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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How long must escape velocity be maintained?

Escape velocity from Earth's surface is 11.2 Kilometres/second. How long would one need to maintain this escape velocity to actually escape Earth's gravitational pull? Must this 11.2 km/s velocity ...
12
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5answers
11k views

Where do magnets get the energy to repel?

If I separate two magnets whose opposite poles are facing, I am adding energy. If I let go of the magnets, then presumably the energy that I added is used to move the magnets together again. However, ...
7
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2answers
1k views

Can a force in an explicitly time dependent classical system be conservative?

If I consider equations of motion derived from the pinciple of least action for an explicilty time dependend Lagrangian $$\delta S[L[q(\text{t}),q'(\text{t}),{\bf t}]]=0,$$ under what ...
4
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0answers
277 views

Conservation of Energy in the Universe [duplicate]

Possible Duplicate: Is energy really conserved? Why can’t energy be created or destroyed? One of the laws of the universe that dazzles me the most is the law of conservation of energy. I ...
4
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4answers
12k views

Where do electrons get their ever-lasting circulating energy?

We all know (or maybe know) that to move, we need to spend energy. If you want to drive a car, you gotta spend gasoline. We also know that energy can't be created (first law of thermodynamics, and ...
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4answers
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Proof of conservation of energy?

How is it proved to be always true? It's a fundamental principle in Physics, that is based on all of our currents observations of multiple systems in the universe, is it always true to all systems? ...
2
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1answer
1k views

A two-level system absorbs a detuned photon. Where does the extra energy go?

Let's consider simple two-level system with frequency gap of $\omega_0$ between ground and excited state. Now, when we turn on external electromagnetic field with frequency $\omega < \omega_0$, ...
1
vote
1answer
202 views

What prevents this rotational perpetuum mobile from working? [closed]

As a newbie in physics I imagine this device and I would like to know why it can't increase its energy. Note I study the sum of energy during one second (or less) after set friction ON. The device: ...
0
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2answers
1k views

force applied not on the center of mass

When applying a force outside of the center of mass of the body, the body will get both linear and angular momentum. Right? Does the linear velocity from this force equal to the linear velocity from ...
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4answers
2k views

On what basis do we trust Conservation of Energy?

I'm happy to accept and use conservation of energy when I'm solving problems at Uni, but I'm curious about it to. For all of my adult life, and most of my childhood I've been told this law must hold ...
7
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4answers
4k views

Kinetic energy with respect to different reference frames

I'm having problems understanding the following situation. Suppose two 1-tonne cars are going with the same orientations but opposite senses, each 50 km/h with respect to the road. Then the total ...
6
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3answers
289 views

Is energy conserved in general relativity? Does $\nabla_aT^{ab}_{\rm matter}=0$ represent the conservation of energy and momentum?

For example, the radiation dominated cosmology, the energy density of radiation is proportional to $a^{-4}$ and the volume is proportional to $a^3$, where $a$ is the scale factor. So the total energy ...
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2answers
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Mass converted to energy in a common fire?

In a common wood fire such as a campfire, is matter converted to energy or is it simply an exothermic chemical reaction and all the mass can be accounted for in the ash and soot?
4
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3answers
426 views

Do photons lose energy due to gravitational redshift? If so, where does the lost energy go? [duplicate]

In the gravitational redshift, the frequency of photons radiated from some source is reduced. As the energy of a photon is given by $\hbar\omega$, if the frequency is reduced where is the lost energy? ...
3
votes
4answers
393 views

What is the energy of a superposition of energy eigenstates?

$\newcommand{\ket}[1]{\lvert #1 \rangle}$Suppose I have a system say SHO in a superposition of energy eigen states $\ket{n_1}$ and $\ket{n_2} $ given by $\ket{\psi} = \frac{1}{\sqrt{2}}\ket{n_1} + ...
3
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1answer
163 views

extracting energy from cosmological expansion

This question is a more concrete reincarnation of an old question about energy conservation in GR. Are there mechanisms to extract energy from the cosmic rate of expansion? putting some extremely ...
2
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2answers
138 views

What is the mechanism of particle anti-particle annihilation

My question is loaded with assumptions so to minimize them, I would like to ask it with respect to an electron and anti-electron annihilating. When I think of annihilation, I think of electron and ...
2
votes
3answers
264 views

The Difference Between voltage and current

I know that this question has been asked many times before, and I have read over several of the threads asking this question, but they do not include the gripe I have with my problem of understanding ...
2
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0answers
200 views

Sum of energy from torques of several disks in double rotation [closed]

Here the study: An external system (not drawn) give energy for rotate disks around themselves and around green axis. All disks have energy at start, at $t=0$ friction is ON and at $t=0$ external ...
2
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2answers
1k views

Are there any other mechanisms that can make virtual particles 'real' other than Hawking Radiation and Universe Births?

As I understand it, if virtual particles do not recombine within the plank time they become 'real'. This is proposed to happen in Hawking Radiation, where one virtual particle crosses the black ...
1
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2answers
316 views

Are photon energies conserved in general relativity?

As I understand it, both Maxwell's wave equation and the null geodesics of general relativity are scale invariant. Thus an electromagnetic wave can be shifted along a null geodesic without changing ...
1
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0answers
138 views

Friction and work from torque

I would like to understand where is the error in the third case, for that I gave 2 easier cases where I'm able to find the energy from heating is equal to the energy lost by torque. Case 1/ Purple ...
3
votes
4answers
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Conservation of momentum but not kinetic energy in inelastic collisions

In inelastic collisions, the kinetic energy of the system is not conserved but the momentum is. Kinetic energy is: $0.5 \times \text{mass} \times \text{velocity}^2$. Momentum is: ...
2
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1answer
437 views

The relation between Hamiltonian and Energy

I know Hamiltonian can be energy and be a constant of motion if and only if: Lagrangian be time-independent, potential be independent of velocity, coordinate be time independent. Otherwise ...
2
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1answer
434 views

How to get the new direction of 2 disks colliding?

I'm developing a 2D game including collisions between many disks. I would like to know how I can get the angle corresponding to the new direction of each disk. For every disk I have this information ...
2
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3answers
356 views

Neutron decay and electron anti neutrino $n\to p + e + \bar{\nu}_e$

Why do we need neutrino to explain neutron decay? Is there any evidence regarding existence neutrinos in the context of $n\to p + e + \bar{\nu}_e$?
2
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1answer
250 views

What is the mass of individual components in a gravitationally bound system?

When material of rest mass M falls from infinity onto a black hole accretion disk, it gets heated and then emits so much light that the energy radiated away can measure up to about 30% or so of M c^2. ...
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2answers
187 views

Matter - Antimatter Reactory Practicality

With current technology, would the energy released by a matter-antimatter annihilation be more than the energy needed to created the antimatter in the first place? Would it be worth it? Just curious, ...
0
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5answers
160 views

Where does particle borrow energy from to tunnel?

Where does particle borrow energy from to tunnel? It is implied that particle can borrow energy and leaped over to the other side wherever that is, the shorter the gap the more energy it borrows my ...
0
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1answer
74 views

We create systems with different values of Energy? [closed]

I understand that Energy is a conserved quantity in a system. A number, that's always the same to the system. However, don't we determine such a number? I mean, we can create systems(Or study them ...
-1
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2answers
763 views

What is a natural movement of a ball on a upward curve (the two arrow lines pointing upward) given no external force?

If you drop a ball down a upward curving parabolic ramp, what is the expected movement of the ball? Assume the ball is set into motion with no external force. Would it go all the way from point A to ...
32
votes
12answers
4k views

Where does the extra force generated by a lever come from?

Given an object being pulled down to earth by a force of 1000 lb: If a machine were to lift it off the ground for me, and I grabbed on to the rope before it was released, there is no way in the world ...
21
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2answers
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Why can't I do this to get infinite energy?

I know that I cannot do this because of conservation of energy, so I am looking for an answer as to why this will not work. So by my understanding of Einstein's whole famous $E=mc^2$ thing it is ...
14
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2answers
431 views

Do accelerated charges radiate or not?

This questions has been asked all over the net (here included) but I can't find a satisfactory answer or discussion. Some say it does not radiate if the acceleration is caused by a uniform gravity ...
13
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5answers
2k views

Does the amount of gravitational potential energy in the universe increase as it expands?

It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
37
votes
7answers
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Why do we need to know the shape of the slide to find the time to slide down it?

In my physics book after this solved example: A child of mass $m$ is initially at rest on top of a water slide at height h = 8.5m above the bottom of the slide. Assuming that the slide is ...
13
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4answers
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Is there a deep reason why springs combine like capacitors?

I was solving a practice Physics GRE and there was a question about springs connected in series and parallel. I was too lazy to derive the way the spring constants add in each case. But I knew how ...
10
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2answers
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How is information related to energy in physics?

I recently attended a talk by Dr. Ravi Gomatam on 'quantum reality', where the speaker suggested, that conservation of energy is not a fundamental law, and is conditional, but the conservation of ...
8
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4answers
974 views

Potential energy in $E_f^2=(mc^2)^2+(pc)^2$?

Let's consider $$E_f^2=(mc^2)^2+(pc)^2$$ where the $mc^2$ is the rest energy due to the rest mass -- in Finnish "lepomassa". $$ \sqrt{(mc^2)^2+(pc)^2} - mc^2~=~(\gamma-1)mc^2$$ is the kinetic ...
7
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2answers
199 views

What can be known about the formulas for energy only from the fact that it is conserved?

The question is to figure out how the energy can be derived knowing just one thing: There is a quantity called Energy that is conserved over time. The goal is to get an equation that somehow ...
6
votes
6answers
713 views

Why is global conservation of energy not considered a tautology?

This question is in reference to my downvoted answer to this active physics.SE question. More than one user has indicated that it is simply wrong and I am having trouble understanding why. My point ...
3
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2answers
804 views

How to understand Feynman's reasoning about perpetual motion?

I'm studying Feynman's Lectures on Physics, and I'm not really understanding his reasoning here: Consider weight-lifting machines $\overline{}$ machines which have the property that they lift one ...
8
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3answers
730 views

Where does the kinetic energy go?

A uniform cylinder was placed on a frictionless bearing and set to rotate about its vertical axis. After a cylinder has reached a specific state of rotation it is heated without any mechanical support ...
4
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4answers
388 views

A confusion regarding an example in The Feynman Lectures

In The Feynman Lectures, In the chapter entitled Work and potential energy, Feynman states: The work done in going around any path in a gravitational field is zero. This is a very remarkable ...
4
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1answer
4k views

How to understand the work-energy theorem?

How to understand the work-energy theorem? I took a short lecture on physics for engineering last week. The lecturer emphasized that the work done on an object will cause the kinetic energy change as ...
4
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2answers
16k views

How efficient is the human body?

This question sort of comes to mind when hearing how efficient an internal combustion engine is turning chemical energy in mechanical energy (something like 20-40%) with lots of excess heat. As an ...
3
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4answers
524 views

Is the principle of Conservation of Energy empirically verifiable?

Before I am inundated by myriad and vociferous claims that conservation of energy is the single most well-attested and experimentally verified principle in all of science, let me say that I am well ...
2
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2answers
1k views

Energy conservation limited by uncertainty principle

The way I learned it from practicing Fourier analysis and signal processing besides quantum mechanics, is that Energy conservation cannot be achieved in short time scales, and that limits energy ...
2
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4answers
949 views

How Uncertainty Principle, Vacumm fluctuations and Energy Conservation coexist in QFT?

Recently I had a debate about the uncertainty principle in QFT that made me even more confused.. Because we use Furrier transforms in QFT we should have an analogue to the usual Heisenberg ...
2
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3answers
714 views

Energy-time uncertainty and pair creation

Usually, the energy-time analogue of the position-momentum uncertainty relation is quoted as $\Delta E \Delta t \geq \frac{h}{4 \pi}$. This has interpretational issues and such. But, with a suitable ...