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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Why is Kinetic Energy = (-) Total Energy and Potential Energy = 2 $\times$ Total Energy?

I came across this relation while reading on the Bohr atomic model. Are there any other forces for which these relations hold good?
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Work, Energy & Power - Body slides down a hemisphere

A small body of mass $m$ slides down from the top of a hemisphere of radius $r$. There is no friction between the surface of the block and the hemisphere. The height at which the body loses contact ...
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79 views

Zero Potential Energy Change in Raising a Mass

Suppose I am able to lift a body of mass $m$ up using a constant force $F=mg$ so that net acceleration is zero on it. I want to find the potential energy and the total energy of the body when I have ...
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Inconsistency in solving energy in free space given electric potential

So we have the equation $$ W= \frac{1}{2} \int_{volume} \rho_v V\,dv$$ and then after applying a vector identity which is $$\nabla \cdot (V\vec{D})=V(\nabla\cdot \vec{D})+\vec{D}\cdot (\nabla ...
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ionizing radiation and energy

At the website "How Stuff Works" an article (radiation sickness) states that when radiation knocks an electron from an atom, energy (specifically 33 electron volts) is released which heats up the ...
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41 views

Wavefunction of a system of particles

A three-dimensional volume $V$ contains a certain number $N$ of electrons and they can't escape the volume $V$. Assume for simplicity that the potential $\mathcal{V}(\mathbf{r})$ is zero in all the ...
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34 views

When should gravitational potential energy be included in the law of conservation of energy

I have a problem that says: A block of mass 0.249 kg is placed on top of a light, vertical spring of force constant 4 975 N/m and pushed downward so that the spring is compressed by 0.090 m. ...
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22 views

Compare a normal spring and a torsion spring

Consider a torsion spring with a known spring constant $K_c$. I would like to calculate the required $K_l$ for an equivalent linear spring, which would store the same amount of energy when compressed. ...
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2answers
89 views

Which coordinate is to be considered for the energy of simple pendulum?

For an simple harmonic oscillator energy can be represented as in picture. Consider in particular picture (b) with the energy as a function of the coordinate $x$. Consider now a simple pendulum. The ...
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1answer
99 views

Do we have a better understanding of what energy is since Feynman's time?

When lecturing about conservation of energy in the 1960s, Richard Feynman remarked: It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a ...
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601 views

How do I derive the formula of coefficient of restitution?

I have learnt in collision mechanics about the term Coefficient of restitution, $\mathrm{e=\frac{v_{2f}-v_{1f}}{v_{1i}-v_{2i}}}$. But the sad part is that, in my book only the formula is there. My ...
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2answers
58 views

How does conservation of energy work?

I understand that the law of conservation of energy states that energy cannot be created or destroyed, it can only change from one form to another. This means that the total energy before a certain ...
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202 views

Climbing Stairs and Calories Burnt

I climb stairs to work (20 floors) every day . The least amount of work my body is doing by gaining potential energy (P.E) = mgh where m is mass , g is acc. due to gravity and h is height . Assuming ...
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1answer
27 views

The highest photon energy ever measured and transplanckian energies

What is the highest energy of we have measured of a photon by any physical experiment? Has a transplanckian energy photon been measured? Can we study them -if they exist- with special relativity?
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1answer
73 views

Statistical Physics: How do we derive this equation?

I'm reading through Statistical Physics by F. Mandl and there is a step in arriving at an equation that I don't follow. He uses $$P = \sum_r p_r \left(-\frac{\mathrm dE_r}{\mathrm dV} \right) ...
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4answers
304 views

Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?

In classical physics we often cast an analogy between translational and rotational systems Force < > Torque Energy < > Rotational Energy Momentum < > Angular Momentum and considering SI ...
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How can work be done during collisions? [closed]

Consider a collision between two material points, with no external forces acting on the system. Linear and angular momentum of the system are always conserved, while the kinetic energy of the system ...
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3answers
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Work energy theorem: variation of kinetic energy as sum of terms given by the work of single forces

Work-energy theorem states that $$W_{\sum F_i}=\Delta K,$$ where I expressed with $W_{\sum F_i}$ the work done by the resultant of forces ${\sum F_i}$ and with $K$ the kinetic energy. In general it ...
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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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What is the abstract idea behind energy? [duplicate]

Many theories use something called energy to explain reason behind certain phenomenon, but I have not been able to grasp the idea of energy. Maybe it is because I have grown in a society where the ...
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1answer
161 views

How to calculate the electric energy caused by eletrostatic field stored in a region, given $V$? [closed]

I have problem in apply the rules to find the energy stored in free space here is the problem Find the energy stored in free space for the region $$0.002<r<0.003m,\quad ...
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23 views

Accelerating voltage relationship to energy

if I have the accelerating voltage, how can I obtain the energy in eV? Basically, I asked someone for the energy, but instead they gave me the accelerating voltage. What more data would I need to ...
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3answers
88 views

Lifting and putting down a weight

A man in a gym lifts a weight and then puts it down where it was before. 1) What can be say about the work done by the man to the weight? 2) Can it be equal in absolute value to the work made by ...
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Does energy produce a gravitational force [duplicate]

$E=mc^2$. From this, I would assume that any form of energy (not just rest-mass energy, but kinetic energy as well) would produce a gravitational force. Am I being too naive in my application of ...
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Turning Work into Thermal Energy in Joule’s Experiment

In Joule’s famous experiment where he determined the specific heat capacity of water, he used a paddle to increase the temperature of the water. That is, the kinetic energy of the paddle went into the ...
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Work done by a non-conservative force and change in potential energy

I know that the work done by a non-conservative force is equal to the change in total mechanical energy (from Work-Energy Theorem). But I read in a place that "Non-conservative forces don't affect ...
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Put energy into void [closed]

what happens when you drop matter or energy of any type into the void , by void I mean a space of absolute emptiness with nothing more than darkness and total absence of every kind of energy , ...
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Physical meanings of kinetic energy

While studying energy on Sears & Zemansky's University Physics, I came up with a doubt on the meaning of kinetic energy. The book gives two possible physical interpretations of this quantity. ...
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30 views

Could Something Infinitely Heat Up?

If there was a perfect environment, and something was able to continue to heat up indefinitely, what would happen?.. What would be the outcome of something continuously heating up?
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1answer
61 views

Work-energy theorem for a system

While studying energy conservation on Morin I found this explanation about the work-energy theorem for a system. The work–energy theorem stated before is relevant to one particle. What if we are ...
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1answer
34 views

Would High Explosive helped free Shackleton's ship, trapped in pack ice?

This question is based on physical concepts regarding presssure waves and crystallography, but I thought it might be helpful to include a brief historical context first. From Wikipedia: Loss of Polar ...
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1answer
36 views

How would you get the velocity in this problem? (and what did I do wrong) [closed]

This problem is supposed to be solved by using the conservation of energy but I do not really know how to solve it that way.
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28 views

Reason behind the heat capacity of water

Water has a high specific capacity. Why can water release a large amount of energy without a great change in temperature?
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24 views

Microscopic Definition of Heat and Work

If I am given a statistical System, then I can define state-variables like Energy, Entropy or other Observables, and then I can (at least for equilibrium states) give the Change of Energy as: ...
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32 views

How to derive kinetic energy from the Lagrange equations? [duplicate]

I'm having trouble deriving the kinetic energy from the Lagrange equations. For reference, I'm following Landau and Lifshitz book, "Mechanics," which can be found for free at Archive. In any case, ...
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1answer
225 views

What prevents this magnetic perpetual motion device from working? [closed]

Two days ago, I thought of this device, which may seem to rotate indefinitely(in all 3 directions). I have two questions. Is this perpetual motion machine already known? If it is, could you please ...
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3answers
177 views

Is work-kinetic energy theorem useless?

This is how my book explains it: $W_{net} = \delta K$ Since the net work is tied to changes in kinetic energy and changes in speed, a mass must accelerate in order for net work to be nonzero. ...
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When space containing an extended mass contracts, what delivers or absorbs energy?

As my understanding goes, general relativity and the many theories that have sprung from it allow for the contraction and expansion of space. When such expanding (or contracting) space contains a ...
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2answers
11k views

Relation between work, kinetic energy and potential energy

We derived two equations in class. The work done between two points $A$, $B$ is equal to the difference between the kinetic energy at the last point and the one at the first point. The work done ...
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1answer
192 views

Where else than Atoms do Photons Disappear and Reappear from? [closed]

Where does photons come from and disappear to? Can photons be destroyed and created? Without atoms would photons exist? What other ways then from atoms can photons be made?
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Energy density inside anisotropic materials

I'm taking some college courses on electromagnetism and there was some talk of energy stored by electromagnetic field and its density. The expression that we use to calculate energy density stored by ...
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1answer
44 views

AC Generator Output Calculations

I am currently doing a university project where we are designing a device that a car drives over it will engage a rack and pinion mechanism, that will change the force from linear to rotational. The ...
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1answer
131 views

relation between potential energy, kinetic energy and sound energy of water

When water flows its PE gets converted into KE so what causes its KE to get converted into sound energy and what the difference will occur in its sound if it is provided with a parallel surface?
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Energy inside a box of air with temperature variation

How to find the energy of a column of air enclosed in a box (assume adiabatic) with a temperature variation $T(z)$ in the vertical ($z$ direction) in a uniform gravitation field? I think the total ...
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3answers
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Fuel usage at the same constant rpm at different gears

I've had a discussion with my father today, about the fuel usage of a vehicle at the same rpm, but a different gear. He claims that the following situations have the same fuel usage: ...
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Does the analogous saying: “matter is essentially 'condensed' energy” have any merit?

Einstein in his derivation of special relativity came upon the equivalence of energy and matter, and given the right circumstances, the relation can go either way - matter can become energy, and ...
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2answers
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Does gravitational force attract bodies with mass or with energy?

On my textbook is written that gravitational force is the force that attracts bodies with mass. But I've seen on a book that It actually attracts bodies with energy. I'm having a class tomorrow and I ...
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1answer
40 views

Work done by friction on a conveyor belt [closed]

At a factory, a 300-kg crate is dropped vertically from a packing machine onto a conveyor belt (I apologize for not having a picture) moving at a speed of 1.2 m/s. The coefficient of kinetic friction ...
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1answer
45 views

How does quadcopter's battery capacity relate to it's maximum flight time?

Suppose that we have a quadcopter weighing 3 kg (without a battery) and a battery (let it be Li-Ion type) weighing 2 kg. With this battery our quad can hover for 20 minutes until the battery runs ...