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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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Why energy is released (most of the time) when electrons are added to atomsg?

[Though initially the question may seem to belong to stack exchange chemistry but I believe that I would get a better physical explanation over here. ] Currently I was reading about electron ...
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Conservation of energy when placing two inductors next to each other

Say there is a circuit with a current source and an inductor. There is a current $i(t) = at$ going through the inductor. We now place a new circuit with an inductor and a resistor next to it. The ...
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How much photon energy has already been destroyed?

We get taught in school that energy can neither be created, nor destroyed. The law of energy conservation is confirmed by many processes. It is an incredibly accurate assumption for every-day life. ...
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How does kinetic energy work in braking a vehicle?

Do the brakes have to do more work (ignoring air resistance) slowing a vehicle from 10 m/s to 8 m/s than from 8m/s to 6 m/s? Say a 1000 kg vehicle is moving at 10 m/s, it has kinetic energy of $$0....
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What is heavier, water or ice?

I was reading this link and I never saw a definite answer. One would think if you have 1000 pounds of water, and you froze it and weight it you would still have the same 1000 pounds of water when ...
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Perpetual Motion Explanation [duplicate]

Hello could someone explain the reason why this machine does not work. There is an explanation in the text but I cannot seem to follow it. Here is the link - it's the buoyant spheres perpetual motion ...
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1answer
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How does 'conservation of law of energy' work in an open circuit connected to a hydroelectric generator?

I am trying to relearn high school physics and having trouble visualizing electricity in an 'open' circuit. I am going to use layman terms so i don't confuse myself! Apologies in advance if i am ...
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3answers
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Inconsistency of PE to KE conversion in moving reference frames

Here's a nice trick question to keep you amused over the weekend. A trolley of unit mass with light frictionless wheels is released to roll down a ramp onto a smooth level surface. The PE lost equals ...
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What quantity remains conserved in a calorimeter when two bodies at different temperatures are mixed?

This is the question: Two identical bodies at different temperatures are mixed in a calorimeter. Which of the following quantities remains conserved? 1) Sum of the temperatures of the two bodies 2) ...
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Why is the momentum always conserved in elastic collision?

As the title above, why is the momentum always conserved in elastic collision (no loss of kinetic energy)? Does it related to the conservation of kinetic energy? If so, how do we prove that? Further ...
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Caculating masses of unstable particles

I learned that if an unstable particle $A$ decays to $N$ particles $i=1,2,3..N$ in the final state, i.e. $$A \rightarrow 1 +2+...N,$$ then the invariant mass W of the final state particles is given by ...
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Conservation of total energy for a system with holonomic constraints

Consider a system with generalized coordinates $u_1, u_2$ and $u_3$ such that $u_1$ and $u_2$ are dependent through the following holonomic constraint \begin{equation} G(u_1, u_2)=0. \end{equation} It ...
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Strange factor of 2 in Hooke's Law [duplicate]

I was working on a PGRE practice problem: A brick of mass $m$ falls onto a masses spring with spring constant $k$ from a height $h$ above it. What is the maximum distance the spring will be ...
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1answer
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Difference between the work done by three masses on different situations

We have 3 particles, with mass $m_{i}$ What's the difference between: i) Putting each one in a positión $x_{i}$ one by one, and ii) Putting the three at the same time in positions $x_{i}$, in both ...
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5answers
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Work done by friction in a complicated path [closed]

A block of mass $M$ is taken from point $A$ to point $B$ in a complex path by a force $F$ which is always tangential to the path. We also have coefficient of friction as $K$. What will be the work ...
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1answer
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Power in different reference frames

I would like to reopen the question asked in this post because I am not quite satisfied with the accepted answer. Imagine observer A stationary (in world reference frame) and observer B moving with ...
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1answer
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Classical Hamiltonian of a free falling particle

I am stuck with the basic question on the classical Hamiltonian for free fall particle (let's say from the infinity). The Hamiltonian can be represented as the total energy of the system, which is the ...
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4answers
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Why when change in kinetic energy equals to zero the work done by two opposite forces is equal even if displacement takes place?

For example if we pull a block on an incline. Then the force of gravity i.e. $mg \sin(θ)$ will be equal to force applied so that the net work done will be zero. But if $mg \sin(θ)$ is equal to force ...
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I want to know more about this specific perpetuum mobile [duplicate]

Sorry for beeing another noob asking stupid questions, but I just wanna know more about this specific perpetuum mobile. What prevents this magnetic perpetuum mobile from working? I don t say I full ...
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2answers
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Mass reduction due to emission of photons

when a torchlight emits a photon of energy E, will its mass reduce by (E/C^2) according to mass energy equivalance? if no what will be the reduced mass?
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Will a body accelerate forever in space?

This is confusing. I have often heard that energy is conserved and it neither be created nor be destroyed. But, if a body is accelerated in space, neglecting every celestial body, it would never stop ...
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4answers
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Moon causing tides violates conservation of energy?

It is a simple fact that the moon causes tides in the earth. But these tides seem to be a violation of the conservation of energy. Why exactly would this be? Well think of it the way I am thinking ...
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1answer
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Linear perturbations of the energy conservation in FLRW spacetime

Recently i have some troubles regarding linear stability analysis in GR, especially matter conservation equation. First order perturbations of the Hubble parameter and energy density are: $$H=H_b(1+\...
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Supernovas create energy?

Ok here we go. Supernovas do have to have energy spent to explode, that is known. That explosion power also equals to the energy spent to cause said explosion, this is also known . Besides creating ...
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Energy loss in sticky collisions

A typical problem of sticky collisions involves an object colliding with an object at rest on frictionless surface, and the two move together. If conservation of momentum is applied we get $ v'=\frac{...
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2answers
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Does action at distance in electromagnetism violate energy conservation?

Consider two charges A and B separated at distance D. charge B is attached on spring and can move towards and away from charge A. Now charge A is brought closer to charge B and then it is taken back ...
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Does a photon's wavelength (and energy) change when reflecting off a mirror?

The momentum of a photon is $\ p=E/c.$ When a photon reflects off a mirror, it is elastic scattering. Elastic scattering should keep the energy of the photon. But radiation pressure states, that ...
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1answer
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In quantum mechanics, is energy conservative statistically or for every single system? [duplicate]

I'm learning from Griffiths' book "Quantum Mechanics". In quantum mechanics, a particle has a variety of accessible energy states. Then is the energy of the particle always conserved? Energy states ...
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Why do things cool down?

What I've heard from books and other materials is that heat is nothing but the sum of the movement of molecules. So, as you all know, one common myth breaker was "Unlike in movies, you don't get ...
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Period of Small Oscillations for Perturbation on SHO

I am trying to find the period of small oscillations of the potential $$ V(x) = \frac{1}{2}m\omega_0^2(x^2-bx^4) $$ It is given that the particle oscillates between $-a$ and $a$ for some $a < \...
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3answers
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Derivation of Conservation of Energy from Newton's Second Law [duplicate]

Given Newtons's Second Law: $$ \frac {d}{dt} (m \boldsymbol{\dot r}) = \mathbf F $$ How is it possible to derive the conservation of energy equation with a constant mass? That is how can you derive $ ...
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2answers
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Prove the Total Mechanical Energy of the System is Conserved via Differential Equations [closed]

Consider the dynamics of a particle P shown: Particle in 3D space with Radius r Newton's second law states that: $$\frac{d}{dt}(m\dot r) = \mathbf F$$ where, $\boldsymbol{r}$ is the position vector ...
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Establishing separate datums

In the following question, how are we allowed to define two different zero potential energy reference points?
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1answer
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Work done by normal in case of a free wedge

In the above image let us assume that block slides along the plane of the wedge and the wedge moves towards left (on a smooth surface). I know that the velocity of the block can be found by simply ...
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3answers
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Confusing work done on a particle

I'm going to ask a simple question, but it's bothering me. We all know that work done by a force which is perpendicular to its path of motion is zero. Suppose I keep a chunk of mass in a perfectly ...
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1answer
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What happens to the extra energy when the photon hits an electron? (+ Compton's Effect)

I understand that the electron needs a specific quantized amount of energy in order to be excited to another state. For example, hydrogen requires $10.2\ \mathrm{eV}$ for its electron to jump from $n=...
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Where does the energy go that is associated with a shift in a FRET spectrum (Forster Resonance Energy Transfer)?

In the commonly used Jablonski energy diagrams, some sort of loss during resonance energy transfer is usually indicated via horizontal lines below the excited state energy of the oscillator (see for ...
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Does the accelerated mirror violate the energy conservation principle within the framework of special relativity?

I have designed a thought experiment for which I could not find a convincing resolution as of yet. Assume that a plane mirror is uniformly accelerated (Born rigid acceleration) along its normal, and a ...
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0answers
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Does rigid body rotation always add a new independent variable?

I want to talk about the constrain added by introducing rotation of a rigid body to a simple case: An homogeneous ring at rest is dropped from height $H$ of an declined surface without any kind of ...
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2answers
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Energy change as ball rolls down a curved ramp [closed]

At the top of the ramp, the ball has gravitational potential energy. As the ball rolls down the ramp, the potential energy converts to kinetic energy. Due to friction, some energy is also converted to ...
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2answers
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Can a conservative force do not conserve mechanical energy?

Let us define a conservative force as being a force whose work is path independent. Then, in particular, a vanishing force is conservative. If a force acting on a particle can be written from a ...
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3answers
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Work - Energy confusion when movement is vertical/up a slope

I've always thought of work done on an object as the energy transferred to it. This explanation, however broke down when I came across the following situation : A block of mass $m$ is transported to ...
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1answer
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What is an isolated system?

I am confused about the concept of conservation of energy in an isolated system. 1) Does an isolated system mean that it does not have any non conservative forces? If yes, then, is mechanical energy ...
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Simple exercise about conservation of momentum [closed]

A block of wood of mass $M$ is dropped, with no initial speed, from a height $h$ with respect to the ground. When it is at altitude $\frac{h}{2}$ it is hit by a bullet of mass m that travels ...
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1answer
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Misunderstanding these particular energy transfers

Given 2 identical iron bars “A” and “B” in deep space. At some distance from them are 2 coils. “A” is moving with constant velocity “V”, “B” stands. Same currents pass through the coils separately, ...
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Fields and Conservation of Energy

I am a bit confused about fields, like magnetic fields, and the conservation of energy. For example, when two magnets are attracted to each other, don't they "exhaust" the field? The moon that orbits ...
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If the first law of thermodynamics ensures conservation of energy, why does it allow systems to lose energy?

I am learning the basics of Thermodynamics. Everywhere I read about the first law, it states "conservation of energy", and talks about how change in internal energy equals heat and work transfer. I ...
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2answers
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Buoyant force of tides and conservation of energy [duplicate]

Imagine an electric power generator in a form of a floating object moving up and down and utilizing the buoyant force of sea tides. Can anyone explain how it doesn't violate the principle of ...
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2answers
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Mechanical energy conservation

Is the mechanical energy conserved when the speed is constant? What if we have a charge that entered an electric field due to an external force -which is making a dynamic equilibrium with the ...
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2answers
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Will momentum be conserved in case of electrostatic force?

If a charged particle approached another particle of same charge held by some external force, then by coulomb's law the particle will start electrostatic repulsion. Thus coming to a closest distance ...