Questions tagged [dissipation]

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Yet another perpetual motion machine: does this imply large selective membranes are not possible? [closed]

So I was just thinking about the buoyant force and came up with what seemed like a simple perpetual motion based on it. Obviously such things are not physically possible so I'm trying to figure out ...
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1 answer
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What does it mean for $\omega$ to not be "real" in practice?

I am reading about landau damping and the author states that $\omega$ is never real (due to collisions). https://cds.cern.ch/record/1982428/files/377-404%20Herr.pdf \begin{equation} 1 + \frac{\...
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0 votes
2 answers
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Piano on the moon

Imagine a piano on the moon. You strike a chord. Since there is no atmosphere, there is no medium for the sound to travel. so where does the sound energy go? Does it just dissipate as another form of ...
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4 votes
3 answers
1k views

Is David Tong incorrect in this remark about classical mechanics in his QM lectures?

In page 11 of his Quantum Mechanics lectures, we have the following quote: It turns out that not all classical theories can be written using a Hamiltonian. Roughly speaking, only those theories that ...
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Do we have to take the dissipation function for the whole system?

In lagrangian formalism, The L also known as the lagrangian is taken account of the whole system, for each of its components and parts! Is this the same for the Rayleigh Dissipation Function? Like If ...
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What would be the dissipative function for a nonlinear nonconservative force in lagrangian? [closed]

For a nonconservative force, What would be the dissipative function for a force $f=-bvⁿ$ in Lagrangian (Where $v$ is the velocity). [For a non-conservative force $f=-bv$, The dissipative function is $...
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2 votes
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Viscous damping in general relativity

In Minkowski spacetime, waves are modeled via the wave equation $$(\partial_t^2-\Delta)u=0.$$ When viscous damping is present, one studies the damped wave equation $$(\partial_t^2-\Delta+W(x)\...
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Lagrangian and Action for a plane pendulum under the dampling force exerted by air resistance [duplicate]

Consider a plane pendulum which is composed of an ideal string and a sphere of mass m and length $l$. As a consequence of the presence of air, it exerts a force proportional to the speed characterized ...
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0 votes
1 answer
30 views

Will the inner gimbal continue to rotate if no friction?

The picture shows a gyroscope with a rotating disk. Adding a force to the outer gimbal from a certain direction causes the inner gimbal (plus the disk with bearings) to rotate as well. When the force ...
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0 votes
3 answers
136 views

Energy dissipated by friction and entropy

Can we compute the entropy increase in some simple dissipative systems? Imagine a block sliding on a frictional floor and that its initial kinetic energy is $K$, let's imagine that the ambient ...
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3 answers
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Modelling friction as a conservative force

Friction is usually considered as a non-conservative force, but by considering the microscopic movement of particles which produce the friction, it seems we can model friction as conservative force ...
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3 votes
1 answer
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Playing cymbals in vacuum [duplicate]

When two cymbals collide each other in our atmosphere (so with the presence of air), the kinetic energy make their atoms vibrate, then this energy is dissipated as sound waves and you here the loud ...
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2 votes
4 answers
267 views

Does classical simple harmonic motion violate thermodynamics?

If SHM allows for motion to occur forever, we can consider it perpetual motion, does this imply that the second law of thermodynamics is violated? Or does the presence of an external force act on the ...
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1 answer
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What does the position $x(t)$ looks like in an overdamped system?

I know that for the position $x$ as a function of time in an underdamped system (such as a mass on a spring) you can use the function: $$x(t)=Ae^{\gamma t}cos(\omega t-\phi),$$ where $$ \begin{split} ...
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-1 votes
1 answer
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Can someone explain why this would not create an infinitely rotating disk (perpetual motion) [closed]

This may be a dumb question and the solution may be super obvious but I can't figure it out as hard as I try. I have drawn a sketch of a device I thought of, now I know that we cant create "Free&...
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1 answer
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Is the Lindblad equation invariant under the unitary transformation?

Let us say we have a Lindblad equation for the density matrix $$ \dot{\rho}=-\frac{i}{\hbar}[H, \rho]+\sum_{i=1}^{N^{2}-1} \gamma_{i}\left(L_{i} \rho L_{i}^{\dagger}-\frac{1}{2}\left\{L_{i}^{\dagger} ...
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0 votes
2 answers
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Why don't perpetual motion machines with superconducting magnets work?

A superconducting magnet works by energizing a superconducting magnetic coil, then short circuiting it to make a closed loop. Would it be possible to transfer the current back and forth between two ...
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0 votes
1 answer
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On the behavior of critically damped oscillators [duplicate]

Is a critically damped oscillator always going to approach the equilibrium position faster that the same system with underdamping or overdamping for a given set of initial conditions, no matter what ...
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0 answers
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Memory effects in clopen quantum systems

A clopen quantum system may be defined as one where there is no global dissipation (e.g. a finite system of interacting spins without photonic coupling.) In the open quantum physics literature, it is ...
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2 votes
1 answer
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What is the real cause of friction? [duplicate]

sir i have studied through various websites. there i found two theories- friction is due to inter molecular bonds which forms when two surfaces kept together. now if it true then why do we not feel ...
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4 answers
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Do you "lose" electricity when you course it through subpar conductors?

Imagine I had a basic circuit - say the classic 9V battery on one end, a couple of wires, and a little light bulb on the other. Of course, in a real world example those wires would probably be ...
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-1 votes
1 answer
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What wavelength should a laser beam have to not interact with the dust in the air for the laser emitter location not to be possibly detected?

What wavelength should a laser beam have to not interact with the dust in the air for the laser emitter location not to be possibly detected by any kind of wave detectors? What is the quantitative ...
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1 vote
1 answer
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General clarifications about the Lindblad equation

I would like to ask some clarifications about the Lindblad equation: Is the system guaranteed to reach a steady state after starting from a generic initial state under both unitary evolution and ...
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0 answers
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Energy dissipated during cycle of damping force

Considering a drive damped oscillator after transients have died out and is being drive close to resonance such that $\omega=\omega_0$, I want to find the energy dissipated during one cycle, $\Delta ...
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2 votes
0 answers
24 views

Systems that Satisfy the Euler–Lagrange equations, and Velocity dependent vs Dissipative forces [duplicate]

"if the force is not derived from a potential, then the system is said to be polygenic and the principle of least action, that the action integral $S$ is stationary at the actual path followed is ...
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1 vote
3 answers
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Does a low resistance make it difficult to run a generator?

Imagine I have a simple AC generator. I am providing energy to rotate the coil, which converts this to electrical energy. If I connect the ends to an electrical component such as motor, bulb, the ...
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4 votes
1 answer
118 views

Considering General Relativity, how long would it take for a ball to stop oscillating dropped through the center of the Earth?

In Newtonian view, if a relatively small mass was dropped through a frictionless tunnel without drag intersecting the center of a non-rotating, spherically symmetric,uniform density Earth, then the ...
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1 vote
1 answer
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When is an impulsive force dissipative?

Let's say you have a ball attached to a string and you let it fall vertically from the point of suspension of the string. There's an impulse when the string becomes taut and the ball comes to rest. ...
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0 votes
2 answers
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Where do BH dissipate heat that aquire from infalling stars or matter from their accretion disks?

Where do BH dissipate heat that aquire from infalling stars or matter from their accretion disks? Is BH temperature increasing due to that events?
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-1 votes
1 answer
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Relationship between voltage amplitude and dissipated heat for current running across a resistor

I'm trying to figure out how voltage amplitude and dissipated heat relate for current running across a resistor at some arbitrarily very high frequency. The numbers used are less important that the ...
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1 vote
0 answers
43 views

Preserving normalisation for non-unitary dynamics in the Heisenberg picture

As the title says. Say I'm trying to calculate some 2 point correlator of some operator $\hat{\sigma}$: $$ \langle \hat{\sigma}(t)\hat{\sigma}(t+\tau) \rangle = \text{tr}[e^{L^{\dagger}\tau}(\hat{\...
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14 votes
7 answers
2k views

Why does critical damping return to equilibrium faster than overdamping?

How can one prove that the critical damping regime returns to equilibrium fastest? Is it true that there are cases where the heavy damped solution will return faster? Consider a free harmonic ...
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0 answers
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Landau Damping explained in Landau's book

In the book Physical Kinetics: Volume 10 from Landau it is explained the Landau damping. The author explain the phenomena considering an electron subjected to a small amplitude longitudinal wave and ...
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2 answers
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Can natural frequency be produced by damped vibrations?

In my book it's written that when the tuning fork A is is struck on a rubber pad then it starts vibrating with natural frequency (though not written but if it's not vibrating with natural frequency ...
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4 votes
5 answers
464 views

Why are the resonant frequencies for displacement, velocity and acceleration different in a damped oscillator?

Consider a driven harmonic oscillator under a sinusoidal force $x''(t) + \gamma x'(t)+ \omega_0^2 x(t) = F(t)$. In the regime of light damping ($\omega_0/\gamma > 0.5$), we find resonance (maximum ...
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2 votes
0 answers
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Lindblad form for a Damped harmonic Oscillator

I'm considering a Lindblad-like master equation for a damped harmonic oscillator $$ \frac{d\rho(t)}{dt} = -i[H, \rho(t)]+\sum_{n,m=1}^2 h_{nm}[A_n \rho(t) A_m^\dagger - \frac12 \lbrace A_m^\...
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3 votes
2 answers
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Finding the resonance frequency for forced damped oscillations

I have a problem regarding a forced, damped harmonic oscillator, where I'm trying to find the resonance frequency. I have calculated the frequency for free oscillations as $$\omega_{free}=\sqrt{\frac{\...
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2 votes
2 answers
149 views

What does $ \mu \nabla^{2} \vec V$ mean in the Navier-Stokes equations?

$$\rho\frac{D \vec V}{Dt}=-\nabla p+ \mu \nabla^{2} \vec V+\rho g$$ In the Navier-Stokes equations there's this term $ \mu \nabla^{2} \vec V $. I don't really understand what this means. What is the ...
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1 vote
1 answer
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Dissipation caused by Gravitational Wave Emission

Two massive bodies orbiting each other can lose energy through gravitational wave emission until colliding. Can a single massive body, moving with constant velocity with respect to an observer, lose ...
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1 answer
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Does damping cause a gradual change in frequency? [duplicate]

Consider a vibrating cantilever in a system where air resistance is taken into account. The air causes the damping of the cantilever's oscillation. As a result, does the frequency of the vibration ...
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0 answers
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How can retardation in flows be determined?

Say that the effect of retardation of flow is to be calculated. How can it be determined? Retardation is understood to occur as a consequence of flow changes to be propagated with the speed of sound. ...
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0 answers
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Provided the heat of vaporization of a liquid, how would one determine the surface area of a plate needed to dissipate a given amount watts? [duplicate]

I am starting on the basic engineering of a DIY two-phase liquid immersion cooling for a personal computer. I have located a few potential liquids for use, notably 3M's Novec 7000 series of products. ...
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-3 votes
2 answers
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Is there a way to put a perpetual motion machine to work in space! [duplicate]

So I had this thought of a spinning disk that would spin forever in space then I imagined that on this disk there were magnets of alternating poles on the disk and then I imagined a bunch of copper ...
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4 votes
1 answer
213 views

Riemannian Manifolds and "Natural" Lagrangians

In V.I. Arnold's Mathematical methods of classical and celestial mechanics, chapter IV, a definition of natural is provided. It is stated that on a Riemannian manifold (i.e. a differentiable manifold ...
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1 vote
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Work done on a harmonically vibrating deformable body, with damping

Explanation The following equation may be obtained by applying Finite Element Analysis to a deformable body considering linear elasticity and a harmonic input force f(t) = Fejωt. We assume that the ...
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1 vote
2 answers
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In an ideal only capacitive or inductive AC circuit where all the wires are ideal and have zero resistance, no energy loss in any form as power = 0?

Is the statement in question true? Work done by the source in one time period ($T$) is =$V_{\text{RMS}} × I_{\text{RMS}} ×\cos(x)×\frac{T}2$, where $x$ = angle between I phasor and V phasor. In case ...
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0 votes
1 answer
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Dissipation function is homogeneous in $\dot{q}$

We have Rayleigh's dissipation function, defined as $$\mathcal{F}=\frac{1}{2} \sum_{i}\left(k_{x} v_{i x}^{2}+k_{y} v_{i j}^{2}+k_{z} v_{i z}^{2}\right)$$ Also we have transformation equations to ...
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4 votes
1 answer
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Inverse Square vs Exponential

I feel a little foolish asking this, but I keep reading sources which say that for an inverse square law relationship, e.g. light intensity vs distance from source, the intensity decays exponentially. ...
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1 vote
1 answer
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Why this buoyancy machine won’t work? [duplicate]

(Check the video for a better understanding of the machine mechanism: https://www.youtube.com/watch?v=zIdn5zQTJAM&t=109s&ab_channel=RenewjouleLLC ) This machine has a different approach for ...
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-1 votes
1 answer
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Why generating energy from the gravity side (air column) in a buoyancy chain machine isn’t possible? [duplicate]

(For simplicity let’s say that just 1 ball will be rotating into the machine.) It's proven that in a buoyancy chain machine the needed energy to submerge the ball into the water column will be (...
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