, Steve (2009). "ω Speed of Rotation [Angular Velocity]". Sixty Symbols. Brady Haran for the University of Nottingham.

Classical mechanics derived SI units

Linear/translational quantities			Angular/rotational quantities
time: t s			time: t s
	displacement, position: x m			angular displacement, angle: θ rad
frequency: f s⁻¹, Hz	speed: v, velocity: v m s⁻¹		frequency: f s⁻¹, Hz	angular velocity: ω rads⁻¹
	acceleration: a m s⁻²			angular acceleration: α rad s⁻²
	jerk: j m s⁻³			angular jerk: ζ rad s⁻³

mass: m kg			moment of inertia: I kg m² rad⁻²
	momentum: p, impulse: J kg m s⁻¹, N s			angular momentum: L, angular impulse: ΔL kg m² s⁻¹ rad⁻¹
	force: F, weight: F_g kg m s⁻², N	energy: E, work: W kg m² s⁻², J		torque: τ, moment: M kg m² s⁻² rad⁻¹, N m	energy: E, work: W kg m² s⁻², J
	yank: Y kg m s⁻³, N s⁻¹	power: P kg m² s⁻³, W		rotatum: P kg m² s⁻³ rad⁻¹	power: P kg m²s⁻³, W