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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Energy of a relativist electron [closed]

I need to find a way to obtain $E = mc^2(\gamma -1)$ by starting with : $E = mc \cdot p = mc \cdot v \cdot \gamma$ with $ \gamma = \frac {1}{\sqrt{1-\frac{v^2}{c^2}}}$
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mass attached to spring rotates in a horizontal circle [closed]

So here is the question I have: A mass of 2kg rotates at 1m/s in a horizontal circle on a table at the end of a spring with an elastic constant of 50N/m. If the original length of the spring is 2 ...
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Why is 7 TeV considered as a big amount of energy?

Considering that $7$ TeV is more or less the same kinetic energy as a mosquito flying, why is it considered to be a great amount of energy at the LHC? I mean, a giant particle accelerator that can ...
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1answer
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Calculating Dark enegy, dark matter and the rest [duplicate]

I have read that Dark energy composes roughly 68% of the Universe and that Dark matter composes about27% and everything else is less than 5%. Where and how did they come up with these figures?
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Electron-Positron Annihilation in a Gravitational Field

When an electron and positron annihilate what happens to any gravitational energy they have? In a way the energy is 'shared' between these two particles and all the matter in space. It must take a ...
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Kinetic energy of a rotating object

in an exercise, a linear molecule is being subject to a force applied on the edge in its axis. Then $K_1=\frac{1}{2}mv^2$, all is well. Then in the second point of the exercise, the force is applied ...
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Basic PE question about moving a planet.

If you lifted enormous masses (eg. many gigantic rocks) at one point (A) of the planet's equator. Then after 180 degree planet rotation, released the potential energy by lowering the rocks (B). ...
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Is energy related to work or is it an entirely different thing?

I was reading about work done and energy, and I got to know some things. Let's take a simple example: Suppose I move an object to a certain height. Here, I apply a force against the force of gravity ...
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Electron energy from beta decay

I read in my IB-physics book that the average energy for an electron in the beta decay of Potassium-40 is 0.44 MeV. However this would imply the electron have a velocity of 3.9E8 m/s, i.e. faster than ...
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38 views

determine the mass of ice that melts during the impact

During a storm, a mass $m = 2 g$ hailstone falls to the ground. Its speed just before coming to ground is $v = 18 m / s$. its speed just after is zero. Assume that the hailstone is pure ice, the ...
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Conservative/Non-conservative nature of hinge force [closed]

Can total mechanical energy be conserved for a hinged rod free to move in the $X-Y$ plane? Because, there's the hinge force. If it is non-conservative, energy won't be conserved. Is it conservative?
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Statistical specific heat as energy fluctuation in spin glasses

Consider the specific heat (in statistical sense, as energy fluctuation in the canonical ensemble) of a complex model, something similar to a spin glass. Is the specific heat defined on fluctuations ...
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Could the Big Bang be the result of a decrease in entropy?

In my freshman engineering physics class I learned that a decrease in entropy, though hypothetically possible, is less statistically likely than either any or almost any other possible single ...
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2answers
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Elevator Cab Problem

I faced this problem in Fundamentals of Physics (Halliday and Resnick): A $0.250\ \mathrm{kg}$ block of cheese lies on the floor of a $900\ \mathrm{kg}$ elevator cab that is being pulled upward by ...
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1answer
102 views

How do momentum get transferred?

Simple Question , Consider two objects namely $A$ and $B$ where $B$ is stationary and $A$ is moving towards $B$ with velocity $v$. When the two objects touch each other what does actually happen ...
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1answer
51 views

Can we measure the energy of one of several identical particles?

Suppose we have a many-particle system described via a many-particle wavefunction that involves single-particle states $\lvert\lambda_{a}\rangle$, $\lvert\lambda_{b}\rangle$, ...
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Simulating Debris Burn Up Upon Atmospheric Reentry

For work I need to model Debris (1 gram to no more than 1000 km) burn-up upon reentry. I can't seem to find much information regarding this. I feel like I am looking for the wrong terms or something. ...
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Can you grow more Biomass in a brickhouse only powered by solar cells than in a greenhouse? Will we ever be able to? [closed]

Right now I read that 1m² of solar cells generates roughly 100Watts of energy. There are really efficient Grow LEDs that use only two spikes in the lightspectrum to make plants grow. I have seen some ...
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Energy contribution due to matter transfer

I have a very important question that lingers in my mind for a long time: In the fundamental relations of Gibbsian Thermodynamics, I simply don't understand how the last term appears. I have ...
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2answers
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Why are the allowed energies a continuum in the region $V_{\_} < E < V_{+}$?

I'm studying quantum mechanics and I don't quite understand why there's an energy continuum in the region $V_{\_} < E < V_{+}$ in the following example: It was explained that because of the ...
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2answers
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Is an x-ray maser possible?

I was wondering if lasers evolved from masers. They applied the same principles to a different wavelength. Would it be possible to create a maser at a higher wavelength, using x-rays, gamma rays, ...
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Why are heavy elements used in nuclear power plants? [duplicate]

I've seen that the energy produced by breaking an oxygen atom's nucleus is equivalent to burning a LOT of coal, 4.6 million metric tons if I remember correctly. (Crash Course Chemistry) If that is ...
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40 views

Reference frame and conservation of energy

Say spaceship $\alpha$ burns a portion of its fuel to leave planet A and is cruising through space at 10 m/s relative to the surface from which it launched. Spaceship $\alpha$ is being observed by ...
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How can I determine maximum efficiency of a process?(like Carnot did) [closed]

I was thinking about developing an "engine" for energy production from "waste". And then I thought why not develop it like Carnot did. So, is there any way I can develop the "most efficient ...
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1answer
48 views

Kerr Effect; Electro-refraction

First post here. is is possible to observe the Kerr effect in transparent materials, at low voltages (such as 3.8V)? If so, can you refract light enough to distort an image, in a way similar to that ...
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1answer
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What happens to a sound wave going from air to vacuum?

Let's say you are in a space ship. The inside is filled with air, a medium in which sound waves can exist and travel. Then there's a wall, still a medium in which sound waves can exist and travel. And ...
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which is efficient on solar electricity by photo voltaic or thermal

As we know solar energy used to generate electricity in two major ways: photovoltaic and thermal energy. The most small applications or personal use are mostly using solar cells. But when it comes to ...
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Calculating torque needed to climb a graded hill [closed]

this is my first post but I'll try to give my best understanding of the problem. I've been designing an electric skateboard which is driven by only one of the rear wheels (for the moment). As I was ...
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1answer
47 views

Help with my bouncy ball lab (I know the factors just not how to approach them) [closed]

In my physics lab we need to determine the factors that account for the energy "loss" during a high bounce ball bounce. I know that energy is "lost" (not really) to heat, air resistance, and sound. ...
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202 views

What is gravitational energy in general relativity?

In GR the curvature of spacetime "is gravity". This curvature is expressed via the Riemann tensor (or the Ricci tensor + Ricci scalar). The curvature is connected via the Einstein Field Equations with ...
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2answers
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Inelastic collisions

During inelastic collision the kinetic energy does not remain constant. In many texts, I have seen it is because there is loss in energy from the system. If there is loss in energy, then how can mass ...
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1answer
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How does energy levels of an electron differ from atoms?

I would like to know how does energy levels of an electron differs from atom. I've read that electrons with almost similar energy orbit in the same orbital and have a discrete set of energy levels. ...
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1answer
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To derive the relation between work function and potential energy

I'm reading "The variational principles of mechanics- Lanczos", The author mentions a relation between Work-Function $U(q_1,q_2,\cdots,q_n,\dot q_1,\dot q_2,\cdots,\dot q_n)$ and the potential ...
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How to detect transient X-ray?

I have a Mercury UV lamp (ENF-260C/FE) which is supposed to emitte UV light with wavelength of 254nm and 365nm (http://www.chescientific.com/ecat/Eng/spectroline.pdf). However it is found that during ...
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Why isn't $E=mv^2$ a general formula for energy?

This is the usual derivation of the formula for kinetic energy: $$\begin{align} \mathrm{d}E &= Fv\,\mathrm{d}t \\ \mathrm{d}p &= m\frac{\mathrm{d}v}{\mathrm{d}t}\mathrm{d}t \\ \mathrm{d}E ...
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Is the integral of $dK$ equal the work done?

$$\frac{dK}{dt} = \frac{d}{dt} (1/2 mv^2) = mv \frac{dv}{dt} = F\frac{dx}{dt}$$ $$\implies dK = F. dr$$ Also, work is defined as $W = \int F. dr$. So does this imply integral $dK = W$?
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The relation between work and time

Work equals the force acting on an object times the distance that object moves. But I am a bit confused about the definition of the distance. is it the distance the object moves under the effect of ...
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1answer
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Which electron is first ionized $n=2,\ell=1, m=?$ [closed]

Eletrons in atoms are described by n,l, m, s quantum numbers? For noble gas 1s2 2s2 2p6 which is the first electron that will be ionized n=2, l=1, m=?, s = ?
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Combination of quantum numbers for a particle in a 3D box

For a second excited state, the three combination of quantum number corresponds to $$n_{1}=2,n_{2}=2,n_{3}=1$$ or $$n_{1}=2,n_{2}=1,n_{3}=2$$ or $$n_{1}=1,n_{2}=2,n_{3}=2.$$ This is from the text ...
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Is the energy always discrete?

In the von Neumann axioms for quantum mechanics, the first postulate states that a quantum state is a vector in a separable Hilbert space. It means it is assumed the Hilbert space has a basis with at ...
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Is a Battery to store photons possible?

@MISC {55768, TITLE = {Why can't we store light in the form of light?}, AUTHOR = {AldCer (http://physics.stackexchange.com/users/20409/aldcer)}, HOWPUBLISHED = {Physics Stack ...
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What is the form of energy that is forming while i am holding a suitcase an not moving at all [duplicate]

If I am lifting a heavy suitcase and I am not moving at all. Abviously I am not doing work because I am not moving or rising the suitcase up and down. But abviously I am losing chemical energy since ...
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2answers
53 views

How much energy could be obtained by freezing water?

I know with our current technology energy is required to freeze water. But if the theories are correct we should be able to freeze water by extracting energy correct? How much energy could be ...
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0answers
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What is the form of energy that is forming while lifting a suitcase [duplicate]

If I am lifting a heavy suitcase and I am not moving at all. Abviously I am not doing work because I am not moving or rising the suitcase up and down. But abviously I am losing chemical energy since ...
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2answers
140 views

Question about distillation

I have got a question about distillation. In distillation as we know the mixture is boiled and the mechanism then goes like substances with high boiling points condense at the bottom and substances ...
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Does it take energy to destroy the surface of a liquid jet? [closed]

Imagine we have a collision of a liquid jet with an obstacle Does it take time for the surface of the jet to be destroyed while collision?? Or it'll be destroyed immediately?
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Does scanning a lightsource affect the scan result?

I'm very much a layman on this kind of topic, so my logic may be wrong, but here goes: Sample situation A 'pass app' on your smartphone, allowing someone to scan your ticket on your device. My ...
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If you lift something does the internal energy increase or remain unchanged?

So change in internal energy $\Delta U = q + w$; so in lifting an object you do work on it, thus increasing its internal energy. Is this correct? Sorry if this is a stupid question.
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Does a man run a same distance with different velocities spend the similar energy?

To simplify the question we assume the same man run on a straight and flat road with constant speed. Personally I don't believe the man would spend a similar energy. But how to prove it in physics? I ...
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Motion in a time-dependent uniform magnetic field

Suppose you have an infinite solenoid generating an uniform magnetic field inside. The field is oriented along the solenoid axis : unit vector $\vec{\bf n}$. The field intensity varies linearly with ...