# 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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### Cosmological energy conservation

The expanding universe is gaining energy through increasing dark energy, and losing energy from red shift. Are these two effects comparable in magnitude?
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### Getting different velocity from momentum conservation and work energy theorem

Consider a system of bullet (of mass 5 kg) and block (of mass 20 kg). Initially the block is at rest and the bullet moves towards it at 30 m/s. After penetrating the block, the bullet comes out with ...
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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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### Difficulty in Work-Energy theorem and basics of work done by a force [closed]

A time dependent force $F=6t$ acts on a particle of mass 1kg. If the particle starts from rest then what is the work done during the first second? In my book first they found final velocity and used ...
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### Velocity of ball sliding on bowl. Need to understand why vertical velocity of bowl is 0 and ball is not

I'm trying to understand something theoretical about this question... There's this ball which falls from height $h$ into a bowl, there are $3$ named points on the bowl, $A$ the first upper point the ...
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1 vote
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### How is energy conserved in interference of light waves? [duplicate]

How is energy conserved during interference of light waves? I already know a textbook explanation that in constructive interference the resulting intensity is four times the initial intensity and for ...
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### Gravitational Aharonov-Bohm Effect and Conservation of Energy

Like for the electromagnetic Aharonov-Bohm effect of QED, the gravitational Aharonov-Bohm effect has recently been proven in an experiment done by Ch. Overstreet (2022), showing that the gravitational ...
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### Is it correct to say that it is theoretically impossible for perfect rigid bodies to exist?

If perfect rigid bodies were to exist, then consider a scenario in which two rigid bodies of equal masses moving with velocities of equal magnitude but opposite in direction colliding against one ...
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### Benefit of Hydrogen Electrolysis

I was considering building a simple solar hydrogen generator for fun and it led me to the question Does the hydrogen produced have more potential energy than the electricity used to get it? Obvious ...
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### Non-Work energy conversion

Are there any conversions of energy that are not considered work, (energy change with a force and displacement)? For example heat transfer does not convert the form of energy, can something such as ...
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### What happens to the potential energy when we connect 2 water tanks with different water levels?

Imagine combining 2 water tanks (with equal cross section areas) with different water levels. I'll call them A (tank with the higher water level) & B. When water is flowing from A to B, what ...
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### To clarify the entropy/ energy/ Gibbs Eqs

Both case are isothermal and internally reversible processes. Why is it wrong to use energy conservation in case7-24 to solve the problem? I referred to the textbook (Aqueous tea An Engineering ...
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### Energy and Momentum conservation in Bhabha scattering

I came across this question here which was asking my exact initial question (I even came from Griffiths problem 2.4 too!). But the given answer gave me a lot more questions which I think constitute ...
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### Why is some mass converted into energy when neutrons and protons combine to form nucleus?

So some of the mass is converted into energy when neutrons and protons combine to form nucleus. Why? And how? And then why do we need to supply the same amount of energy to separate them?
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### Which law of thermodynamics is broken here? [closed]

This question came in the Dhaka university admission exam 18-19. A heat engine in each cycle does positive work and loses energy as heat with no heat energy input. Which law of thermodynamics does the ...
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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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### Where does all the energy in black holes go?

The temperature inside of a black hole is almost absolute zero, so particles inside a black hole have almost zero motion. So if they don't give out any heat or light, where does all the energy it ...
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### Why is the friction in the motor wrong as an answer to wasted energy in this question?

I do not understand why the answer to part b)ii) cannot be energy lost to friction within the motor as the work done by the motor should be the total work done to move the cyclist and should include a ...
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### Pressure wave speed $v=\sqrt{\frac{B}{\rho}}$ rearranged $\rho v^2=B$ is similar to kinetic energy: $E_k=\frac{1}{2}mv^2$. Is this a coincidence?

The speed of a Pressure wave speed is given by: $v = \sqrt{ \frac{B}{\rho}}$, which seems rearranged very similar to kinetic energy: $\rho v^2 = B$. How can this be understood, is this a coincidence? ...
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### Free energy from the vacuum?

An electric field with an intensity of the Schwinger limit $$E=\frac{m_e^2c^3}{e\hbar} \approx 10^{18} \hbox{ V/m}$$ is strong enough to create electron-positron pairs out of the vacuum by separating ...
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### Where does an electromagnetic wave's energy come from?

tldr; doing work on a neutron would only increase kinetic energy, but doing work on a proton would increase kinetic energy as well as emit energy in the form of EM waves. How does that work? I know ...
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### Conservation of energy and work done by a torque [duplicate]

Suppose you let a solid roll down an incline without slipping, from height $h$. My textbook gives the following conservation of energy relation $$mgh = \frac{1}{2}mv_{cm}^2 + \frac{1}{2}I\omega^2.$$ ... 69 views

### Angular Momentum Conservation and Conservation of Energy

A heavy solid sphere is thrown on a horizontal rough surface with initial velocity $u$ without rolling. What will be its speed, when it starts pure rolling motion? So using conservation of energy, I ...
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### What exactly is the limiting factor of the efficiency of a heat engine?

In an imagined scenario, $n$ moles of steam at a temperature $T_h$ and pressure $p$ enter an engine. The steam leaving the engine is at a temperature $T_c$ and pressure $p$. In this scenario, the heat ...
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### Zel'dovich, Raizer - derive energy equation

Consider following excerpt from Zel'dovich, Raizer, Physics of Shock Waves and High Temperature Hydrodynamic Phenomena: I wonder how exactly can I derive (1.10) from (1.9)? For example, (1.9) ...
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### Can Energy and Momentum Conservation prevent Particle Interactions?

I understand that Quantum Numbers must be preserved during particle interactions, which prevents certain interactions from occurring. However, as Energy and Momentum must also be conserved, are there ...
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### How does general relativity resolve the fact that energy is not positive definite in Newtonian gravity?

In Newtonian gravity, the energy of the gravitational field $\vec{g}$ is $$U = -\frac{1}{8\pi G}\int |\vec{g}|^{2} d^{3}x$$ (assuming we don't have any point masses that lead to singularities and ...
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### Two final velocities for a mass launched from Earth

i'm trying to find the final velocity of a mass that is launched vertically from the earth, only considering the gravitational force. My first attempt was with angular momentum: $r$ is the distance ...
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### When a solid is melting how do the potential energy and kinetic energy of the particles in the solid change?

When a solid is melting, does the kinectic energy and potential energy of the particles in the solid change? Will they remain the same or increase or decrease.
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When a load P is applied gradually on an object, the work done on it is the average of the initial and final loads, which is generally P/2 multiplied by the deflection Δ1. In the case of impact load, ...
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### A Tough Mechanics Problem [closed]

Here's the problem diagram: The situation is set up on a fixed frictionless horizontal table. We are required to find the time after which the block B crosses the dotted line. The diameter of the ...
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### Will the heat generated from solar panel electricity eventually generate wind energy?

I'm curious about limits for extracting solar energy, but I never studied this stuff so I'm going to speculate a lot. My first question deals with the things like wind energy. I assume this energy is ...
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### What makes energy "the" conserved quantity associated with temporal translation symmetry?

This kind of relates to my prior question about the non-triviality of temporal translation symmetry and will use some of the same concepts: How is energy conservation & Noether's theorem a non-...
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### Problem with work done by normal force

A block of mass $M$ with a semicircular track of radius $R$, rests on a horizontal frictionless surface. A uniform cylinder of radius $r$ and mass $m$ is released from rest at the top point A(see fig)....
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### Total energy of the universe calculation [duplicate]

If the total energy of the universe is zero, how is the positive energy calculated--is it mass energy, kinetic energy or both? Also if dark energy is causing the universe to expand, how can the total ...
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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 ...
1 vote
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### Are there non time-symmetric systems that increase total energy over time?

According to Noether's theorem, systems that are not time-symmetric have $\frac{\mathrm{d}E}{\mathrm{d}t}\neq0$. I have essentially two questions, then: Are there any real systems (discovered or ...
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