Questions tagged [energy-conservation]

The law of conservation of energy, which states that the amount of energy in a system is constant. For questions about Earth's environment, see the climate-science tag instead.

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Is this short paper correct?: “Heating the coffee by looking at it. Or why quantum measurements are physical processes”

I've just found the following paper in arXiv written by a Spanish politician who is also a physicist. In it he claims that you can setup a sequential Stern-Gerlach-type experiment in such a way that ...
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Energy conservation in cosmological phase transitions

Let us consider a cosmic phase transition, in which fermions $\psi_f$ condense and the vacuum expectation value $|\langle \bar{\psi}_f \psi_f\rangle |$ of the resulting fermion-bilinear field gives ...
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200 views

Brownian Ratchet Plausibility

Alright I'm going to throw whatever reputation I have on the line here. And yes this is a serious question. Apologies for the shoddy imagery. I had a couple ideas to get the Brownian Ratchet to work. ...
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680 views

Attractive gravity has negative energy, what about repulsive gravity in the inflation phase?

Alan Guth's cosmic inflation theory posits that about 10^-38 second before the Big Bang which led to our particular universe, a tiny patch of space doubled in size more than 100 times, from sub-atomic ...
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Swing: why does the body position modify the amplitude?

When a person swings, why does the amplitude of oscillations increase if the person changes the body position ? That is, when descending and approaching the vertical position, if the person extend his ...
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If negative mass should exist, how would conservation of momentum work?

Imagine that a particle with negative mass has been discovered. It is known to obey both the equivalence principle and Newton's second law. This means that the particle is a subject to repulsive ...
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Inflation and dark energy have constant energy density - energy is created out of nothing without any source or mechanism?

I can understand that a photon in an expanding universe can be redshifted and lose its energy by its wavelength being stretched from space expansion. But in the case of inflation and dark energy, it ...
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77 views

could a force with corolios and centrifugal terms be written as a potential gradient?

I have an exam in classical mechanics next week, so I came across this problem which I did not fully understand nor any of my colleagues (it was a bonus problem in an old exam) I just want some hint ...
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Why does the law of conservation of energy not hold true when the work-function $U$ depends explicitly on $t$?

[...] the infinitesimal work $\overline{\mathrm dw}$ comes out as a linear differential form of the variables $q_i$: $$\overline{\mathrm dw}= F_1~\mathrm dq_1 +F_2~\mathrm dq_2+ \ldots + F_n~\mathrm ...
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1answer
258 views

Fermi's golden rule and Fock states

I am having trouble understanding the derivation of the rate of spontaneous and stimulated emission given in this link. We have a perturbation that takes the form: $$ \hat H=\sum_{\vec k}f(\vec r,\...
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161 views

Non-locality of gravitational energy

Gravitational energy is non-local which is essentially because of the equivalence principle. The equivalence principle says that you can always transform your frame so that you feel like in a ...
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197 views

Non-local gravitational energy tensor

The well-known derivation of the Landau-Lifshitz gravitational energy pseudotensor, relies on several requirements: 1) that it be constructed entirely from the metric tensor 2) that it be index ...
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Avoiding Pseudo-tensors when addressing global conservation of energy in GR

Discussions about global conservation of energy in GR often invoke the use of the stress-energy-momentum pseudo-tensor to offer up a sort of generalization of the concept of energy defined in a way ...
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What quantum states can be measured using entanglement?

I have been reading about quantum entanglement, and I was wondering what quantum states can be "sent" using entanglement. I know you can measure the spin of one of the particles, and know the spin of ...
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1answer
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Do emitted photons have a well defined frequency or just a spread as per HUP?

I have read this question: Conservation of Energy in photon exchange between two atoms where Kurshal Shah in a comment says: As per the energy-time uncertainty relation, the emitted photon ...
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Max velocity before toppling risk

I want to find out the maximum velocity a machine can travel before there is a risk of toppling. Say, worst case, is that the castors hit a dead stop and the machine is brought to rest '...
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Can an Opposing Current Create another Opposing Current?

In Inductors when current increases it's magnetic field induces a voltage which causes an opposing current that slows down the rise of the current that initially creates it, but can this opposing ...
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35 views

Why energy plays such an important role among other conservative quantities

I was reading books where I realized an interesting thing: In quantum, the evoluation of states/wave funciton is governed by hamiltonina operator $H$, which is bascially the energy operator. In ...
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Internal energy of a system and its conservation

In textbooks of thermal physics, we usually find internal energy of a system defined as the energy measured in the center of mass frame. Now, in mechanics, we study the conservation of mechanical ...
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Does Friedmann fluid equation prove that energy of the fluid is not conserved?

The Friedmann equation in question is:$$ \dot\rho + 3H \left(ρ + P\right) = 0 \,.$$ I’m a layperson so I don’t know why this equation is sometimes called fluid equation, energy conservation equation, ...
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2answers
200 views

Confusion about Noether's theorem on conservation of energy

Assume a gravitational field, with area A having some gravitational forces while area B having no gravitational force. The Lagrangian of particle moving along this field is obviously time invariant, ...
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143 views

How to estimate the kinetic energy loss of a rigid cube after it colliding with a heavy spring at rest?

In an environment without gravity, there is an unfixed, motionless, uniform spring with non-negligible mass m, length L, and stiffness k suspending and a rigid cube with mass M moving with velocity v ...
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Power in a wind turbine

An wind turbine starts turning by a wind speed of $v_b$ and it stops turning (rated wind speed, to prevent overloading) by a wind speed of $v_g$. By wind speeds between $v_b<v<v_g$, the power ...
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Violation of energy conservation by counter-rotating terms in Rabi model for atom-field interaction

When we describe the interaction between two-level atom and single-mode electromagnetic field in quantum optics, we have the so-called Rabi-model before the rotating-wave approximation, by which the ...
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399 views

Conservation of Energy in Capillary Tube

The capillary action lets a liquid rise in a narrow tube to a certain height. In this, the liquid gains some potential energy. According to the Conservation of Energy, the energy must come from ...
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249 views

What would happen if I took empty space and stretched it?

There was a talk at my school by Rocky Kolb and he claimed that they derived (or it might have been ''experimentally found'', I don't remember) that the mass of empty space must be/is on the order of $...
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2answers
186 views

How is potential energy lost when a water droplet is dropping down slowly on a wall?

When a water droplet is on a vertical wall, it usually drops slowly, which is different from free falling. As the dropping speed is slower than free drawing, so I guess some energy must be lost. I ...
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1answer
74 views

Potential Energy in a System and its Relation to the Total Energy

In this answer it was stated that potential energy is a property of a system and not an individual particle. If we have two particles (1 and 2) interacting via a conservative force, we can write an ...
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Fluid approximation for inflaton?

In cosmology there is a widely used fluid approximation which applies when the mean free path is very small compared to the scale of observation and then all the properties of stress energy tensor can ...
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365 views

Friction and work from torque

I would like to understand where is the error in the third case, for that I gave 2 easier cases where I'm able to find the energy from heating is equal to the energy lost by torque. Case 1/ Purple ...
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1answer
86 views

How many properties like charge, mass etc any quanta or particle must have?

How many properties are required to measure full energy of a fundamental particle? I know $E=mc^2$, but what about charge, spin, etc? Which full equation would give me all parameters of any particle?
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Momentum of capacitor in a uniform magnetic field

We are observing ideal, charged, parallel plate capacitor placed in uniform magnetic field parallel to plates. Whole system is at rest and isolated (we have forces that hold plates separated, but net ...
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Would a large, small mass object in orbit experience induced rotation

Imagine a large (multiple earth radii), very small-mass ring orbiting The Sun. Half of the ring would be closer to The Sun than the outer half. Since orbital velocity decreases with distance, two free ...
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299 views

Under what conditions can we consider the coefficient of restitution to be independent of velocity?

In high-school mathematics textbooks a bouncing ball is often considered as an example of an exponential decay. One can easily derive this if one assumes that the coefficient of restitution is ...
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James Joule's Conservation of Energy

Is there a manifest or documentation of the original Conservation of Energy as stated by James Joule word for word? Edit 1.1: I found Joule's Memoir on google, skimming through to find any conclusive ...
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How do I calculate the energy balance of a trompe?

I was quite fascinated by the concept of an ancient type of air compressor, called a trompe. It entrains air bubbles into a falling stream of water via the Venturi effect, and extracts the air at a ...
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1answer
321 views

Ball, Fixed Curved Track, and Non-Fixed Curved Track

I wonder whether the way I approach and solve this question is correct or not. Question: Case 1 As illustrated, a track of mass $M$ is fixed on a horizontal table. $AB$ is horizontal while $CD$ is ...
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2answers
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Escaping the Gravity well of the Moon

How much energy would it take to get 1 kg from the Moon's surface to Earth? Would You aim for Lagrange Point 1 or would you launch in the opposite direction that the moon is orbiting?
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Mechanics of a particle problem (potential energy vs. work approach)

I'm trying to reconcile two methods of approaching a problem, see picture below: Disregarding the angle of the surface and any friction, calculate the velocity $v$ of a particle pulled back a ...
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How to evaluate the period of a particle in a system with potential energy $U=-U_0/\cosh^2(\alpha x)$?

I am working through the textbook "Mechanics", from the series "Course of Theoretical Physics " by Landau and Lifshitz. In Chapter 3, where the authors talk about integrating the equation of motion $E=...
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3answers
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$\Delta K=\Delta U$ vs $\Delta K = -\Delta U$

While doing physics homework, I noticed that in some problems, the change in kinetic energy(KE) is equal to the change in potential energy(PE), even though I learned that conservation of energy shows ...
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1answer
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Energy conservation in induced emf in open loop

We have a closed loop in the influence of magnetic fields of electromagnets somewhat like this. (Kindly ignore the currents and forces shown - the coil is stationary and initially no current flows) ...
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1answer
49 views

Perpetual magnetic attraction

There are two magnets, M1 and M2. Let's say that there is an attractive force existing between M1 and M2. M1 is attached to a surface, while M2 is free to move. Now slow this motion down, and M1 is in ...
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1answer
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Kinetic Energy change in particle to wall collision

The Question A particle of mass m is moving in the +x direction with a speed v. It is approaching a massive wall which is also moving in the +x direction, but with a speed u < v (see figure) The ...
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Question about Helmholtz's paper “ON THE CONSERVATION OF FORCE”

Below follows the exact extract from Helmholtz's paper "On the conservation of force". Let us now imagine, instead of the system $A$, a single material point $a$, it follows from what has been just ...
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Do conservative forces obey Newton's laws of motion?

Do conservative forces obey Newton's laws of motion? Is the force between two charge particles a counterexample?
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Help understanding a spring-mass system where mass is not attached

This is the scenario: We have a spring resting on a surface with no mass on it. Let the $y$ position of the top of the spring be $l_0$. When we put an object with some mass on top of the spring, the ...
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How to calculate energy (KWh) from Temperature (T),Air quality (Co2), and illumination (lux)?

I have data of temperature, air quality, and illumination.I want to calculate energy consumption (KWh) from these parameters.My data is shown in the image . Also, examples are also shown in image 2 .
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5answers
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Derivation of the work-energy theorem

We state the following version of work-energy theorem : $$ K_2-K_1=Fd=W $$ Where acceleration is assumed to be constant, so is the force $F$. Then the physicists proceed by writing $K_2-K_1=F[x(t_2)...
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When can I go in the reference frame of a moving object?

Two particles separated a distance r, each of mass m, are being launched at opposite directions with the same speed v. If we’re in the reference frame of the center of mass of the particles, what ...