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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Strange conservation of energy in Navier-Stokes Equations

We want to roughly model the fluid flow of a star; consider the following proposition: In the absence of viscosity, and heat conduction, the Navier-Stokes Equations for a steady, spherically symmetric,...
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How does protium-protium fusion work?

How does protium-protium fusion work? As far as I know, a proton turns into a neutron by emitting a positron. How does that work? Shouldn't a proton be slightly lighter than a neutron? This seems to ...
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What would happen if you drew electricity from a magnet in orbit?

So suppose you had a big, dense, heavy magnet that we got into orbit. In its path of orbit would be a tube of copper wire running all the way around the planet. So it would be orbiting the planet, ...
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Intuition for the time/energy connection?

I find the following analogy reasonably intuitive: translational symmetry : conservation of linear momentum :: rotational symmetry : conservation of angular momentum In contrast, I find the analogy ...
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Energy Conservation in Hydrodynamics [closed]

The problem statement is as above, with the figures and everything. Now while solving it i considered the process as a mass transfer of liquid from the tube with the bigger cross section to the two ...
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This clever device moves heavy rocks upwards against gravity using buoyancy. Where is 'the work' coming from to move the rocks upwards?

So I saw this machine whilst researching the pyramids, and I've become completely fascinated by it. It seems to beautifully demonstrate many of the conundrums of work done, but remains alluring enough ...
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Is energy conserved during the expansion of the universe? [duplicate]

Consider the following thought experiment. Suppose we initialize a parallel plate capacitor with the plates of area $A$ separated by a distance $d$ in empty space. The energy in the capacitor is ...
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Will energy ever run out? [closed]

Gravity is the source of all energy and it will never go away. So that means there will always be energy for us right? I have seen some places say that energy will go away eventually and the last star ...
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Why do we need large time assumption for energy conservation in electron transitions?

For electron absorption calculations (with an electric field perturbation $\Delta H = eE_0x \cos(\omega t)$) we end up with an integral like: $$c_2(t) \propto \int \rho(\omega) \left( \frac{\sin(\...
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Problem 5.2 from David Morin (classical mechanics) [closed]

This problem is from Introduction to Classical Mechanics by David Morin: and this the answer: I cant understand why the highlighted part is obvious (or not obvious) I have no idea!
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How do I apply the principle of conservation of angular momentum in this case?

I came across a question: A disc of mass 2M and radius R lying in a vertical plane which is free to rotate about a fixed horizontal axis passing from its centre. A particle of mass M is tied to a ...
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Conservation of Energy with Chemical and Kinetic Energy

There is a rocket $R_1$ that is traveling with velocity $V$ with respect to another rocket $R_0$ of the same mass. $R_1$ has $J$ joules of kinetic energy with respect to $R_0$. $R_0$ has fuel onboard ...
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How to show that momentum is conserved when tachyon interacts with normal matter?

The problem is as follows: Show that when tachyons interact with normal particles, then a normal particle can emit a tachyon so that the total energy and 3-momentum are conserved. Does anyone know how ...
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Photodisintegration of deuterium

The photodisintegration reaction is given by $$\gamma\ + d\rightarrow n \ + \ p$$ I'm trying to find out the minimum energy of gamma photon $E_\gamma$ so that the reaction can occur. Writing the ...
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Why does SHM happen in vacuum?

Very simply put, I don't understand why SHM happens. If a spring or a pendulum can attain lowest potential energy by stopping at the mean position, why don't both of them stop at the mean position ...
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Concept of Potential in STR

How can we think of Potential Energy (with analogus to Classical Mechanics) in Special Relativistic Dynamics?
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Intuitive explanation for why time symmetry implies conservation of energy?

According to Noether's Theorem, every physical symmetry leads to a conservation law. For example, time-translation symmetry (the laws of physics don't change over time) implies conservation of energy,...
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In empty space what happens to the lights energy that is lost to the doppler effect go to? [duplicate]

When light makes a long distance journey it's frequency shifts toward longer wavelengths. With $E = hc/\lambda$ being the amount of energy in any given photon (see photon energy); as it travels trough ...
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How can a rigid body's weight do work on it to make it rotate?

Consider a cylinder that rolls without sliding on an inclined plane. If it's placed at the top of the plane, with its center of mass at a height $h$ from the bottom, it will have a potential energy $...
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Infinite acceleration without loss of energy in a vacuum, gravity-less void?

Suppose we have a void that is free of any other objects or forces, even gravity (impossible I know but hypothetically). If one were to propel an object forward, would that object be able to ...
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Fluid Mechanics/Fluid Dynamics help - Conservation of Mass (Mass Flow Rate)

I need to figure out how exactly mass flow rate relates to volume, height and pressure difference with respect to time. I have a gas can that is pouring out water at a mass flow rate of 3 gal/min. As ...
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Professors gave two different solutions for the same problem. What is the right one?

"A ball $m$ moves horizontally with velocity $v_0$, colliding with a spring $k$ fixed to a block mass $M$ such as shown in the diagram. The block $M$ has no friction with the ground. What is the ...
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Is Energy conserved in General Relativity [duplicate]

I have seen a few posts here on stack exchange that suggest that energy is not conserved in general relativity on a large scale. I am a pretty confused with this. I don’t understand how such a ...
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Does it take work to bend light?

We all know that light always travels in a straight line. Would it not then stand to reason that changing the path of light requires energy? If so, would this not violate Newton's laws of motion if ...
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Does creating information cost energy of itself (sorry layman question) [closed]

Say I wanted to heat a room with either resistors or cpus mining bitcoin, if I give either the same watts, will they both produce the same watts into heating the room ? (obviously the cpus ...
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Total energy and momentum in general relativity

Discussing the energy-momentum tensor in presence of gravitational field, our professor stated that, except some particular cases, it’s not possibile to define a “global” energy and momentum in ...
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Integral form of energy-momentum tensor conservation (Stokes' theorem)

Is there a way in which the conservation law of the energy momentum tensor $\nabla _\nu T^{\mu\nu}=0$ can be written in integral form using Stokes' theorem, namely as something roughly similar to: $$ ...
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Doubt in Poynting's theorem

In Griffiths's Electrodynamics, to explain Poynting's theorem, he writes: Suppose we have some charge and current configuration which at time t produces fields E and B. In the next instant dt the ...
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How to show that at resonance a damped mass-spring system gains no energy?

I am having trouble understanding the topic of resonance in vibrations. For a simple spring-mass-damper system, I need to calculate the energy from the drive function and compare it with the energy ...
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Are heat emissions as bad as greenhouse gasses?

Most of the technology we use generates excessive heat. Like cars, computers, smartphones, batteries etc... All of these needs to be cooled down in some way. Is all this extra heat adding to rising ...
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Atom in superposition: is it an atom-photon system, and what is the role of the environment?

Let's assume an atom would get from its ground state $|g\rangle$ to its first excited state $|e\rangle$ by absorbing a single photon. Now, let's assume the atom gets from $|g\rangle$ to $\frac{1}{\...
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Why ratio of photon energies at two locations are not given by ratio of metric values at those locations?

What I know I am told that in an inertial frame, the 4-momentum of a photon is: $$p^{\mu}=\frac{E}{c}\left(1, \frac{\vec{p}}{|\vec{p}|}\right).\tag{1}$$ I can also show with the geodesic equation, ...
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Why the zeroth component of a photon 4-momentum is constant?

I am aware that the 4-momentum of a photon can be defined as: $$p^{\mu}=\frac{dx^i}{d\lambda}\tag{1}$$ ie it is tangential to its worldline. Geodesic equation for such a case: $$\frac{du_i}{d\lambda}=\...
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Where does $W_{net} = \Delta K_e + \Delta P_e$ come from?

I was learning how to derive Bernoulli's equation and saw that he was using the work-energy theorem as $$W_{net} = \Delta K_e + \Delta P_e.$$ What I know is that Work-energy theorem says that, $W_{...
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When a car is moving at constant speed, is chemical energy in the engine still being converted to kinetic energy?

I know for the car to be moving at constant speed, the driving force equals the magnitude of all the opposing forces so kinetic energy is not increasing and net force is zero, but I was reading about ...
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Conservation of energy when an object comes to rest

Can someone explain conservation of energy to me? Here is what I don't understand: if one particle is moving towards another (say one has a mass much larger than the other, so it is like the Earth and ...
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Can we create energy from nothing?

Energy came from nothing as many scientists know. So, is it possible to create energy from nothing and break the laws of physics.
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Have anyone seen an unbound electron absorb/emit a photon via Bremsstrahlung process?

I know free electron cannot absorb or emit photon as it cannot satisfy both the conservation of energy and momentum at once but how about in Bremsstrahlung process? The presence of a positively ...
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Finding radius of the loop in minimum time

This was my method in solving this question : Since it is mentioned minimum time, the ball B ( after recieving 'v' velocity from A) hits C transferring the same v velocity. Due to friction, the ...
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If the definition of work done is change in kinetic energy, when I am pushing a box along the floor at constant speed am I not doing any work?

And can I still say the chemical energy in my muscles is being converted to kinetic energy? Sorry if this is a stupid question, but I was doing some physics questions in a textbook, and one question ...
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Can I use energy conservation to find out the new amplitude of SHM? [closed]

A mass $m_1$ connected to a horizontal spring performs S.H.M. with amplitude $A$. While mass $m_1$ is passing through its mean position another mass $m_2$ is placed on it so that both the masses move ...
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Is work a transfer of energy in general, or is it a transfer of kinetic energy?

the net-work is the change in kinetic energy, so is every one of the "works" done on an object solely transferring energy through kinetic energy? In other words, if I was to lift an object ...
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Constructive interference of light waves: Formula give $4x$ the intensity instead of $2x$

For constructive interference, I look at two of the same phase shift. They may arise form a thin interference layer of $\lambda/2$ thickness. The result of the E component should be double the ...
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Why was a third particle, the neutrino, necessary to explain the continuous energy spectrum of beta-decay? [duplicate]

Question pretty much in title. I am confused as to why it couldn't just be that the energy is shared between the 'leftover' particle and the beta particle hence creating a continuous energy spectrum ...
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Cannibalism and energy conservation [closed]

This question in biology forum asks why cannibalism is not an evolutionarily stable situation. In this respect one has to distinguish occasional and exclusive cannibalism - in the latter case one ...
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Can 1st postulate of special relativity break conservation of energy? [closed]

1st postulate of relativity has emerged from the fact that there is no distinction/difference between state of rest and moving with a constant velocity. Thus, they are part of single frame, inertial ...
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Force from wall during SHM and Conservation Laws

Please consider a standard horizontal spring mass oscillator system (with no friction). When doing SHM physics problems I would assume energy is conserved. However, I never got why it was conserved. ...
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How would the expansion of space affect a very long rope? Why would the effect not violate the law of conservation of energy?

This question isn't fundamentally new, but I haven't found one that elicits a clear answer on the conservation of energy problem. Another question asks: "How Does Hubble's Expansion Affect Two ...
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Elasticity of spring and conservation of energy [closed]

I have a problem of one of my students mechanics homework that I am unsure on and could do with resolving by tomorrow is possible! A spring of natural length $0.5m$ is attached to the ceiling and a ...

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