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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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528 views

Justification for excluding gravitational energy from the stress-energy tensor

I did general relativity years ago at Uni and was just revising with the aid of Dirac''s brilliant book; the beauty of this book is that it is so thin and concise. On reading this book I find that I ...
24 views

Drawing the orbits in the phase space of a particle with the following properties

Considering a 1D space, I'm given the graph where $U(\pmb{q})$ is the potential energy as a function of position $\pmb{q}$, and each $E_0, E_1,E_2$ is a different amount of total energy in the system....
470 views

Unruh radiation and conservation of energy

Consider the Minkowski spacetime filled by some fields in their Minkowskian vaccum state. Now consider a Rindler observer carrying with him, say, one liter of water. According to Unruh formula, the ...
67 views

Problem on conservation of energy

A parcel is launched at speed of $3.0\; \mathrm{ms}^{-1}$ up a rough plane inclined at an angle of $35^o$ above the horizontal. The coefficient of kinetic friction between the parcel and the plane ...
3k views

How does kinetic energy work in braking a vehicle?

Do the brakes have to do more work (ignoring air resistance) slowing a vehicle from $10\ \mathrm{m/s}$ to $8\ \mathrm{m/s}$ than from $8\ \mathrm{m/s}$ to $6\ \mathrm{m/s}$? Say a $1000\ \mathrm{kg}$ ...
38 views

Why the gravitational potential energy at infinity is zero?

If an object is taken from earth's surface to infinity, it's gravitational potential energy becomes zero (always taken as zero), but it doesn't make any sense as energy can never be destroyed so where ...
44 views

Conservation of energy in decay

A Λ0 particle of rest mass 1115.7 MeV and kinetic energy 172.6 MeV decays into proton of mass 938.8 MeV/c^2 and momentum 428.8 MeV/c and a "mystery" particle - calculate mass of mystery particle and ...
17 views

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 ...
129 views

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. ...
38 views

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 ...
6k views

The physical observation of the conservation of energy?

Aside from Noether's Theorem, how do we know energy is conserved? Energy is the capacity of a system to do work. It's the number that tells me how much "force" a system can apply over a distance. For ...
107 views

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 ...
25 views

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 ...
11k views

Why doesn't this perpetual motion machine using the buoyant force work?

I realize this isn't possible, but I can't see why not, especially if you change the model a little bit so that the balls simply travel through a tube of water on the way up, rather than exactly this ...
1k views

When is mechanical energy ever conserved for inelastic collisions?

So as I was taking a test today I encountered an intriguing question, the question came in two parts. The first parts, which I answered quite easily, was about whether or not mechanical energy is ...
267 views

What is the relationship between flow work and velocity of gas?

As we know that for open systems, we have energy conservation equation as $$h+\frac{v^2}{2}+zg=const.$$ $$h=u+pV$$ $$u+pV+\frac{v^2}{2}+zg=const.$$ Here $pV$ is flow energy. We know that all these ...
24 views

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 ...
149 views

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 ...
83 views

Does Friedmann fluid equation prove that energy of the fluid is not conserved?

The Friedmann equation in question is:$$\dot\rho + 3H \left(ρ + P\right) = 0 \,.$$ I’m a layperson so I don’t know why this equation is sometimes called fluid equation, energy conservation equation, ...
182 views

Transport equation for temperature in a fluid: Heat capacity at constant volume or pressure

I want to understand the temperature equation for a Newtonian viscous fluid from first principles. In the following, the section "Background" describes how I arrive to the problem, and the section "...
53 views

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 ...
39 views

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) ...
12k views

Photons in expanding space: how is energy conserved? [duplicate]

If a photon (wave package) redshifts (stretches) travelling in our expanding universe, is its energy reduced? If so, where does that energy go?
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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 ...
248 views

How does an atom's electric field overcome an electron's inertia?

An electron has mass, and therefore has inertia. How does an atom's electric field perpetually overcome an electron's inertia, necessary to hold it in its shell? Does this require continual work to be ...
36 views

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 ...
37 views

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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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 ...
189 views

How is potential energy lost when a water droplet is dropping down slowly on a wall?

When a water droplet is on a vertical wall, it usually drops slowly, which is different from free falling. As the dropping speed is slower than free drawing, so I guess some energy must be lost. I ...
171 views

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 ...
546 views

Magnetic potential energy in a Gauss gun

How does magnetic potential energy have an effect on the velocity on the final ball of the gauss gun (magnetic linear accelerator)? it is said that the kinetic energy of the last ball (the ball that ...
81 views

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 ...
55 views

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 ...
47 views

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 ...
175 views

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 ...
5k views

When a planet is heated through gravitational pull, where is the energy taken from?

Jupiters moon Io is heated through the gravitational pull of Jupiter, but when Io is heated because of this, where does that energy come from? How does conservation of energy work for this effect, ...
15k views

What prevents this magnetic perpetuum mobile from working?

As a child, I imagined this device, which may seem to rotate indefinitely. I have two questions. Is this perpetual motion machine already known? If it is, could you please give some references? What ...
133 views

$\Delta K=\Delta U$ vs $\Delta K = -\Delta U$

While doing physics homework, I noticed that in some problems, the change in kinetic energy(KE) is equal to the change in potential energy(PE), even though I learned that conservation of energy shows ...
44 views

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 ...
6k views

How much additional mass does the Earth gain each day from solar radiation?

According to this answer, energy has some (minimal) mass associated with it. Therefore, when lots of energy hits the earth (such as solar radiation in a 24 hour period) shouldn't the earth gain some ...
81 views

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?
64 views

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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Brownian Ratchet Plausibility

Alright I'm going to throw whatever reputation I have on the line here. And yes this is a serious question. Apologies for the shoddy imagery. I had a couple ideas to get the Brownian Ratchet to work. ...
34 views

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{...
862 views

Is “Total Energy = Kinetic + Potential” always true in a harmonic oscillator?

Consider a object executing simple harmonic motion in one dimensions due to a variable external force (like a spring maybe). I like to think that at any point in it's motion at any time the mechanical ...