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 do electrons and protons attract each other? [duplicate]

In the end, are they not a condensed form of energy? I want the 'Why' to go as far as it takes. Where it ends? What are the constituents of the electron and proton, that make them attract each ...
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558 views
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150 views

Classical disintegration of particles, Landau-Lifshitz series on Physics

i read Landau's book recently. In this book p.43 Landau says from (16.1) (16.2) can be write down $T_10$= $p_0^2$/2$m_1$=($M-m_1$)($E_i-E_1i-E_i'$)/$M$ For me, it is hard to understand the factor ...
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172 views

Where does energy in a field come from?

Let us consider for example Earth's gravitational field. If we put a ball somewhere in this field, the ball starts to accelerate due to the gravitational force exerted on it. I understand the ...
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1answer
69 views

What becomes of the massive clouds expelled from Quasars?

What becomes of the massive clouds expelled from Quasar jets? Do they simply disperse into intergalactic diffuse clouds? Or do they on occasion have enough gravity to hold together and or collapse? If ...
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2answers
792 views

Why aren't solar panels efficient?

Why can't solar panels produce 1 Kw per 1 square meter? This is the energy of the Sun's radiation per square meter on Earth but solar panels don't come close. Why can't we trap all that energy? ...
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Melting ice in a tray in space shuttle

We have an ice cube in a tray in a space shuttle. As the ice cube melts, the centre of mass shifts downward, so the tray should move downward to maintain the position of the centre of mass. Now, some ...
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2answers
157 views

The velocity of light changes from one medium to another, but does its energy change?

Does energy of light change when a light travels from a medium to another of different optical density?
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3answers
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Where is the energy lost in a spring?

Thinking about springs, and their extensions, I recently came to a confusion which I hope this wonderful community can help me solve. The question is this. When the block is initially attached to ...
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1answer
695 views

Where does an object get kinetic energy?

Where does an object "obtain" kinetic energy? I understand that an object often gets kinetic energy from another object. Where does the first object get the energy?
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1answer
2k views

Potential energy of the human body if all its atoms were split

I am working on a sci-fi story in which a character has control over matter on the atomic level (think Dr. Manhattan from Watchmen). In thinking up potential tragedies for this character I thought, ...
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55 views

What is the nature of energy? [duplicate]

In science fiction adventures, spacecraft are sometime equipped with "energy beams", i.e. devices that can project pure energy unassociated with matter. Is this possible? Is energy a "thing" in the ...
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0answers
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Is heat energy from electronics wasted in the winter? [duplicate]

In the winter, are not all electronics heating the house - thus no energy is wasted? Considering the energy I'm paying for: If I leave my computer running while I'm out, how much energy is actually ...
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496 views

Which electron gets which energy level?

Electrons sit in different energy levels of an atom, the farther the higher energy is. Every electrons have the same structure, they can gain energy from environment, electrons which gained energy ...
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2answers
153 views

Dark light filters letting through less light

I'm looking for an explanation on why darker light filters let through less light than lighter filters (secondary/high school level). Although this seems to the layman like the obvious outcome, our ...
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3answers
2k views

Is kinetic energy a scalar? [closed]

Is it correct to say that kinetic energy is a scalar?
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2answers
138 views

How come weight doesn't affect the rebound height on a net?

According to my text book two unequal masses, (80kg and 50kg) fall on a net and the work done by that net is calculated by $\frac{1}{2}kx^2$. This work is then transformed into kinetic energy for both ...
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2answers
464 views

Matter and anti-matter collision energy problem

From Beyond Einstein, by Michio Kaku and Jennifer Thompson, Chapter 13, Antimatter : Dirac, also focused on the fact that Einstein's equation $E=mc^2$ wasn't totally true. (Einstein was aware that ...
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2answers
952 views

Energy transfer in elastic collision [duplicate]

In a given reference frame where object 1 (with known mass and velocity) collides elastically with object 2 (with known mass and velocity), can we identify which object loses kinetic energy? Is it ...
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2answers
202 views

Derivation of rest energy in Landau & Lifshitz

In Landau & Lifshitz The Classical Theory Of Fields there's a statement: $$\mathscr E=\frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}\tag{9.4}$$ This very important formula shows, in particular, that ...
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1answer
471 views

Is it true that a Space Shuttle launch is powerful like an “average” nuclear explosion?

Kind of an odd question here. First let's define what an "average" nuclear explosion is, because that's quite a controversial term - "average". Considering that the peak of nuclear tests was at the ...
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1answer
1k views

What is the maximum theoretical efficiency of heat to electricity conversion?

I know that heat engines (heat to kinetic) are limited by Carnot cycle and that kinetic energy to electric energy conversion via standard generator reaches over 90%. However I would like to know ...
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1answer
175 views

Perpetual motion in spaces of different gravity?

Imagine two locations with different amounts of gravity. I carry up a weight in low gravity, move it on this height over to the other place, and let it fall down there with higher gravity. Wouldn't ...
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1answer
127 views

What happens to matter when in a thermodynamic equilibrium?

I am trying to gain a better understanding of thermodynamic equilibrium. Here's what (I think) I know: If a system is in Thermal, Radiative, Chemical, and Mechanical Equilibrium, then it is in ...
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0answers
346 views

How can one calculate the distance a particular sound will travel?

What do you need and how to calculate a distance traveled by sound? For example if you hit a bell with a specific amount of power how far will it travel?
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3answers
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Conservation of Momentum/Energy collision Problem

I'm working on a physics problem in preparation for the MCAT and there's this particular problem that's troubling me. I don't know if it's a bad question or if I'm not understanding some sort of ...
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0answers
128 views

Conduction– conservation of energy

So I know that conduction is transfer of thermal energy by direct contact via molecular collisions. For conduction, $Q/t$ is the rate of heat flow and is heat current ($I$). My textbook says that ...
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0answers
314 views

Free Energy of N Spin 3/2 Particles

This question is from the book "Introductory Statistical Mechanics" by Bowley and Sanchez. The question is as follows: Calculate the free energy of a system with N particles, each with spin 3/2 with ...
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2answers
103 views

What causes different decays?

Nuclei spontaneously decay according to a certain decay rate. There are however different kinds of decay, alpha, beta, gamma... What causes then the nuclei, when they decay, to do so in one way of ...
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1answer
382 views

What determines the distribution of the energy between the beta-particle and the neutrino in beta decay?

When beta-decay occurs an electron or a positron is emitted along a neutrino or an antineutrino. The energy spectrum of the beta particles is continuous because, as I read, the energy is shared ...
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2answers
318 views

Question related to work done and energy

Suppose you lift a box from the ground over your head. You will be using energy as there is work done. But when you keep the box lifted over your head, aren't you using any extra energy?
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3answers
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Can antimatter be used as fuel for nuclear reactors?

I completely understand the difficulties of making and storing antimatter, so I am not talking about the mechanism or the way of doing it here, I am just talking about the concept. As far as I know, ...
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0answers
200 views

Helmholtz free energy minimization using Lagrange [closed]

can you please help me with that one: Minimize free energy for a liquid crystal: $F = \int (K_{11} (div(n))^2 + K_{22} (n*rot(n) + q)^2 + K_{33} (n*rot(n))^2 ) dV$ in the case $n = cos(\alpha)*e_z ...
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2answers
213 views

How do you make a collision more elastic?

Currently, as described in this question, collisions cannot be elastic. However, is there a way to make a collision more elastic? Background: We're working on a project right now involving high ...
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1answer
108 views

What does (simple) $j/cm^2$ represent AND how does this result $6.959j/cm^2$? [closed]

According to the image shown below, this specific Laser Hair Treatment device claims that it has a concentration of $6.959 j/cm^2$. So far by research I have found that it needs around $6\mbox{ to }7 ...
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2answers
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Mass Effect on Slingshot Motion?

For my physics class (I'm a high school student), we created slingshots. Our task is to predict the distance a projectile, launched from a slingshot using surgical tubing, would land. We aren't given ...
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6answers
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What keeps mass from turning into energy?

I understand the energy and mass can change back and forth according to Einstein. It is fluid; it can go from one to the other. So, what keeps mass from just turning into energy? Is there some force ...
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2answers
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Where does a spinning figure skater's energy go when she slows down?

Today in physics class we were talking about angular momentum and rotational kinetic energy. My teacher used the classic example of a figure skater spinning on ice - when she pulls her arms in, her ...
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3answers
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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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Troubles with the Nucleon Bound Energies

I was reading my quantum mechanics text and I have a doubt. I have the energy levels well defined for the finite square well and the author suddenly compares (I believe) those levels with the levels ...
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1answer
227 views

Virial theorem and variational method: an exercise (re-edited)

I have a hydrogen atom, knowing that its Hamiltonian has been modified turning the standard potential $$ V_{0}(r) = -\frac{Z}{r} $$ into $$ V(r) = -\frac{g}{r^{\frac{3}{2}}} $$ with $g$ a positive ...
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3answers
398 views

Is it wasteful to use a heating element, instead of doing useful work?

Consider a computer CPU consuming electrical energy to perform calculations and consequently emitting heat. Assumption: That a CPU consuming x Watts of power, emits the same amount of heat as an ...
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1answer
93 views

Can the technology behind Particle accelerators can be used for energy storage?

As I understand, the kinetic energy of the proton beam in a hadron collider is quite large. Can you build a energy storage system that is based on accelerating a proton beam to relativistic speeds to ...
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What is the amount of EM energy from the sun reaching the earth as a function of wavelength?

The question stems from a desire to know if more light from the sun in the infrared wavelengths has more energy than light at say yellow or violet wavelengths - at the Earth's surface. I'm guessing a ...
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2answers
3k views

Is any work done if I walk in a circle?

My friend and I were arguing about this and I was wondering if someone out there could settle this for us. Basically, he and I were walking to buy some stamps. When we were on our return trip he ...
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3answers
569 views

Explain internal energy and enthalpy

Internal energy and enthalpy. I am finding it hard to distinguish between the two. Confused. Can someone explain me the two terms and difference between them? I tried learning from wikipedia but it ...
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2answers
252 views

Why is a vacuum cleaner not as good heater as an electric radiator?

I've read this question and answer: How efficient is an electric heater? , but still don't understand. If I have an electric radiator it heats the room with 1000 Watts of power. And I feel the room's ...
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2answers
2k views

Where does the lost energy go in a rubber band powering a rotating shaft?

Okay, I'm no physics whiz, and this has me stumped. You know those toy airplanes you can get with the rubber-band driven propellers? You twist the propeller a bunch of times, and this stores ...
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Gravity from energy [duplicate]

Both energy and mass gives has gravity. If an object receives energy, it will appear heavier and space will curve slightly more around that object. That energy could be potential energy, or static ...
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183 views

does a rocking kettle cause the water to boil more slowly?

When I boil water in the kettle on my electric stove, sometimes it rocks back and forth making an annoying sound at a frequency of about 6Hz. When that happens, I move the kettle slightly to make it ...