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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Can the vibrational energy of a engine be used to increase efficiency?

In whole mechanics, we never talk about wasted vibrational energy! As we see in our daily life, a lot of energy is wasted as vibrational energy in every engine, for example as we fire our car's engine,...
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2answers
5k views

What is the relationship between keV and kVp?

On X-ray topics we get used to talk about "energy", but what is keV? and what is the relationship between keV and kVp?
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1answer
113 views

What's the difference between vacuum particle-antiparticle annhiliation and regular annihilation?

Why does vacuum particle-antiparticle creation and annihilation result in nothing rather than photons? What is the difference between that and regular annihilation that does result in photons.
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3answers
302 views

How can Kinetic energy formula and the Work formula be derived without assuming the other to be true?

After searching how to derive the formula for Kinetic energy, I found that most derivations required the use of the "work" formula. After searching how to derive the work formula for a bit, I found ...
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1answer
301 views

Kinetic energy as $\pi k_B T$

In the derivation of the Thermal De Broglie Wavelength on Wikipedia, I come across the following: "In the nonrelativistic case the effective kinetic energy of free particles is $E_K=\pi k_B T$ https:...
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2answers
136 views

If there is no definite speed in the universe, only relative speeds, how does energy increase when velocity approaches the speed of light?

Is the concept of energy increasing as it approaches the speed of light based on the fact that this is only true relative to the observer? Lets say, there's a scenario where a person in a rocket ...
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2answers
277 views

Is it possible to measure temperature using sound?

Is it possible to measure the temperature of something using sound, by blasting sound waves at solid objects, liquids and gasses, then measuring how much sound is reflected or absorbed, to give an ...
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2answers
69 views

The correspondence of potential energy with reality

I'm a bit in doubt about whether the concept of potential energy has any correspondence to the real world. Real world entities can't change their magnitude in an instant, nor without being affected ...
3
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1answer
91 views

Why is gravity sensitive to absolute energies?

In QFT absolute energies play no role in the physical set-up, only relative energies (i.e. energy differences) are important. However, in general relativity this doesn't appear to be the case, I've ...
3
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1answer
100 views

4 momentum in particle physics, collision of positron and electron

If I have a positron striking an electron at rest to create 2 pions( + and -) and I want to calculate the minimum kinnetic energy that the electrons can possess to create these pions, then the created ...
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1answer
179 views

Differentiating between standing waves (stationary waves) and progressive waves [closed]

The question is ''Differentiate between standing waves (stationary waves) and progressive waves?" I do not understand what exactly they would like to hear. Should we talk about the frequency, ...
0
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1answer
68 views

Along with electrons, do molecules also directly take in photon energy from a source?

Let's take a black object for example. When waves of light (whether from the sun or a lamp) come into contact with the object, is it just the electrons (not the molecules) of the object that directly ...
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3answers
158 views

What's faulty in my reasoning about energy and momentum in this problem?

The question (from Sparknotes SAT Physics): An athlete of mass 70.0 kg applies a force of 500 N to a 30.0 kg luge, which is initially at rest, over a period of 5.00 s before jumping onto the ...
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1answer
109 views

Does energy exist? [closed]

None of this is going to be rigorous or have a right answer, so please don't take it as though I'm demanding one. I'm just interested in everyone's thoughts. It feels to me (and possibly you) that ...
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12answers
29k views

What happens to the energy when waves perfectly cancel each other?

What happens to the energy when waves perfectly cancel each other (destructive interference)? It appears that the energy "disappear" but the law of conservation of energy states that it can't be ...
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7answers
2k views

How can energy be useful when it is 'abstract'?

The topic which haunted me for two years until I gave up on it. But now I am doing engineering and this topic suddenly popped out of my textbook from nowhere. I seriously need to understand this topic ...
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0answers
165 views

What is the relationship between the velocity of an object and its Sound Intensity Level in an inelastic collision?

Assuming the mass, acceleration, and height of the dropped object are constant and the area of the measured sound wave is constant, would the relation be linear, exponential, logarithmic, square root ...
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3answers
245 views

Work - constant or zero speed

Say I'm riding a bicycle at speed $v$. There's air that causes drag force, let's suppose it's constant, equal $F$, and it doesn't change with speed (we know it does increase quadratically with speed, ...
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4answers
1k views

Is $E^2=(mc^2)^2+(pc)^2$ correct, or is $E=mc^2$ the correct one?

I have been having trouble distinguishing these two equations and figuring out which one is correct. I have watched a video that says that $E^2=(mc^2)^2+(pc)^2$ is correct, but I do not know why. It ...
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1answer
38 views

How can I interpret $P(t) = \frac{1}{Q(t)} \frac{dW(t)}{dt}$ physically?

Generally speaking, one can calculate the work, $W$ (energy) required to compress a volume, $V$ of gas by integrating the pressure-volume 'loop' in a phase-space as $$W=\oint PdV$$ where $P$ is the ...
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2answers
2k views

Internal energy of an ideal gas as a function of volume

Okay so I've been reading a bit on Thermodynamics and I found something that I couldn't wrap around my head. For an ideal gas, the change in internal energy is equal to $$\Delta U = Q + W$$ And also,...
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2answers
93 views

Energy in the Relativistic COM Frame

I have been taught that in Classical Mechanics, the total energy of a system of two particles in the Centre of Mass Frame is given by $$ E_\mathrm{total} = \frac{1}{2}MV^2 + \frac{1}{2}\mu v_r^2 $$ ...
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2answers
180 views

Conservation of Energy vs Conservation of Momentum in Rotational Dynamics

It is clear to me why angular momentum is always conserved, and how in some cases energy is not necessarily conserved within the system (in those cases where bodies deform, or friction is involved). ...
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2answers
128 views

How did physicists arrive at conclusion that the product of mass and velocity is equal to momentum?

How did physicists arrive at conclusion that the product of mass and velocity is equal to momentum? What is the intuition behind $p=mv$? I had trouble finding any sources that state the actually ...
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0answers
92 views

How much of this 'Slow Death of the Universe' was predicted?

In short: the energy generated in today's universe, is "about half of the produced two billion years ago" and found that "this fading occurs at all wavelengths" as yesterday news: ESO - Charting the ...
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1answer
103 views

work and energy dilemma in an inclined [closed]

An example problem done in my book: A driver was driving a car of mass 1000 kg through an inclined plane which makes an angle of 30° with plane with a velocity of 25 m/s . Then he saw a boy 50 m ...
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6answers
3k views

Will drinking ice cold water and eating cold food cause weight loss (over a period of time)? [closed]

A friend of mine has the idea that drinking cold water and eating cold food will assist them in losing weight. The core temperature of a human body is 37$^{\circ}$ C. If they drink water, at a ...
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1answer
63 views

Could light be limiting towards a more fundamental speed limit? [closed]

Could it be possible (as in strictly speaking, no evidence going against this) that light could be limiting towards a more fundamental speed, and that light truly does slow down when a mass goes a ...
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0answers
55 views

Mass, Spin, Internal Energy and 1-Particle States in Galilean Quantum Mechanics

I have been reading an article discussing the unitary representation of Galilean group and non-relativistic quantum mechanics. The link to the article is given below. http://arxiv.org/abs/1107.2442 ...
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1answer
245 views

conversion of units

I have a quantity that I need to convert to different units. First, I need to note that the value is in the natural units system. The quantity is in cm^-1 (or, to be accurate, $cm^{-1}\hbar c /k$, ...
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2answers
1k views

Energy source of electrons?

I am aware that electrons are moving in an empty space so basically there is no friction to slow it down and its velocity stays the same. However where did the electron get its energy from in the ...
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1answer
164 views

Energy for acceleration towards speed of light is relative?

This seems a very simple question - and I guess it will turn out to be so. It's a given that accelerating particles with mass towards the speed of light takes more and more energy (ultimately an an ...
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1answer
57 views

Reproducing electricity [closed]

We all know reproducing solar energy is possible. Same stands for mechanical energy (air, water, coals) - they are all reproducible. But what about other types of light ? A diode light for example ...
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3answers
87 views

Why can you turn on your bike without adding energy and still be moving?

There's a physical effect that I can't seem to figure out. When you are biking (on a bicycle), you have some speed. Then let's say you stop petaling and make a 90º turn to the right. You slow down, ...
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2answers
73 views

Why is there no formula for some energies? [closed]

There is no formula for solar energy, mechanical energy etc, so Why there is no formulas for some energies?
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0answers
25 views

Minimal possible energy to make calculation [duplicate]

Are there any restrictions to energy that must be consumed to make calculations? Can calculations theoretically be done without consuming it?
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19 views

How much energy will fall into the eye given luminosity of display

The smart-glasses have tiny display. The manufacturer says the display has luminosity of $2000 cd/m^2$. The area of display is around $0.0002 m^2$. The reference smartphone display has luminosity of $...
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1answer
99 views

The only Energy is Kinetic Energy, isn't it? [closed]

I'm really looking for a discussion on this and don't really have a specific question. But this seems to be the best forum for such a topic. I could try to phrase this in a question, but I think that ...
0
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3answers
239 views

Lightning Power?

What is the power source of a lightning? It doesn't consume fuel, nor an atomic fission/fusion, but it discharges a large amount of energy. Is it feasible to create an artificial lightning using the ...
2
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3answers
405 views

Shouldn't solar cells be nonreflective?

If solar cells take energy from light, then why do they reflect it? How come they can take energy without actually affecting the source? Shouldn't reflected light (as a minimum) have a lower ...
18
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1answer
545 views

Positivity of Total Gravitational Energy in GR

I read the following statement in the introduction to an article: Over the last 30 years, one of the greatest achievements in classical general relativity has certainly been the proof of the ...
2
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2answers
305 views

How much energy does lowering an object into a black hole generate?

An object of mass m is slowly lowered into a black hole of mass 1000 m. Is the amount of braking energy larger than $0.6 mc^2$? Now what if, after lowering the mass close to the event horizon, we ...
0
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1answer
69 views

How is the kinetic energy of an object in collision converted to work done in deforming itself?

Usually, in a perfectly inelastic collision, maximum amount of KE is lost. I guess it depends on the rigidity of that object collision if any KE at all will be converted to work done to deform. ...
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1answer
93 views

What's Wrong With This Quantum Analogy?

"Sometimes the idea of the quantum is compared to the units we use for money. A dollar can be divided into smaller units, where the cent is the smallest possible unit." A question I came across ...
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0answers
21 views

Why does carbon help with electron transfer in a dye sensitized solar cell?

what property of carbon(soot) makes it easier for electron transfer within a dye sensitized solar cell?
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1answer
295 views

Is average kinetic energy equal to the total thermal energy of a gas?

"Average KE" as in this equation: $$K_{average} = \frac{3}{2} kT$$ Since potential energy in ideal gas model is eliminated, I guess this equation is also for the total thermal energy of a gas/a ...
0
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1answer
96 views

Why we can't define energy in accelerating frame?

In my old unanswered question about accelerating frame, I have found (later after editing that question), that one of the keys is that energy is not defined in accelerating frame. What I do not ...
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0answers
51 views

What is the most energy efficient way to run an A/C unit?

I recently installed a 12,000 BTU Mini-Split A/C unit with an inverter - http://www.tempstar.com/products/dfac.html I was told that it uses approximately 0.6Kw/hour. What I am really curious about ...
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2answers
256 views

Does a mass falling towards the earth obey conservation of energy?

I'm trying to grasp basic concepts of energy and I have a question regarding conservation of energy. According to Feynman's book and Wikipedia, the law of conservation of energy states that ...
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2answers
91 views

How is finite energy spread wholly in the infinite universe

We all know that energy is never lost, but it transforms into another form. Doesn't that mean that energy is not unlimited? I mean, why is it that way if it was infinite? However, if energy is spread ...