# Questions tagged [energy]

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 the [energy-conservation] tag if it is specifically about its conservation.

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### Noether's theorem and energy in four-momentum

In Newtonian physics, momentum and energy are often treated as distinct entities, which happen to be separately conserved. In relativity, energy is regarded as the "time" component of the ...
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### What is the energy density of Cf-252 compared to Pu-239 and U-235? [closed]

Cf-252 has a critical mass of 2.7 kg compared to U-235 (52 kg) or Pu-239 (10 kg). one Cf-252 atom releases 3.7 neutrons. One U-235 atom releases 2.5 neutrons and one Pu-239 atom releases 2.9 neutrons.
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### A rolling stone on a rollercoaster

A ball rolls from point $A$ ($v_0 = 0 \; m / s$) to the right along a friction-less path. We can then calculate the speed of the ball at the points $B$ and $C$ using the law of conservation of ...
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### Why is there Energy? [closed]

I am a layman and this is my first ever question! Although, it is simplistic in nature, I hope that there is a beauty in its simplicity which could give a simple answer. Alan Watts, the philosopher, ...
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### Could you use a chaotic pendulum to generate energy? [closed]

Could you use a chaotic pendulum to generate energy?
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### Relation between kinetic energy and momentum in relativity

In a recent discussion a friend of mine claimed that kinetic energy ($K$) an momentum ($p$) in relativity can me expressed by $$K=\frac{p^2}{(1+\gamma)m} \tag{1}$$ This equation if holds, has some ...
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### Franck-Hertz experiment

In the Franck-Hertz experiment why are the peaks much sharper than the troughs? Also our data analysis suggested that the distance between the peaks is smaller (nearer to $4.9 \$ eV ) than the ...
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### Kinetic energy of an electron inside the sun [closed]

I have to tell if an average proton or electron at 15 million degrees kelvin be treated by relativistic mechanics. The criterion is that the kinetic energy differ from the classical kinetic energy by ...
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### Potential energy at a certain height

If an object is levitated above the surface of the earth with constant velocity so the net work done is equal to zero then the object must possess no potential energy =0 at a certain height $h$ ...
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### If Potential Energy Is only defined For Conservative forces, can Non-conservative forces Increase spring potential?

Consider Two Cases Of collision When Energy is conserved before And after collision,we can say That The forces that acted between them were Conservative. During deformation their kinetic converted to ...
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### Saturated vapor and liquid densities (VLC curve) from Helmholtz free energy equation of state

I have been reading about the statistical associating fluid theory (SAFT) which computes the Helmholtz free energy ($A$) from the SAFT EoS (a hell lot of equations) for a molecule of interest, and is ...
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### Current from guiding center drift under gravity

Wikipedia says: when there is a force on the particles perpendicular to the magnetic field, then they drift in a direction perpendicular to both the force and the field. This is called "guiding ...
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### What happens to Kinetic Energy in Perfectly Inelastic Collision?

I’m going over a chapter on linear momentum in my physics course right now and am somewhat puzzled with what happens with some of the kinetic energy that is lost in a perfectly inelastic collision. ...
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### Confusion regarding ball bouncing off a wall with respect to Newton's Third Law

This is more of a confirmation post whether my ideas are correct or not. Suppose a ball of mass $5 kg$ travelling with uniform velocity $5 ms^{-1}$ through vacuum hits a block of mass $200kg$ with ...
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### Why work can't be extracted from thermal energy?

As I understand it, thermal energy is simply molecular agitation, and therefore nanoscopic mechanical energy. So why can't this form of energy be used to extract work? I consider here an isolated ...
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### Energy in nuclear fusion - mass change vs potential energy explanation

Trying to understand how the energy related to nuclear reactions. In particular why particular fusion of small nuclei release energy. So far, there seems to be two explanations which I have ...
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### Could photons generated from the many trillions of stars be some how contributing energy to dark energy? Where do they go and what becomes of them?

What is dark energy and can photons contribute to it. When the total number of stars start to diminish will dark energy perhaps begin to diminish as well. What does happen to all those photons with ...
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### Kinetic Energy of wavefunction

Suppose we have been provided the form of some wavefunction on a graph, but not the exact mathematical expression of the wavefunction $\langle x|\psi\rangle$. Now I'm asked to find the average ...
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### How can we make the energy levels of an one-dimensional infinite square well potential equispaced?

We know that the energy values for an one-dimensional infinite square well potential is given by $$E_n = \frac{n^2{\pi}^2{\hbar}^2}{2ma^2}$$ where $a$ is the width of the well. Now, as we can see that ...
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### Why is the half of the total height taken as effective height in case of certain mathematics regarding gravitational potential energy?

A problem gives the depth and diameter of a well full of water, states that a pump can empty the well within a certain given time, and asks to find out the power of that pump. In the solution, while ...
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### Confusion Regarding Reaction Forces and Newton's Third Law of Motion [closed]

So I was confused about reaction forces for a LONG TIME and I have finally been able to clear some of my confusions, but the concept is still not totally clear to me. Can someone confirm whether my ...
We have an exercise where we have to work with energy as a legendre transform in the form $$E(B) = \tilde{E}(m) - mB$$ where $\tilde{E}(m)$ are the lowest energy of states with given magnetization $m$....
Suppose you squat. As you stand up, about how much mechanical work do your muscles do? Your weight is $500 N$. Your center of mass moves at a constant speed from a height of $H_{CM}=0.2 m$ to \$H_{CM}^′...