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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From knowing just the change in kinetic energy, can we find the friction force and engine power?

I understand this topic well enough to get all the task done because they aren't very creative. But for my exam I think I should have this clear. During the acceleration the force from the engine is ...
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1answer
251 views

Decay of particle (for example $\pi^+$); Find energy [closed]

I don't understand how to solve this: A $\pi^+$ decays into a muon and neutrino. Find the pion's energy if max $E_\nu$ / min $E_\nu$ = 100/1; $m_\nu = 0$ $m_\pi*c^2 = 140\text{ peta-eV}$
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1answer
189 views

Is energy “destroyed” when walking?

Conservation of energy states energy can't be destroyed, but isn't energy used up when walking in a straight line? If your not walking up a slope, kinetic energy isn't converted to gravitational ...
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2answers
909 views

I have a slight problem understanding the concept of “work”?

What I understand is that work is not the same as a car using gas or a crane lifting a car high up into the air. Let's use the crane as an example. And let me write out a few lines from the book. ...
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1answer
115 views

Is it safe for astronaut to do this “dangerous” movement?

So, if the astronaut is working outside the space station in thew space, could he not tie himself onto the space station and if he is going further away of the space station, i think he could just ...
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2answers
2k views

What's the relationship between an object's color and it's energy?

All else held equal, which has more energy: a violet-painted teacup or a red-painted teacup? I know that violet light has more energy than red light, but does this mean that the inverse holds true ...
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1answer
256 views

Could ions emitted by an ion thruster represent any realistic danger?

As ion thurster designs improve, the ions emitted could have a velocity (relative to the spacecraft) of well above 10^5 or even 10^6 m/s. It the likelihood of any such ions ever hitting a human ...
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1answer
2k views

What does the Fermi Energy really signify in a Semiconductor?

In understanding the behavior of semiconductors, I'm coming across a description of the Fermi Energy here and at Wikipedia's page (Fermi Energy, Fermi Level). If I understand correctly, the Fermi ...
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4answers
239 views

Why does it require such little energy to create the fastest thing in the universe?

I have noticed when I turn on the light switch in my house light comes from the bulb. How is this light created?(process occurring in the bulb) and why is this small amount of electricity enough to ...
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3answers
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Why do we need the quantity momentum?

Why do we need the quantity Momentum in physics when we have the quantities like Force and Energy? Isn't it possible to substitute the usage of Momentum with equivalent of Force and Energy?
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0answers
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Taylor approximation of e(v) [closed]

Relativistic mass $\displaystyle m(v)=\frac{m_o}{\sqrt{(1-(v/c)^2}}$ $m_o$ = mass of object measured at rest $c$ = speed of light ($3\times 10^8\;m/s$) $v$ = speed If the total relativistic energy ...
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1answer
607 views

Conservation of Energy in Special Relativity

In classical Newtonian mechanics, from what I understand, conservation of energy stems from the fact that all known forces are conservative forces, and vector calculus tells us that they can be ...
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1answer
317 views

Does friction act in the same way in different directions?

A physics textbook is launched up a rough incline with a kinetic energy of $X$ joules. When the book comes momentarily to rest near the top of the incline, it has only gained ($X-20$) joules of ...
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2answers
579 views

how does heat energy start to speed up a gas molecule?

If it was possible to place a single gas molecule in a cell and freeze it to near absolute zero. What would the molecule do as it thawed out? Would it translate the heat energy into it's electrons ...
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6answers
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Why is there a $\frac 1 2$ in $\frac 1 2 mv^2$?

For elastic collisions of n particles, we know that momentum in the three orthogonal directions are independently conserved:$$ \frac{d}{dt}\sum\limits_i^n m_iv_{ij} =0,\quad j=1,2,3$$ From this, it ...
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2answers
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The energy of an electromagnetic wave

The intensity of an electromagnetic wave is only related to its amplitude $E^2$ and not its frequency. A photon has the same wavelength as the wave that's carrying it, and its energy is $h f$. So ...
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0answers
228 views

Simple: What do these ballistic impact numbers represent?

I bought physics for game programmers, found here, today to study ballistic impact, but I'm confused as to where a few numbers are coming from. I'm basically looking to understand what the values ...
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2answers
185 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, ...
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1answer
153 views

Proportion of energy compared to mass in universe

In Wikipedia (http://en.wikipedia.org/wiki/Dark_matter), there is a picture that shows us the proportion of matter and energy in the current universe - CMB. It seems to suggest there is nothing that ...
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1answer
261 views

Mathematical question on Collisions [closed]

A 2.5kg ball travelling with a speed of 7.5m/s makes an elastic collision with another ball of ...
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1answer
9k views

Water Electrolysis Calculations

From Wikipedia (http://en.wikipedia.org/wiki/Water_electrolysis#Efficiency): The electrolysis of water requires a minimum of 237.13 kJ of electrical energy input to dissociate each mole. Each ...
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1answer
4k views

How do kinetic energy and linear momentum relate?

It took me quite a long time to click my gears in place and even then I'm not sure it's completely correct. The problem is that I need to understand these concepts (physics concepts; not just these ...
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1answer
78 views

is it possible to gain from charging cold battery

Is it possible to get higher load of energy into a battery if you charge it when its cold eg. +9C? the battery is emptier, but does it let the power in? Is it safe?
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Rotational kinetic energy during vertical circular motion of a particle

Why is it not necessary to take into account rotational kinetic energy when using the Law of Conservation of Mechanical Energy to solve vertical circular motion problems? After all, the particle is ...
2
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2answers
195 views

Which new insight did $E=mc^2$ give us?

I had a special relativity course at university. Now I'm trying to extract what new insight $E=mc^2$ did give us. I mean that moving mass has/is energy (kinetic) not new. The energy merely changed ...
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3answers
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How does energy depend on frequency in an alternating current circuit?

In what relation is the energy input in an alternating current circuit to its frequency? I'd guess I have to compute something like $$E=\int P(\omega,t) dt=\int U(\omega,t) I(\omega,t) dt, $$ but ...
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2answers
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Connection between entropy and energy

An isolated system $A$ has entropy $S_a>0$. Next, the isolation of $A$ is temporarily violated, and it has entropy reduced $$S_b ~=~ S_a - S,\space\space\space S\leq S_a.$$ Is it true to say: the ...
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1answer
89 views

System moves away from equilibrium $\rightarrow$ it has energy added?

Suppose there is an isolated system $A$ at time $(-\infty, t_1)$, whose entropy is $S=S_{max}$, i.e. it is at thermodynamical equilibrium. Between moments $[t_1, t_2)$ the isolation is violated and ...
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2answers
1k views

How does the energy of a sound wave decrease with the distance

More precisely, how small is the potency a listener hears, compared to the potency of the emitter. I'd like to present a simple and yet reasonable approximation, to a high school audience (I am a ...
46
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12answers
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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 ...
2
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2answers
2k views

Are neutrinos affected by gravity?

Layman here, but EE and BS physics. I know that light is affected by gravity. But are neutrinos? During the collapse of a star into a neutron star, as the electrons join protons to form neutrons ...
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3answers
223 views

Is the free energy bounded

Can we say that the free energy of a mixture of a finite number of fluids situated in a bounded container $\Omega$ is bounded? I was thinking of this as an optional hypothesis for one ...
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Is it possible to be changed Energy Unit in future or it is strict reality in nature?

Could you pease tell me why energy unit must be $Energy=Mass . \frac{Distance^2} {Time^2}$? (I tried to write general form of Energy unit) What is the strong proof of that unit? Does it just depend ...
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1answer
130 views

What are some ways that humans could have influence over what sequence a star was in?

How would a society go about either preventing our sun in its primary sequence from going into a Red Giant a billion years from now? Or perhaps, accelerating the process of going from main sequence ...
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1answer
453 views

Why is $L^2$ norm of the gradient called kinetic energy?

I'm reading Lieb-Loss's book 'Analysis', chapter 7. The authors refer to the following integral: $$\tag{1} \lVert \nabla f\rVert_2^2=\int_{\Omega}\lvert \nabla f(x)\rvert^2\, d^nx $$ as the kinetic ...
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How did the inflaton field “add” energy to the universe?

How did inflation add energy to the universe? What mechanism did this occur by? In other words, where did that energy come from? Was it due to the quantum fluctuation (or that scalar field rolling ...
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1answer
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Neutrino beam energy

Neutrino is one of the most mysterious particles in todays physics. Even when values of some parameters like for example mass associated with it are not known (or there is great range of possible ...
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2answers
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Physical interpretation of Parseval's theorem

I have read that Parseval's theorem, relating the norm of a function $f$ and the norm of its Fourier transform $g(k)$: \begin{equation} \int |f(x)|^2 dx=\int|g(k)|^2 dk \end{equation} has the ...
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2answers
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Zero energy universe

Regarding this thread: http://www.physicsforums.com/showthread.php?t=506985 Why is the idea that the total energy in the universe "zero" so popular (re: Laurence Krauss) and why is the flatness of ...
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1answer
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How did the energy/entropy/volume/pressure/temperature relationship exist at the Big Bang and how did it evolve thereafter?

According to the current Big Bang with inflation cosmological model? I was under the mistaken impression that there was very low volume, very high temperature/pressure, very low entropy and the Big ...
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1answer
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How much is 1 electron-volt (eV)?

I am interested in knowing how much is one eV of energy. Everywhere I found are the technical definitions. Can anybody please tell me how much is this much energy. I need something which I can feel. I ...
2
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1answer
170 views

Why does elastic energy only depend on first derivatives?

Say we have an elastic material that is deformed with displacement function $u : \mathbb{R}^n \rightarrow \mathbb{R}^n$. It is reasonable to assume that the energy required for such a displacement is ...
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0answers
53 views

Is energy applied by the muscle when it holds a body in the air? [duplicate]

Possible Duplicate: Why does holding something up cost energy while no work is being done? We all know the following Newtonian formulas: PotentialGravitationEnergy=m*g*h Kinetic Energy = ...
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3answers
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Conservation of Energy and Momentum Regarding Forces - clarification needed

The other day, my teacher stated something along the lines of, "Conservation of momentum is not violated by the actions of internal forces, but the conservation of energy is violated. Energy is ...
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0answers
115 views

Is there analysis library for stress-strain data?

I have three column data that has time-displacement-force from 1D tensile/compression test. Now I would like to get the standard mechanical properties of the material, like Young modulus, yield ...
4
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1answer
268 views

Is this formula for the energy of a configuration of 3 fluids physically reasonable?

I have studied for a couple of months now a mathematical model of the energy of a configuration of immiscible fluids situated in a fixed container such that the fluids fill the container. In other ...
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What are the calculations for Vacuum Energy?

In wiki the Vacuum Energy in a cubic meter of free space ranges from $10^{-9}$ from the cosmological constant to $10^{113}$ due to calculations in Quantum Electrodynamics (QED) and Stochastic ...
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1answer
279 views

2d or 1d conduction in this scenario?

There is a rectangular fin attached to a heat exchanger with a base temperature of 350K. The fin has uniform properties and experiencesa uniform heat generation. It also experiences heat transfer with ...
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1answer
395 views

Energy versus free-energy diagram

Energy versus free energy diagram. I haven't been able to find an adequate definition of these two terms in relation to each other. Could someone point me in the right direction, please? From Borrell ...