Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding [tag:energy-conservation] if it is specifically about its conservation.

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If humans were able to catch all sun energy reaching the earth for their use, will the climate change?

I guess that energy will be used up and, at the end, will contribute to heat the earth, so I see no big differences... please explain your point of view.
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Proof that Energy Momentum Tensor of Scalar Field Theory satisfies Weak Energy Condition

It's a question on Sean Carroll's Spacetime and Geometry, where we are supposed to prove that the energy momentum tensor of scalar field theory satisfies Weak Energy Condition (WEC). The energy ...
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Conservation of Momentum/Energy collision Problem

I'm working on a physics problem in preparation for the MCAT and there's this particular problem that's troubling me. I don't know if it's a bad question or if I'm not understanding some sort of ...
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217 views

Violation of conservation of energy and potential energy between objects

I would like to clarify my question. I have numbered them to be independent questions For any conservative fields, $\vec{F} = -\nabla U$. Which means the restoring force is opposite to the ...
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Certain material heating water in a recipient

I don't know how to resolve a problem, but I don't want the answer since I'm almost going to have it resolved. What the problem says is we have 85 liters of water at 7ºC in an iron pot of 29kg. We ...
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Why can't energy be created or destroyed?

My physics instructor told the class, when lecturing about energy, that it can't be created or destroyed. Why is that? Is there a theory or scientific evidence that proves his statement true or ...
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Practical personal aircraft today? [closed]

Is it possible to build, today, a personal aircraft that not use an impracticable amount of fuel for everyday use? What are the physical concepts that could be used to build it?
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How much energy is in a lightning strike?

According to Wikipedia an average lightning has 1TW, the whole world used 16TW in 2006. (I suppose this means the same as 16TWh in one year?) Sometimes the lightning reaches 100kA. This peak last for ...
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Could someone remind me what this means again? $\nabla U = \pm F$

You know that for a potential function (conservative force/fields) that $\nabla U = \pm \vec{F}$ In math, we don't have that minus sign, we have only the plus one. What does it mean if you get rid ...
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443 views

Spin angular momentum of a system of particles : Is there any energy associated with it?

Consider a system of point particles , where the mass of particle $i$ is $μ_i$ and its position vector is $\vec{r}_i$. Let $\vec{r}_\text{cm}$ is the position vector of the center of mass of the ...
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546 views

Thermal energy generated due to loss in kinetic energy when observed from two different frames of reference

A body is moving with a velocity $v$ with respect to a frame of reference $S_1$. It bumps into a very heavy object and comes to rest instantaneously, its kinetic energy $$\frac{1}{2}mv^2$$ as ...
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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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What is a single word that describes the idea of the second time derivative of energy?

I think about position, its time derivative speed, and its second time derivative, acceleration. I would like to identify a single word that can be used as a handle for the second time derivative of ...
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1answer
232 views

Is there a theory which treats particles as classical point singularities?

Is there a published theory that looks at all matter as occupying no space and only being felt because of its gravitational pull? We've been taught in school that matter has mass and occupies space. ...
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My math says this is wrong, but what part of energy conservation am I violating?

Basically energy of conservation says it doesn't matter what path I take, energy will be the same from before as it is after. So I took the block at the top's energy equal to the energy when it ...
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Why is the work positive here?

Problem http://apcentral.collegeboard.com/apc/public/repository/ap11_frq_physics_b_formb.pdf Please refer to question 1f Solutions ...
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378 views

Momentum Energy and Higgs

So, as an object accelerates it gains energy. And energy is mass. So an object becomes more massive as it approaches the speed of light. But, if mass is ONLY due to an object's interaction with the ...
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376 views

Is the energy coming from sun to earth completely sent back?

So energy comes to earth from sun in the form of EM waves. Some of it is reflected back but some of it remains on earth and is used by plants to create food and some is used in the atmosphere creating ...
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431 views

Einstein's Mass-Energy Equivalence versus Quantum Kinetic Energy

Using a naive view of Einstein's Energy Mass Equivalence $E=mc^2$ (where m is mass and c is the speed of light), it seems tempting to interpret this as a quantum mechanical version of the inherent ...
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Energy transferring question

If I have a spring being compressed by two bodies, A and B, with different masses, how much energy would be transferred to each one when they are released and the spring expanded?
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1answer
110 views

Is the Energy Sharply or Fuzzily Defined in Quantum Mechanics?

According to quantum mechanics, energy of a state is uncertain within a small range in hydrogen atom. But we also know that energy of a state is quantized which is contradictory to the first. Which ...
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1answer
397 views

What happens to a rotating rod that breaks in two?

I know that the approximation for the moment of inertia of an infinitely thin rod of mass $m$ and length $L$ spinning around an axis perpendicular to its own axis at its center is $\frac{mL^2}{3}$: ...
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271 views

What “I” should use in Rotational Energy formula $(I \omega^2)/2$

$\text{Rotational Energy} = \frac{1}{2} I \omega^2$. What $I$ should be used? $I$ as a inertia tensor matrix = stepRotation * inverse moment of inertia * inverse stepRotation; Or I as moment of ...
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164 views

2 streams going into an engine and 2 come out. Find the enthalpy?

Problem: Steam supply to an engine is made of two streams that mix before entering the engine. Stream 1 flows at a $.01\frac{kg}{s}$ with an enthalpy of $2952\frac{kJ}{kg}$ and a velocity of ...
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671 views

How do I determine the Internal Energy of ammonia at a pressure in temperature when the steam table doesn't say it

I just bought a steam table for my thermodynamics class since they don't allow use to use the one from the book for the tests. This one is different from the one in my textbook as it doesn't give the ...
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123 views

Is stopping something work?

If somebody pushes against a mass moving with $3 \frac{m}{s}$ to slow it down to $2 \frac{m}{s}$, he will drain the moving system of kinetic energy. Does he do work then or does he consume work? My ...
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610 views

Is there something like the Poynting vector for hydraulic circuits?

The Poynting vector is a representation of the energy flux in electromagnetics, showing the amount and direction of power flow at different points in space. In electric circuits, the energy is not ...
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901 views

Energy non-conservation for time-dependent potentials

Written in a book I read that the "total energy is not preserved when the potential depends explicitly on time", i.e. $U=U(x,t)$. Is there any proof or explanation for this?
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Is there a measure of internal energy flow?

A system might have internal energy and/or kinetic energy. Kinetic energy in classical mechanics is a form of energy the object has, only because of its relative movement to other objects. If you ...
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686 views

Bungee jump physics

Question: A bungee jumper jumps from a bridge. The length of the loose rope is 30 m. When the jumper reach the lowest point possible, the rope stretches 10 m. What is the final stretch of the rope, ...
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Energy is actually the momentum in the direction of time?

By comparatively examining the operators a student concludes that `Energy is actually the momentum in the direction of time.' Is this student right? Could he be wrong?
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106 views

Mathematical Modelling for Efficiency of PV thermal technology

As the efficiency of PV thermal or wind energy decreases with time(from the installment). Is there any mathematical model that gives its relation with time. i.e., whether it is a linear decrease or ...
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When does 'energy' turn to matter?

I always hear about matter converting to 'energy' - fusion, fission. When does it go the other way around? What conditions lead to it? Are there reproducible experiments on this topic?
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Why is energy in a wave proportional to amplitude squared

I'm a mathematics student trying to grasp some basics about wave propagation. A sentence I find very often in introductive physics textbooks is the following: In a wave, energy is proportional to ...
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Force as gradient of scalar potential energy

My text book reads If a particle is acted upon by the forces which are conservative; that is, if the forces are derivable from a scalar potential energy function in manner $ F=-\nabla V $. I ...
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492 views

why egg cooks slowly in mountains?

A quick Google tells me "Because water boils at a lower temperature at high altitudes". But I am not being able to understand this answer and fill-in the gap. Like, how does an egg cook in the first ...
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How do we perceive hotness or coldness of an object?

Some objects, especially metallic ones, feel cold on touching and others like wood, etc. feel warm on touching. Both are exposed to same environment and are in their stable state, so some kind of ...
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How does liquid convert to gas on getting thermal heat energy?

Say for example, when we heat, water converts to steam gas. How does it happen? What happens underneath giving rise to breaking of bond between molecules in liquid state and spreading them in gas ...
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Net work done on the body when we lift it and put it on the table is zero?

I'm little confused here. Work done on the body when we lift it and put it on the table is zero, because according to work energy theorem, change in kinetic energy of the body is zero. So, the net ...
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Is the E-cat by Andrea Rossi et al. for real?

Does this thing really do what they say? http://www.youtube.com/watch?v=EhvD4KuAEmo If it does, it looks like this will probably be the biggest breakthrough in science ever :)
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Difference between momentum and kinetic energy

From a mathematical point of view it seems to be clear what's the difference between momentum and $mv$ and kinetic energy $\frac{1}{2} m v^2$. Now my problem is the following: Suppose you want to ...
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Which subjects in physics should I choose if I want to help tackling today's energy and environment related problems? [closed]

I was wondering what subjects a freshman in mathematics ought to choose in the future if s/he wanted to help working on energy and environment-related issues we are currently facing, and will very ...
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If electric charges accelerate towards lower potential energies, why do opposite charges attract?

I know my logic must be wrong but I can't figure out why. I know that charges must accelerate towards lower potential energies simply because that's a general rule of nature. However, when you release ...
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Depalma Free Energy fields

Few years ago I read some papers about Free Energy, written by Bruce de Palma, a physicist who is said to be the inventor of the N-Machine, which is an device that works with free energy latent in the ...
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Do asymptotically similar potentials yield similar energy levels asymptotically?

Let there be given two Hamiltonians $$H_1~=~ p^{2}+f(x) \qquad \mathrm{and} \qquad H_2~=~ p^{2}+g(x). $$ Let's suppose that for big big $x$, the potentials are asymptotically similar in the sense ...
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166 views

Is there a theortical limit to the amount of sound-energy air can contain?

is there a theoretical limit to the amount of sound energy air can contain? In case, there is a limit, what is that limit?
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Do we have control on what an electron transition emits: light or heat?

I don't know quantum mechanical model. So, I'm referring to just bohr's model of atom. Any atom emits energy when it makes transition from higher excited state to lower excited state. Now, some times ...
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480 views

Kinetic energy puzzle

System S1 moves at constant speed V with respect to S0 in one dimension: ...
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When will the moon fall down? [duplicate]

Possible Duplicate: Are tidal power plants slowing down Earth's rotation? I've heard that there are turbines converting energy from tidal waves. Tidal waves are created by the moon's ...
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Is the continuous transformation of energy from one form to another one free? Or it consumes some quantity of who knows what?

Sorry for the unclarity. I probably created some bias in your mind having tagged my question with energy-conservation and conservation-laws. I simply consider that, relatively independently of ...