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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I don't understand why combustion releases energy?

So I've tried to wrap my head around fire. To keep it simple, I explain it with cellulose combustion. But to explain exotermic reactions, I show the example of methane combustion. Here is the detail ...
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Dark energy, potential energy and energy conservation...?

The universe has an accelerated expansion (due to dark energy, according to the standard cosmology model). There are some papers (like this one: https://arxiv.org/abs/astro-ph/0507195) that mention ...
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Why doesn't this buoyant free energy device work?

Okay so I've seen a few of these but the setup here is a bit different. Suppose we set up an electrolysis plant at the bottom of the ocean and for some constant energy cost $E_1$, we can get a bunch ...
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Function for total energy expenditure by a laser of a wattage W feeding a black hole from mass $M$ to $M_1$ (accounting for Hawking Radiation) [closed]

As stated in the title, I need an equation that will tell me the total energy expenditure of a laser with the a wattage of W feeding a black-hole from mass M to M1 WHILE accounting for Hawking ...
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Thermodynamic work done on a free-falling object

By definition the thermodynamic work is the opposite of the mechanical work (engineers'convention). Consider then an object of mass $m$ falling at constant velocity above the Earth surface (imagine ...
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Does energy conservation law conflict with Active Noise Cancellation

Assume the picture below: From inlet port In1 and In2 we can have electromagnetic or acoustic energy entrace single sine wave with same amplitude and frequency. Assume we have aligned the waves to ...
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Prove that kinetic energy is conserved for simultaneous collisions if Newton's impact law holds

5 months ago I asked a question about simultaneous collisions Derivation of collision rules. I've been reading about the subject, but, unfortunately, some things are still not clear to me. I ...
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Faraday's law of induction and circuits which move and deform

In this wikipedia article there is a description of the difference between the Maxwell-Faraday equation $$ \vec\nabla\times\vec E = -\frac{\partial\vec B}{\partial t} \tag{1}\label{uno} $$ and the ...
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How do velocities become equal when potential energy is maximum? [closed]

I was given this diagram and asked to find the maximum potential energy during the subsequent motion if one of the blocks is projected with a kinetic energy K initially. So I did reach upon the answer ...
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Will an ideal bouncing ball ever stop bouncing?

By an ideal ball I mean a bouncing point mass with the property that it loses half of the velocity (50%) after each bounce. Now according to my definition of ideal ball, it is impossible for the ball ...
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Do objects always speed up as they fall? [closed]

In the thermodynamics video I watched, it was stated that objects at higher elevations have more energy. And that objects must speed up as they fall. My question is, "what happens if an object ...
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Where does energy go in an induced electric field? [closed]

The induced electric field is produced due to changing magnetic field field is non-conservative in nature. However this is in contradiction to my prior understanding that all fundamental forces are ...
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Proof Of Conservation Of Energy

Yesterday I was trying to prove The law of conservation of energy, But in a different approach: Using the concept of vector field here is how it went: I assumed a force field $\mathbf{F}=f(x,y,z){\...
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Why this problem setup seems to violate conservation of momentum? [closed]

Problem: An object $A$ with mass $m_A=2$ kg is at rest with velocity $v_A=0$ m/s. A second object $B$ with mass $m_B=1$ kg approaches it with velocity $v_B=3$ m/s. $B$ slides over $A$ and pulls it ...
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Why don't we consider the displacement relative to the equilibrium point when calculating the maximum displacement of a mass dropped on spring? [closed]

I'm doing a homework differential equation homework problem (but my question is regarding basic energy conservation). There is a tldr at the bottom. A mass is dropped on a spring (spring constant k) ...
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Energy Conservation when two permanent magnets attract each other

I have a very basic doubt related to energy conservation in a system of magnets. Suppose I have two magnets (bar magnets) of magnetic moment $M$ kept along the line with an initial distance $X_0$ such ...
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Conservation laws on a block with friction

If a bullet strikes a block which is sitting on a table (with friction) is it correct that momentum will not be conserved because the friction is force on the system? I'm assuming that I could then ...
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How to find velocity at natural length when total mechanical energy is negative? [closed]

I'm attempting to answer the following questions: 'A particle of mass $1/2$ kg is attached to one end of a model spring which is hanging vertically from a fixed point A. The spring has stiffness $4 \, ...
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Velocity required for pendulum to make a full swing

I have a conceptual question about the problem, I'm not requesting a solution. I just don't know how to implement the condition of the full swing. I know that the tension at the top is zero. But if I ...
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Conservation of energy confusion [closed]

Ideal Zorro (mass $M$, no height) swings down on a vine of length $L$ from a height $H$ and grabs a kid of mass $m$ (zero height, standing on the ground) and together they barely reach safety at a ...
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How can an airfoil create more lift than drag?

According to law of conservation of energy, energy can only be converted, not created. Airfoils convert airspeed to lift, or to put it differently, they create a positive lift force by creating a ...
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Is tidal energy an infinite source of energy?

As tidal waves are caused due to gravitational force which acts infinitely until the presence of mass (the Moon) which make me think of an infinite source of energy. So what's wrong in here?
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Lever in deep space

Can you tell me where my logic goes wrong below? Lever on Earth: if I drop down mass 2m at distance d from fulcrum a certain amount h (i.e. velocity v and KE mv2), then it will lift mass m at ...
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In adiabatic process, is work done in compression is equal to the work done in expansion? Please suggest

Assume a reversible adiabatic process where is there no exchange of heat from cylinder to environment. I assume that the work done on the system will be equal to the work done by the system. So, the ...
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Why do we say that gravity did work on an object to give it velocity, when it already had that energy as a potential?

I'm having a crisis of intuition here. Scenario A person takes a dive from a cliff. The person has a potential energy of $E_p = mgh$. As the person falls, they build kinetic energy, and lose potential ...
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From my reference frame, why does the Earth need a huge kinetic energy [$0.5M_E(11200)^2=3.75*10^{32}$ joules] to escape me? [closed]

I wake up screaming into the void, when I see the Earth almost touching me but going away from me at 11.2 $km/s$. After waiting forever, the Earth comes to a stop at infinity. I don't remember what ...
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Why wouldn't this perpetual motion machine based on Archimede's principle work? [duplicate]

I found this today on the interwebs: Obviously this cannot work, but what bugs me is that I cannot figure out the part that would stop it and bring it to equilibrium. I suspect that it has to do ...
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Do rigid bodies experience work greater than or equal to that of point particles?

Consider the following two scenarios in outer space: A yo-yo whose string is pulled by a constant force. The same yo-yo as before although this time, the string is fully unwound and attached to the ...
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Can gravitational waves gain energy in an expanding FRW spacetime?

I was reading this paper (Green's functions for gravitational waves in FRW spacetimes: https://arxiv.org/abs/gr-qc/9309025) I had a specific question about one statement in the paper that I would like ...
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Am I destroying or disrupting the foundation of physics by inquiring if it has overlooked the exclusion of mutual inductance from conservation? [closed]

By attempting to ask this question ... Has it ever been proven, or disproven, that a coincident set of mutual inductances are always conserved? ... at Quora, I got this first response ... https://qr....
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Why exactly does the interaction of a positron and an electron result in energy?

Forgive the extremely basic question, but I'd like to know exactly why the collision of these two particles results in the release of energy (EM radiation?)
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Is 100% of the energy consumed by a light bulb eventually converted into heat?

I understand that any light bulb, whether incandescent or LED, transforms a part of the energy it consumes into heat and the rest into radiation (mainly visible light). Assuming a 10W LED light bulb ...
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Illogical conclusion about a mass on a spring [closed]

If you place a weight on a spring and it is an equilibrium, then you have this equation: $mg = kx$ you would solve for k and get: $k = mg/x$ but, if we used conservation of energy, assuming that when ...
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Derive the Law of Conservation of energy from Newton's 3rd Law of Motion

I'd like to derive a conservation principle for the mechanical energy, as sum of kinetic and potential energy, for a multi-body point mass isolated system using the $2^{nd}$ and $3^{rd}$ Newton's ...
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Why does it happen that $mv^2/2$ - kinetic energy - is quantity associated with movement that transforms into other forms of energy? [duplicate]

I'm having a hard time figuring out why it is that, of all possible scalar quantities associated with the movement of things, $\frac{mv^2}{2}$ is the one that neatly wraps up conservation of energy ...
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Problem with energy conservation in field theory

As it's known, for conservation of energy by Noether Theorem it's neccesery that equation below is satisfied $\partial_\mu T^{\mu\nu}=0$ Where $T^{\mu\nu}=\frac{\partial\mathcal{L}}{\partial(\partial_\...
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Why does a photon can only be absorbed by an electron if the energy of the photon equals to the energy difference between two energy level levels?

As my title suggests, I am a bit confused as in why does a photon require exactly amount of energy in order to be absorbed by an electron? What will happen if the photon has more energy than the ...
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Conservation of Energy with respect to Lenz's Law with moving coil

My knowledge of electromagnetism is quite elementary but I think I understand WPE and the Law of Conservation of Energy well enough. Here's what I understand of electromagnetic induction in a coil in ...
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Is energy conserved under perturbation (Classical System)?

Surprisingly, I don't see this question on here, so here goes. If I have a classical system and perturbation is applied is the energy before and after equal. I want to say no. However, I am not sure. ...
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How to derive that total kinetic energy is conserved during the collision? [closed]

How to derive the below equation from scratch? What law support this equation? What is the name for it? Does it refer to the conservation of kinetic energy? Like law of conservation of momentum ...
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How to find the distance travelled by a pendulum powered car? [closed]

I'm having trouble finding an equation that find the distance travelled by a pendulum powered car based on the weight of the pendulum bob, the length of the pendulum and the angle where the pendulum ...
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What is the relation between Newton's third law and law of conservation of energy?

Using newton's third law we can find law of conservation of momentum. $F_{12} = - F_{21}$ $\hspace{2cm}$ Newton's 3rd law $F_{12} t = - F_{21} t$ $\hspace{1.8cm}$ Multiplying by "t&...
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How to determine the extension of a spring? [closed]

The spring extended by $x\ m$, and the object moved downward from the mean position due to the force of gravity and reached an equilibrium. Find $x$. My attempt: I used a free body diagram to find $x$:...
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Linear and angular velocity of a falling rod in the corner of a room [closed]

A uniform rod of length l and mass m is sitting vertically in a frictionless environment in the corner of a room, having a floor beneath it, and a wall to its left. It gets slightly off balance and so ...
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Gravitational energy and its conservation in quantum field theory

How does conservation of energy in quantum field theories reconcile with gravitational energy that is not modeled with quantum field theories? Does this mean that conservation of energy is only ...
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How is the total energy conserved when an electron moves through a plate capacitor and gets accelerated?

We assume that the electron enters the plate capacitor with a constant velocity. If we also assume that the electric field is homogeneous and edge effects can be neglected, a force along the y-axis ...
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Why is energy not conserved in some cases? [closed]

I've got a lot of cases where the energy of the whole system is not conserved, that's probably because of some non conservative force. One reason is that, the energy is transferred to heat, which is ...
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Doesn't $F=ILB$ violate the conservation of energy?

Assume we have a current carrying conductor of finite-length $l$, carrying a current of $I$ (w/ a negligible cross-sectional area) under the influence of a Magnetic field, whose constant Flux density ...
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Oscillations of a cylinder inside a cylinder [closed]

Please read the whole thing I'm asking for a concept not the problem itself, but I have to show the problem to explain myself Find the period of the small oscillations of a cylinder of radius r that ...
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Conservation of energy in quasar outflows?

I found this article about the behavior of quasar outflows in cosmology and how they can create a magnetic field. In section 2.1.4., the authors say that when a quasar produces a "wave" or ...
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