Formula for group velocity

Solved Example on Velocity Formula Now let’s test the equation with the help of examples. Example 1-Suppose there is an object traveled a distance of 10 meters in the left direction and the time taken by the object is 2 minutes. Find the velocity of the object. Solution. Now recall the formula which is velocity = displacement ÷ time. v = a / t

In analogy with the refractive index, the group index (or group refractive index) n g of a material can be defined as the ratio of the vacuum velocity of light to the group velocity in the medium: n g = c υ g = c ∂ k ∂ ω = ∂ ∂ ω …The group velocity is always less than the speed of light, we like to think of that this is because the EM wave is ping-ponging back and forth as it travels down the guide. Note that group velocity x phase velocity = c 2. Group velocity in a waveguide is speed at which EM energy travels in the guide. Plotted below as a percentage of the speed ...The expression for Phase Velocity is presented below -. Vp = λ T V p = λ T. Here, Vp V p. is the Phase Velocity, λ λ. (read lambda) is the Wavelength, and T is the time period. The expression for Group Velocity is -. Vg = δw δk V g = δ w δ k.The minimum fluidization velocity that was calculated by Leva formula was only suitable for particles with diameter smaller than 0.75 mm (d p < 0.75 mm) and value of Re mf number smaller than 10 (Re mf < 10). The regime of air flow through the particle layer is laminar flow.This velocity is called the group velocity, since it’s the velocity of the envelope of a group (in this case, 2) of waves traveling together. The velocity of the envelope function given by equation 14 is v g=!"!k, [15] which, using equation 11 yields: v g=v o This agrees with our starting assumption the particle has a mean velocity of v o.The Doppler effect or the Doppler shift describes the changes in the frequency of any sound or light wave produced by a moving source with respect to an observer. Doppler effect in physics is defined as the increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other.Imagine you want to determine the speed of oxygen molecules at 27 °C. To do this, proceed as follows: Input the temperature of 27 °C into the root mean square velocity calculator.It will convert to kelvin, or you can specify the temperature in kelvin straight away: K = °C + 273.15 = 27 + 273.15 = 300.15 K. Select the name "oxygen" from the list.With given elastic moduli aijkl and a known wave direction ni, one can apply these equations to calculate the phase velocities for three wave modes in any class ...

According to the formula for wave speed, wave speed = wavelength/time period. Since the S.I. unit of wavelength is meter (m) and that of time period is second (s). The S.I. unit of wave speed is m/s. ... or by using the mass and velocity of the object along with the radius of the curved path. Angular Momentum Calculator. Capacitor charge time.The time dependence of (10.21) is animated in program 10-2. Note the way that the carrier waves move through the signal. In this animation, the group velocity is smaller than the phase velocity, so the carrier waves appear at the back of each pulse of the signal and move through to the front.The acceleration calculator is based on three various acceleration equations, where the third is derived from Newton's work: a = (vf − vi) / Δt; a = 2 × (Δd − vi × Δt) / Δt²; and. a = F / m. where: a — Acceleration; vi and vf are, respectively, the initial and final velocities; Δt — Acceleration time;This expression for group velocity is the slope of a frequency versus wavenumber graph. In the case of adding many waves, each with its own angular velocity and wave vector, ω ω and k k …Asset velocity tells how efficiently a company is using its assets to generate revenue. For example, a company with an asset velocity of 2.1 produced $2.10 for every $1 worth of assets on its books. Capital asset velocity, meanwhile, slices...A dispersion relation relates the wavelength or wavenumber of a wave to its frequency. Given the dispersion relation, one can calculate the frequency-dependent phase velocity and group velocity of each sinusoidal component of a wave in the medium, as a function of frequency. In addition to the geometry-dependent and material-dependent ...d t = t f – t i = 30 – 0 = 30sec. The formula of tangential acceleration is. a t = d v / d t. a t = 60 / 30. a t = 2 m/s 2. Stay tuned with BYJU’S to learn more on Physics-related concepts. FORMULAS Related Links. Hooke's Law Equation. Factorization Formula.

Cannot use vdisplace in variable formula between runs. This is a function of elapsed time. Cannot use velocity bias command without temp keyword. Self-explanatory. Cannot use velocity create loop all unless atoms have IDs. Atoms in the simulation to do not have IDs, so this style of velocity creation cannot be performed.escape velocity, in astronomy and space exploration, the velocity needed for a body to escape from a gravitational centre of attraction without undergoing any further acceleration.The escape velocity v esc is expressed as v esc = Square root of √ 2GM / r, where G is the gravitational constant, M is the mass of the attracting mass, and r is the …The formula for finding the (tangential) velocity is v=rω This comes from taking the derivative of both sides of s=rθ (where s is arc length) with respect to time. We also know that the engine has an angular velocity of 300 but then is reduced by a factor of 50.A convenient equation can be derived easily by considering the kinetic energy of individual molecules rather than moles of gas: Knowing that kinetic energy is proportional to temperature, if the two gases are at the same temperature, Dividing, m1 m2 = (urms,2)2 urms,1)2 m 1 m 2 = ( u r m s, 2) 2 u r m s, 1) 2. Example 9.15.1 9.15. 1 : Molar Mass.Displacement Formula. Displacement is calculated as the shortest distance between starting and final point which prefers straight-line path over curved paths. Suppose a body is moving in two different directions x and y then Resultant Displacement will be. It gives the shortcut paths for the given original paths. Here, u = Initial velocity

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Group velocity: is nothing but the phase velocity of an envelope of the above mentioned wave. So the equation of envelope is: Cos⁡ (Δω/2t−Δk/2x) So here we will consider two points that would be the Crest A & B of the wave. The Crests are in phase ( both point phase= Δω/2t−Δk/2x= π/2) so we can conclude the phase is constant in the ...Phase, group, and signal velocities. Chapter \(3\) discussed the wave equation and solutions for linear systems. It was shown that, for linear systems, the wave motion obeys superposition and exhibits dispersion, that is, a frequency-dependent phase velocity, and, in some cases, attenuation.The group velocity is velocity of the envelope. For two waves group velocity is defined by vgroup = Δω/Δk v group = Δ ω / Δ k where Δω = ω1 −ω2 Δ ω = ω 1 − ω 2 and Δk = k1 −k2 Δ k = k 1 − k 2. This expression for group velocity is the slope of a frequency versus wavenumber graph. In the case of adding many waves, each ...Group velocity is the speed at which wave energy travels radially outward from a point source in a homogeneous elastic anisotropic medium [1]. Phase velocity is the velocity in the direction of the phase propagation vector, normal to the surface of constant phase [2]. Field measurements of traveltime and distance often yield group velocity ...Solving this for the drift speed results in. (11.7.2) v d = E B. Figure 11.7. 1: In the Hall effect, a potential difference between the top and bottom edges of the metal strip is produced when moving charge carriers are deflected by the magnetic field. (a) Hall effect for negative charge carriers; (b) Hall effect for positive charge carriers.Group Velocity in a Waveguide For light propagating in a waveguide such as an optical fiber, the group velocity can be calculated by replacing the wavenumber k with β (the imaginary part of the propagation constant) (or replacing the refractive index n with the effective refractive index) in the equation given above.

The concept of group velocity arises when a waveform is Fourier analyzed into a set of harmonic waves, f(x, t) = (k)ei kx−ωt ( ) dk,The group velocity is velocity of the envelope. For two waves group velocity is defined by vgroup = Δω/Δk v group = Δ ω / Δ k where Δω = ω1 −ω2 Δ ω = ω 1 − ω 2 and Δk = k1 −k2 Δ k = k 1 − k 2. This expression for group velocity is the slope of a frequency versus wavenumber graph. In the case of adding many waves, each ... So, at 1 s the velocity is 2 m/s, at 2 s the velocity is 8 m/s, at 3 s the velocity is 18 m/s, etc. Let's say that we want to find the acceleration of the particle at the instant t = 3 s . The first thing we need to do is to choose an interval of time Δ t which includes the instant 3 s .Group Velocity The phase velocity of a wave is and comes from the change in the position of the wavefronts as a function of time The waveform moves at a rate that depends on the relative position of the component wavefronts as a function of time. This is the group velocity and is which can be found if you have" " " " " " " " " giving 7 v =! k v g =This formula, which expresses the equivalence between group velocity and energy velocity in a loss-less medium [Citation 4, Citation 5], tells us that the group velocity for a given mode is proportional to the integral of the Poynting vector of that mode over the unit cell. It can, therefore, be calculated in a straightforward manner from a ...If an object changes direction in its journey, then the average speed will be greater than the magnitude of the average velocity. Speed is a scalar, and average velocity is a vector. Average velocity indicates direction and can be represented as a negative number when the displacement is in the negative direction.Group velocity, effective group index and effective phase index vs normalized frequency at plasma frequency = (5.6 ÿ 10 11) ÿ 1, N = 15 and n = 12.25 (Al 2 O 3). Fig. 11 , Fig. 12 are depicted the optical properties of plasma photonic band gap structure of plasma frequency 5.6 ÿ 10 11 and refractive index 2.35.Group A powders. Geldart Group B. These particles have a particle size range of 150 μm to 1,000 μm. Group B particles tend not to undergo smooth fluidization, and bubbles form at the onset of fluidization. Thus, the minimum fluidization velocity and the minimum bubbling velocity are similar. Group BThe average velocity formula describes the relationship between the length of your route and the time it takes to travel. For example, if you drive a car for a distance of 70 miles in one hour, your average velocity equals 70 mph. In the previous section, we have introduced the basic velocity equation, but as you probably have already realized ...Group velocity and phase velocity Propagation of a wave packet, with the motion of a single peak shaded in purple. The peaks move at the phase velocity while the overall packet moves at the group velocity. ... which agrees with the formula for the classical velocity of the particle. The group velocity is the (approximate) speed at which the ...x = v0t + 12at2. constant α, a. ω2 = ω02 + 2αθ. v2 = v02 + 2ax. constant α, a. Table 6.3 Equations for Rotational Kinematics. In these equations, ω0 and v0 are initial values, t0 is zero, and the average angular velocity ω¯¯¯ and average velocity v¯¯ are. ω¯¯ = ω0 + ω 2 andv¯¯ = v0 + v 2. 6.11.

Phase and group velocity are two important and related concepts in wave mechanics. They arise in quantum mechanics in the time development of the state function for the continuous case, i.e. wave packets. Discussion. Harmonic Waves and Phase Velocity. A one-dimensional harmonic wave (Figure 1) is described by the equation,

the group velocity. Phase and group velocity, example Consider Schrödinger equation iut +uxx = 0: ... Suppose a linear equation has solutions u(x;t) = exp(˙t +ikx) where ˙= ˙(k) is the (real exponential form) dispersion relation. If Re ˙(k) <0 for all k, then equation isstable.FILE - A can of toddler nutritional drink sits on a shelf in a grocery store in Surfside, Fla., on Friday, June 17, 2022. Powdered drink mixes that are widely promoted as …Phase, Group, and Signal Velocity . The velocity of a wave can be defined in many different ways, partly because there are many different kinds of waves, and partly because we can focus on different aspects or components of any given wave. The ambiguity in the definition of "wave velocity" often leads to confusion, and we frequently read stories about experiments purporting to de• The group velocity defined by the equation-. Page 5. • where ω is the wave's angular frequency (usually expressed in radians per second), and k is the.Many people get speed and velocity confused. It's no surprise because the terms are often used interchangeably. But they're not quite the same thing. So how do you find the velocity of an object? Advertisement Far be it for us to criticize ...Phase and group velocity are two important and related concepts in wave mechanics. They arise in quantum mechanics in the time development of the state function for the continuous case, i.e. wave packets. Discussion. Harmonic Waves and Phase Velocity. A one-dimensional harmonic wave (Figure 1) is described by the equation,Nov 6, 2016 · The expression of the group velocity. vg = ∂ω ∂kz v g = ∂ ω ∂ k z. for the waveguide with wave vector kz k z in propagation direction is correct. And the phase velocity is. vph = ω kz v p h = ω k z. According to the dispersion relation of the wave guide, when ω ω approaches the cut-off frequency ωc = cπ a ω c = c π a, the wave ... This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for example, the crest) will appear to travel at the phase velocity. The phase velocity is given in terms of the wavelength λ (lambda) and time period T as. Equivalently, in terms of the wave's angular ...FILE - A can of toddler nutritional drink sits on a shelf in a grocery store in Surfside, Fla., on Friday, June 17, 2022. Powdered drink mixes that are widely promoted as …The formula to calculate displacement is x = ½(v + v0)t. X represents the actual displacement, while V is the velocity. V0 defines the initial velocity, while T represents the time taken.

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Dispersive waves are waves in which the phase speed varies with wavenumber. It is easy to show that dispersive waves have unequal phase and group velocities, while these velocities are equal for non-dispersive waves. Derivation of Group Velocity Formula [edit | edit source] We now derive equation (1.36).Looking at the thrust equation very carefully, we see that there are two possible ways to produce high thrust. One way is to produce high thrust is to make the engine airflow rate as high as possible. As long as the exit velocity is greater than the free stream (entrance) velocity, a high engine airflow will produce high thrust.Phase velocity is the speed at which a point of constant phase travels as the wave propagates. 1 For a sinusoidally-varying wave, this speed is easy to quantify. To see this, consider the wave: A cos(ωt − βz + ψ) (6.1.1) (6.1.1) A cos ( ω t − β z + ψ) where ω = 2πf ω = 2 π f is angular frequency, z z is position, and β β is the ...Phase and group velocity are two important and related concepts in wave mechanics. They arise in quantum mechanics in the time development of the state function for the continuous case, i.e. wave packets. Discussion. Harmonic Waves and Phase Velocity. A one-dimensional harmonic wave (Figure 1) is described by the equation,Solution: The product of the phase and the group velocities is given by the square of the speed of light. Thus Vp x Vg = c2 is the relation. Example 2: Find the group velocity of a wave with a phase velocity of 60 x 109 (in 106 order). Solution: Phase and Group Velocities are given by Vp x Vg = c 2. Thus, upon substituting for Vp = 60 x 10 9 ... Group velocity calculation with the finite difference method is still able to be activated using GV_DELTA_Q tag or --gv_delta_q option. \(\Delta\mathbf{q} = (\Delta q_x, \Delta q_y, \Delta q_z)\) is described in Cartesian coordinated in reciprocal space. In the implementation, central difference is employed, and \(+\Delta q_\alpha\) and \(-\Delta q_\alpha\) are taken …The maximum height is reached when \(\mathrm{v_y=0}\). Using this we can rearrange the velocity equation to find the time it will take for the object to reach maximum height \[\mathrm{t_h=\dfrac{u⋅\sin θ}{g}}\] where \(\mathrm{t_h}\) stands for the time it takes to reach maximum height. From the displacement equation we can find the maximum ...The time dependence of (10.21) is animated in program 10-2. Note the way that the carrier waves move through the signal. In this animation, the group velocity is smaller than the phase velocity, so the carrier waves appear at the back of each pulse of the signal and move through to the front.Wavenumber Definition. In theoretical physics: It is the number of radians present in the unit distance. In chemistry and spectroscopy: It is the number of waves per unit distance. Typically, it is measured using cm -1 and is given by-. In complex form: The complex values wavenumber for a medium can be expressed as –. ….

The group velocity surface, aka energy surface, ray surface, wavecrest surface, is plotted in Fig. 6.33 for the three wave modes, P-wave, SV-wave, SH-wave. The group velocity surface for the quasi-P mode has a very elongated shape as shown in Fig. 6.33A. This elongated shape is due to the fact that the group velocity along the fibers, that is ...The equation for the distance traveled by a projectile being affected by gravity is sin(2θ)v 2 /g, where θ is the angle, v is the initial velocity and g is acceleration due to gravity. Assuming that v 2 /g is constant, the greatest distance will be when sin(2θ) is at its maximum , which is when 2θ = 90 degrees.23 พ.ค. 2566 ... Group Velocity and Phase Velocity, Relationship between phase velocity and group velocity using the de Broglie postulates, Wave packets ...Explore math with our beautiful, free online graphing calculator. Graph functions, plot points, visualize algebraic equations, add sliders, animate graphs, and more.Example 8.6 Drag Forces at High Speeds. An object of mass m at time t = 0 is moving rapidly with velocity V→ 0 V → 0 through a fluid of density ρ . Let A denote the cross-sectional area of the object in a plane perpendicular to the motion. The object experiences a retarding drag force whose magnitude is given by Equation (8.6.1).Equation 11.1. M V = nominal GDP M V = n o m i n a l G D P. The equation of exchange shows that the money supply M times its velocity V equals nominal GDP. Velocity is the number of times the money supply is spent to obtain the goods and services that make up GDP during a particular time period.Apr 13, 2016 · Group velocity, for any kind of wave, is defined as $$\boxed{v_g=\frac{\mathrm dω}{\mathrm dk}}.$$ ... Asymptotic formula for ratio of double factorials The formula to calculate displacement is x = ½(v + v0)t. X represents the actual displacement, while V is the velocity. V0 defines the initial velocity, while T represents the time taken. Formula for group velocity, Pitch Angle calculator uses Pitch Angle = Phase Velocity/[c] to calculate the Pitch Angle, Pitch Angle refers to the angle between the axis of the helix and the direction of propagation of the electron beam. The helix is a key component of the TWT, which consists of a cylindrical metal wire wound in a helical pattern along the length of a ..., Phase and group velocity are two important and related concepts in wave mechanics. They arise in quantum mechanics in the time development of the state function for the continuous case, i.e. wave packets. Discussion. Harmonic Waves and Phase Velocity. A one-dimensional harmonic wave (Figure 1) is described by the equation,, 23 พ.ค. 2566 ... Group Velocity and Phase Velocity, Relationship between phase velocity and group velocity using the de Broglie postulates, Wave packets ..., An open source textbook on applied electromagnetic geophysics. Aimed at providing background and physical understanding for steady state Maxwell equations ..., From this formula, we see that the group velocity is equal to the phase velocity only when the refractive index is a constant dn / dk = 0. When this occurs, the medium is called non-dispersive, as opposed to dispersive , where various properties of the medium depend on the frequency ω ., Velocity defines the direction of the movement of the body or the object. Speed is primarily a scalar quantity. Velocity is essentially a vector quantity. It is the rate of change of distance. It is the rate of change of displacement. Speed of an object moving can never be negative. , Given these assumptions, the following steps are then used to analyze projectile motion: Step 1. Resolve or break the motion into horizontal and vertical components along the x- and y-axes. These axes are perpendicular, so A x = A cos θ and A y = A sin θ are used. The magnitude of the components of displacement s along these axes are x and y. The …, The basic formula for velocity is v = d / t, where v is velocity, d is displacement and t is the change in time. Velocity measures the speed an object is traveling in a given direction., F net = Δ p Δ t. to be. Δ p = F net Δ t . F net Δ t is known as impulse and this equation is known as the impulse-momentum theorem. From the equation, we see that the impulse equals the average net external force multiplied by the …, 1 Answer. I believe there should be a second derivative of refractive index with respect to the wavelength in the GVD definition as otherwise the values would look different. As you have already indicated, inverse of the group velocity is defined 1 vg = dk dω 1 v g = d k d ω and k = ωn(ω) c k = ω n ( ω) c. Therefore, we can find the value ..., Equation 11.1. M V = nominal GDP M V = n o m i n a l G D P. The equation of exchange shows that the money supply M times its velocity V equals nominal GDP. Velocity is the number of times the money supply is spent to obtain the goods and services that make up GDP during a particular time period., The subscripts 2 and 1 indicate the final and initial velocity, respectively. This theorem was proposed and successfully tested by James Joule, shown in Figure 9.2.. Does the name Joule sound familiar? The joule (J) is the metric unit of measurement for both work and energy. The measurement of work and energy with the same unit reinforces the idea that work and energy …, Angular Velocity Formula. There are three formulas that we can use to find the angular velocity of an object. 1st option. This one comes from its definition. It is the rate of change of the position angle of an object with respect to time. So, in this way the formula is. w = \(\frac{\theta} {t}\) Derivation of the formula. w = refers to the ..., The derivation above shows that the group velocity ($\omega_0^\prime$) and phase velocity ($\omega_0$) are decoupled, and both are tied to the enveloppe and the carrier respectively and the latter is monochromatic, therefore, even though we have a 'dispersive' media because 'phase velocity depends on frequency', there's only one frequency that ..., Group velocity synonyms, Group velocity pronunciation, Group velocity translation, English dictionary definition of Group velocity. or n physics the speed at which energy is propagated in a wave. This is the quantity determined when one measures the distance which the radiation travels..., With given elastic moduli aijkl and a known wave direction ni, one can apply these equations to calculate the phase velocities for three wave modes in any class ..., speed. If you think of phase velocity to be like the speed of an individual runner, then the group velocity is the speed of the entire group as a whole. Obviously and most often, individual runners can run faster than the group as a whole. To stretch this analogy, we note that the phase velocity v p of waves are typically larger than the group ... , The linear velocity v v of the point P P is given by. v = s t = rθ t (1.4.10) (1.4.10) v = s t = r θ t. where θ θ, measured in radians, is the central angle subtended by the arc of length s s. Another way to measure how fast an object is moving at a constant speed on a circular path is called angular velocity., You can write the wave speed formula using this value, and doing as physicists usually do, exchanging the period of the wave for its frequency. The formula becomes: c = \frac {\lambda} {T} = f × \lambda c = T λ = f ×λ. Since ‌ c ‌ is a constant, this equation allows you to calculate the wavelength of the light if you know its frequency ..., An open source textbook on applied electromagnetic geophysics. Aimed at providing background and physical understanding for steady state Maxwell equations ..., In terms of source frequency and observed frequency, this equation can be written as. (5.8.1) f o b s = f s ( 1 − v c) ( 1 + v c) Notice that the signs are different from those of the wavelength equation. Example 5.8. 1: Calculating a Doppler Shift. Suppose a galaxy is moving away from Earth at a speed 0.825 c., Relation Between Group Velocity And Phase Velocity. Waves can be in a group and such groups are called wave packets, so the velocity with which a wave packet travels is called group velocity. The velocity with which the phase of a wave travels is called phase velocity. The relation between group velocity and phase velocity is proportionate. , Jun 15, 2021 · 1 Answer. I believe there should be a second derivative of refractive index with respect to the wavelength in the GVD definition as otherwise the values would look different. As you have already indicated, inverse of the group velocity is defined 1 vg = dk dω 1 v g = d k d ω and k = ωn(ω) c k = ω n ( ω) c. Therefore, we can find the value ... , Solution: Phase and Group Velocities are given by Vp x Vg = c2. Thus, upon substituting for Vp = 60 x 109 and the speed of light, we get Vg = 1.5 x 106 ..., Finally we swap sides to get the formula for the group velocity vg = dE dp (1.2-7) Thus we can draw the following conclusion Group Velocity The group velocity of any particle (massive or massless) is equal to the derivative of its total relativistic energy with respect to its relativistic momentum., Group velocity definition, the velocity of finite numbers of waves undergoing simple harmonic motion, equal to the phase velocity when it does not vary with the wavelengths of the waves. …, Looking at the thrust equation very carefully, we see that there are two possible ways to produce high thrust. One way is to produce high thrust is to make the engine airflow rate as high as possible. As long as the exit velocity is greater than the free stream (entrance) velocity, a high engine airflow will produce high thrust., Group velocity dispersion is the phenomenon that the group velocity of light in a transparent medium depends on the optical frequency or wavelength. The term can also be used as a precisely defined quantity, namely the derivative of the inverse group velocity with respect to the angular frequency (or sometimes the wavelength), called β 2: β 2 ..., In analogy with the refractive index, the group index (or group refractive index) n g of a material can be defined as the ratio of the vacuum velocity of light to the group velocity in the medium: n g = c υ g = c ∂ k ∂ ω = ∂ ∂ ω ( ω n ( ω)) = n ( ω) + ω ∂ n ∂ ω. For calculating this, one obviously needs to know not only the ..., The first animation featured a group velocity which was larger than the phase velocity. But there are, mathematically speaking, other possibilities. For example, the phase velocity may be larger than the group velocity. Image taken from the video Group Velocity / Phase Velocity Animation - Case 5: Positive Phase and Negative Group Velocity ... , 1 Answer. I believe there should be a second derivative of refractive index with respect to the wavelength in the GVD definition as otherwise the values would look different. As you have already indicated, inverse of the group velocity is defined 1 vg = dk dω 1 v g = d k d ω and k = ωn(ω) c k = ω n ( ω) c. Therefore, we can find the value ..., p=mv c) The formula of the relativistic mass (1.1-1) (1.1-2) m= 0 2 v (1.1-3) c2 where = total relativistic energy of a particle m= relativistic mass of a particle 0= rest mass of a particle = …, This vector is parallel to r r, so that it points in the wave’s direction of propagation through spacetime, and tells us its group velocity (∂C ∂k)/(∂C ∂ω) ( ∂ C ∂ k) / ( ∂ C ∂ ω). In our example of water waves, a calculation shows that the group …