# 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 that reasonable to use kinetic energy and potential energy of a fluid system to define the laminar, transition and turbulent region?

This is my assumption ( I don't know whether it exists or not ): I suppose a fluid system always contains two types of energy, i.e., kinetic energy and potential energy. The kinetic energy of a ...
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### Potential, Kinetic, Thermal Energy?

When things move, potential energy turns into Kinetic energy, right? Then how does Kinetic energy convert to thermal energy?
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### Why can't we make a perpetual motion machine by using a magnet to pull up a piece of metal, then letting it fall back down? [duplicate]

Obviously, a perpetuum mobile isn't possible by any law in physics, because energy can't be "created" or "destroyed", only transformed. This said, I've had an idea for a perpetuum mobile and can't ...
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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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### How is energy conserved during simultaneous refraction and reflection of light?

My question is regarding the partial reflection and refraction of light when incident on a refracting surface. Here, the energy of Incident Ray is E=hv of Reflected Ray is E=hv and that of Refracted ...
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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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### Could the flex in a skyscraper due to wind be used to produce power?

While browsing 'Hot Questions' on Stack Exchange I came across this question: Can we use the stored gravitational potential energy of a building to produce power? It got me thinking about the use of ...
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### Conserved and invariant quantities in a nuclear reaction

For a reaction $$a+X\rightarrow Y+b$$ in the book, Introductory Nuclear Physics by Krane, he has written that the total energy and the total momentum both in RHS and LHS remains conserved in Lab frame,...
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### Can we use the stored gravitational potential energy of a building to produce power? [closed]

There are skyscrapers sitting and pushing on the ground with tremendous weight. Is it possible to convert this weight/force to harness energy to power the building? Maybe build the building on top ...
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### Concerning the energy stored in electromagnetic fields

How do we know that $$u = \frac{1}{2}\left(\epsilon_0E^2 + \frac{1}{\mu_0}B^2\right)$$ gives the energy density of electromagnetic fields? Is it a postulate of classical electrodynamics? Griffith ...
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### Does it take energy to move something in a circle?

Work can be calculated by $W = Fd\cos\theta$ where $\theta$ is the angle between the force $F$ and the displacement $d$ Let's say there's a ball and a (physically ideal - no friction etc.) robotic ...
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### Do orbiting planets have infinite energy?

I know that planets can't have infinite energy, due to the law of conservation of energy. However, I'm confused because I see a contradiction and it would be great if someone could explain it. ...
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### Radiation pressure and conservation of energy

TL;DR: How can radiation pressure conserve energy, if we consider the case where the atom absorbs all the Energy of the incident photon via its newly excited electron, and stills gains additional ...
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### An energy/model based controller to minimize overshoot and response time of a mass spring system

Consider a very simple system: $$m a + k x = F \, , \tag{1}$$ where $m$ is mass, $a = \ddot{x}$ is acceleration, $k$ is the spring's elasticity, $x$ is position of the mass and $F$ is the force from ...
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### How does Destructive Interference redistribute energy?

I have tried looking at the many other questions on a similar topic, however, they don't provide a definitive answer to my question. What I managed to gather so far, is that if a light wave is super ...
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### Applying energy and momentum conservation to the problem of pulling a bent carpet at a constant speed

Consider this system. A long, thin, pliable carpet is laid on a floor. One end of the carpet is bent back and then pulled backwards with constant speed $v$, just above the part of the carpet ...
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### Harnessing molecule vibration energy

Disclaimer: I am learning physics for fun, please dont kill me but explain where I am wrong. As I understand all molecules have a lowest energy state in which they posses some amount of momentum even ...
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### Why friction causes energy to be lost in terms of heat when it appears to be an energy transfer mechanicsm?

For example, when we move/walk, we apply a force (via friction) on earth, and the earth in turn on us. So essentially I see it as an energy transfer as follows: Suppose I move in same direction as ...
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### Why can't a particle interaction $A + B \rightarrow C$ conserve energy and momentum at the same time?

I learned that in the Feynman diagram for electron photon scattering, the intermediate particle represented by the middle solid line must be a virtual particle because it violates energy-momentum ...
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### On inserting a thin capillary tube into a large tray of water, does the total surface energy and the total potential energy of the system increase?

In both cases, the system is the liquid in the (tray+tube). I have made reasonably accurate calculations for the gravitational potential energy part. It follows from the calculations that the ...
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### Energy in Many-Worlds Interpretation [duplicate]

A few answers on here seem to suggest to something like the following: In the Many Worlds Interpretation, the amplitude of a branch exponentially decreases over time and so the energy of that branch ...
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### Unclear assumption in deriving fluid energy conservation laws

I am currently working through Alexandre Chorin's Mathematical Introduction to Fluid Mechanics. In the first chapter, he treats the change in Kinetic energy of a fluid region $W\subset D$ subject to ...
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### How do we define the quantity $Q$, in the conservation of energy? And what does it rely on?

Noether's theorem to me explains how a certain defined quantity (Q) is conserved (locally) in time due to the time translation symmetry, and to be more specific; if we had a ball that is placed in a ...
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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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### Energy can be created

Consider a device which converts kinetic energy into electrical energy ( just like piezoelectric generator). Now I install this on two trains of very high mass approaching each other (moving in ...
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### Does the force on magnet depend on coil, when it is being moved relative to the coil?

I am new to electo-magnetism, so i prefer simple language, the problem is: When we move a magnet relative to a coil such that one of its face is towards or at the coil, an EMF is induced in the coil ...
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### Proving that gravitational potential is work done by the object against gravity while KE increases and PE reduces

There's a person in my class who thinks that the formula for gravitational potential (-GM/r) represents the work done by gravity to move an object from infinity to any point in the gravitational field....
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### How do I derive the angular frequency of a simple pendulum through conservation of energy? [closed]

Is it possible? I'm not exactly sure what I'm doing wrong. So far I've gotten: $mgl(1-$cos$\theta) = \frac12\omega^2l^2$ Which then gives $\omega = \sqrt\frac{2g(1-cos\theta)}{l}$ which is ...
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### obtaining a force equation using the time derivative of energy

i saw this trick for obtaining a force equation but i don't understand why it works lets suppose our system conserves energy then $$E=U+E_{k}=const$$ $$\frac{dE}{dt}=\dot E_{k}-F=0$$ and then to ...
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### gravitation (kinetic energy and angular momentum)

When a satellite in a circular orbit around the earth enters the atmospheric region it encounters small air resistance to its motion. then (1) kinetic energy increases (2) angular momentum decreases ...
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### Acceleration downhill, fastest trajectory for a ball

Given 3 ways of going downhill, like in this image: Would a ball behave like that in real life? Intuitively, it makes no sense. The shortest path here is not the fastest. Any hints to the math ...
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### Is Poynting vector conserved or Intensity conserved for reflection and transmission of electromagnetic wave?

When an electromagnetic wave meets a boundary, does energy conservation mean Poynting vector of reflected + Poynting vector of the transmitted wave is equal to Poynting vector of incident wave or just ...
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### Transformation of energy [duplicate]

Consider a man rotating on a frictionless turntable , initially with his hands folded . When he opens his hands the moment of inertia increases and as the angular momentum is conserved the kinetic ...
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### Dissipation of energy in inductors

Please explain the dissipation of energy in a circuit containing a resistance and inductor after the switched in the circuit is opened.I mean how do you explain that the energy that is across inductor ...
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### Why does the potential energy of the system decreases when two charged particles are attracted to each other?

I've been reading about lattice energy, it says that during the formation of ionic compound the ions formed due to electrostatic force are attracted to each other and libration of energy takes place ...
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### Confusion on a phrasing of the First Law of Thermodynamics

According to Finn's Thermal Physics Third Edition, the First Law of Thermodynamics is described as: If a thermally isolated system is brought from one equilibrium state to another, the work ...
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### From the perspective of QM, how can a thermodynamically isolated system can have a constant internal energy $U$?

In thermodynamics, we assume that an isolated system has an internal energy $U$, and if there is no heat given, or work done by/to the the system, this energy is constant. However, in QM, we do know ...
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### Can a neutron decay into a proton + muon (+ muon-neutrino)

Can a neutron, if it has enough kinetic energy, decay like: $\hspace{0.3cm} n \rightarrow p + \mu^- + \bar{\nu}_\mu$ I assume it's not allowed since it's not on the possible decay modes on the ...
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### Why is electric power needed at all when elevator is descending?

If the ropes are doing negative work on lift, why is electric energy needed? And why is there a limit on number of passengers in a lift in this case?? Please explain in terms of energy conservation ...
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### In Schwarzchild solution what happens between $r=2M$ and $r=3M$?

Numerical Relativity by Tomas Baumagrte and Stuart Shapiro page 10. By adapting $h=c=1$, so in schwarzchild solution the areal radius $r=2M$ is the event horizon, and $r=3M$ is the photon orbit. But ...
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### Is a photon emitted from Earth less energetic that a photon emitted from the Moon

Two similar looking photons (locally) are emitted, one from Earth and the other from the Moon, and they are observed at some point X out in space. On Earth, time is slightly more dilated due to ...