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Questions tagged [energy-conservation]

The law of conservation of energy, which states that the amount of energy in a system is constant. For questions about Earth's environment, see the climate-science tag instead.

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19
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Noether Theorem and Energy conservation in classical mechanics

I have a problem deriving the conservation of energy from time translation invariance. The invariance of the Lagrangian under infinitesimal time displacements $t \rightarrow t' = t + \epsilon$ can be ...
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Do accelerated charges radiate or not? [duplicate]

This questions has been asked all over the net (here included) but I can't find a satisfactory answer or discussion. Some say it does not radiate if the acceleration is caused by a uniform gravity ...
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Can a car get better mileage driving over hills?

Two towns are at the same elevation and are connected by two roads of the same length. One road is flat, the other road goes up and down some hills. Will an automobile always get the best mileage ...
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Does the Banach-Tarski paradox contradict our understanding of nature?

Since the Banach-Tarski paradox makes a statement about domains defined in terms of real numbers, it would appear to invalidate statements about nature that we derived by applying real analysis. My ...
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Hubble's law and conservation of energy

If all distances are constantly increasing, as Hubble's law say, then lots of potential energies of form ~$\frac{1}{r}$ changes, so how is the total energy of the Universe conserved with Hubble's ...
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How efficient is the human body?

This question sort of comes to mind when hearing how efficient an internal combustion engine is turning chemical energy in mechanical energy (something like 20-40%) with lots of excess heat. As an ...
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Conservation law of energy and Big Bang?

Did the law of conservation of energy apply to the earliest moments of the Big Bang? If so, what theoretical physics supports this? I hear that Einstein's theory of relativity disputes the law of ...
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Contradiction between law of conservation of energy and law of conservation of momentum?

Yesterday, while studying a simple question, a rather strange doubt hit my mind: Consider a ball of mass $m$ moving with velocity $v$ that collides with a wall in a perfectly elastic collision and ...
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Is the energy conserved in a moving frame of reference?

Consider this situation: When the box is at the bottom of the frictionless incline, it will have a velocity of $v_f$. The person is an inertial frame of reference that moves at a constant velocity ...
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Conservation of Mathematical Constraints when deriving Energy and Momentum from $F=ma$

Background: Starting from $F = ma$, integrating with respect to time, and using basic calc, one can derive $\int Fdt = m (v_f - v_i)$ Starting from $F = ma$, integrating with respect to distance, and ...
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Applying energy and momentum conservation to the problem of pulling a bent carpet at a constant speed

Consider this system. A long, thin, pliable carpet is laid on a floor. One end of the carpet is bent back and then pulled backwards with constant speed $v$, just above the part of the carpet ...
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How does kinetic energy work in braking a vehicle?

Do the brakes have to do more work (ignoring air resistance) slowing a vehicle from $10\ \mathrm{m/s}$ to $8\ \mathrm{m/s}$ than from $8\ \mathrm{m/s}$ to $6\ \mathrm{m/s}$? Say a $1000\ \mathrm{kg}$ ...
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Is it possible all matter in the universe emerges from nothing?

If the Universe is flat and the total energy of the universe can be zero (we don't know if it is, but many theorists support the idea, i.e. at BB initial conditions: t = 0, V = 0, E = 0) then is it ...
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Can nuclear fusion alone account for the energy output from type 1a supernova?

Unlike other supernova, which can vary in their size, Type Ia supernova are all about the same size. This is due to the fact that they are caused when a white dwarf star gains enough mass from its ...
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Is the law of conservation of energy still valid?

Is the law of conservation of energy still valid or have there been experiments showing that energy could be created or lost?
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Can I take heat from the air and convert it to electricity?

Its a summer day and the air in my house has been heated up. I could switch on my air conditioning, but then I'd be using energy from the grid in order to reduce the amount of energy in my house. ...
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Popular depictions of electromagnetic wave: is there an error?

Here are some depictions of electromagnetic wave, similar to the depictions in other places: Isn't there an error? It is logical to presume that the electric field should have maximum when magnetic ...
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Is the total energy of the universe constant?

If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
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Energy conservation and quantum measurement

Consider a particle in a potential well. Let’s assume it’s a simple harmonic oscillator potential and the particle is in its ground state with energy E0 = (1/2) ℏω0. We measure its ...
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Conservation of Energy vs Expansion of Space [duplicate]

My question pertains to a possible inconsistency between the first law of thermodynamics and the general expansion of space itself. First, the knowns: Space itself has, and continues to expand - ...
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How can a planet gravitationally capture objects?

I would expect that any asteroid or other object originating far away but passing near a planet would pick up speed and energy as it approaches, but unless it comes into contact with the atmosphere (...
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Does the amount of gravitational potential energy in the universe increase as it expands?

It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
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Can we use the stored gravitational potential energy of a building to produce power? [closed]

There are skyscrapers sitting and pushing on the ground with tremendous weight. Is it possible to convert this weight/force to harness energy to power the building? Maybe build the building on top ...
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Does a rock use up energy to maintain its shape?

A rock sitting on land, the ocean floor, or floating in space maintains its shape somehow. Gravity isn't keeping it together because it is too small, so I'm assuming it is chemical or nuclear bonds ...
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How to explain independence of momentum and energy conservation in a 2-body collision in elementary terms?

I'm trying to explain to someone learning elementary physics (16 year old) that linear momentum and energy are conserved independently in a 2-body collision. I'm not a professional physicist and haven'...
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Why doesn't a ball thrown horizontally have a greater speed upon hitting the ground than one thrown vertically if they are thrown with same speed?

The question below shows the velocity of three balls projected in different directions will be the same upon hitting the ground. Why isn't the velocity of the ball thrown horizontally the greatest? ...
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Does the fact that energy is not conserved in cosmology open the possibility of new matter/atoms being created in the universe?

Does it open the possibility of new baryonic matter/atoms being created in the universe and avoiding the Heat Death of the universe?
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Compressed Spring Dissolving in Acid

This is just an interesting question a friend's uncle asked me that I was somewhat annoyed I couldn't answer. When a material dissolves in acid there is a chemical process that causes the changes in ...
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As the universe expands, the wavelengths of photons are stretched, and energy is lost. What about electrons?

Will electrons, and other particles, also loose energy as they travel through the cosmos? They have wavelengths. Do they get "stretched"? My guess is that the EM force, somehow, counteracts this ...
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Have red shifted photons lost energy and where did it go? [duplicate]

I think the title says it. Did expansion of the universe steal the energy somehow?
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Why can't we make a perpetual motion machine by using a magnet to pull up a piece of metal, then letting it fall back down? [duplicate]

Obviously, a perpetuum mobile isn't possible by any law in physics, because energy can't be "created" or "destroyed", only transformed. This said, I've had an idea for a perpetuum mobile and can't ...
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Does it take energy to move something in a circle?

Work can be calculated by $W = Fd\cos\theta$ where $\theta$ is the angle between the force $F$ and the displacement $d$ Let's say there's a ball and a (physically ideal - no friction etc.) robotic ...
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Humans have an average energy budget of $100$ Watts, but the power radiated from the body is $1000$ Watts?

On average a human consumes around $2000$ kilocalories per day. This converts to roughly $2000000$ calories / $86400$ seconds or around $100$ joules / second giving roughly $100$ watts. But if you ...
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Where do electrons get their ever-lasting circulating energy?

We all know (or maybe know) that to move, we need to spend energy. If you want to drive a car, you gotta spend gasoline. We also know that energy can't be created (first law of thermodynamics, and ...
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What happens to light after it enters an eye

What happens to the light [energy] after it enters an eye and hits the rods and cones? I presume the energy becomes electrical, and it must be near 100% perfect, else our eyes would heat up? Or am I ...
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Does matter with negative mass exist?

Or does it exist mathematically? Is it really inconsistent with a common-sense, mathematics or known physical laws? As far as I understand, if it exists, it must be far away from the "positive" ...
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A problem of missing energy when charging a second capacitor

A capacitor is charged. It is then connected to an identical uncharged capacitor using superconducting wires. Each capacitor has 1/2 the charge as the original, so 1/4 the energy - so we only have 1/2 ...
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Which sphere is fastest? [closed]

I was doing a physics problem, and it seemed simple at first, but now it doesn't at all, and I need help. Here it is. Three identical spheres are thrown from the same height above the ground. Sphere ...
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When they say that the universe cooled after the big bang, where did the heat go?

Layman here, Stumbling through some physics stack posts and started reading the Wikipedia for the chronology of the big bang. In it, it states The very earliest universe was so hot, or energetic, ...
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What kind of energy does superfluidity use?

Liquid helium (and other similar fluids) can "climb up" the walls of their containers. Who does the work in this case, and what kind of energy does it use? I'm sure we can't make a perpetuum mobile ...
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Does a cooling object lose mass as it radiates?

Assume we have a red hot cannonball in space. It starts off with mass M at 1000K. Later it has cooled by radiation to 100K. Has the mass decreased?
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Is energy conserved when things fall into a black hole?

We all know in physics, the law of conservation of energy states that the total energy of an isolated system remains constant—it is said to be conserved over time. Energy can neither be created nor ...
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What happens to the energy in an ocean wave after it breaks on a reef or the shore?

I understand that the conservation of energy essentially says that "energy can neither be destroyed or created but that it is transferred from one form to another." Given that, on the ocean, wind ...
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Are mechanical energy of an element of a rope and energy density constant in the case of mechanical waves?

I'm confused about energy driven by a wave. Consider a sinousoidal wave moving in a rope. In my view each element $dm$ of the rope follows a simple harmonic motion in time. That means that the ...
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2answers
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How is dark energy consistent with conservation of mass and energy?

I initially thought that dark energy must in some way violate conservation of mass and energy since the component of the energy density of space that comes from dark energy is constant, and space is ...
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Where does the electricity, generated by a solar panel, go if you don't use the electricity?

I'm sorry if this question is too trivial for this Q&A forum. I am a layman when it comes to physics (though I did cover the high-school physics courses). I was wondering what happens to the ...
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Where does tidal energy come from?

Kind of an odd, random question that popped into my head. Tidal energy - earth's ocean movement, volcanism on some of Jupiter's moons, etc. - obviously comes from the gravitational interaction between ...
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Do all forms of energy fall under kinetic and potential energy?

I know that energy is recognized through motion. Even in the mass-energy equivalence a velocity is present even though it is a rest-energy (Not really sure if this would count as a potential energy ...
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Would Portal-style portals transmit gravity? [closed]

In the video game Portal, there are often puzzles which must be solved by gaining a large amount of momentum. Typically, this is accomplished by putting one portal on the ground and another directly ...
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Can an object falling in vacuum generate electricity by itself?

When an object falls through vacuum, gravitational potential energy is converted to kinetic energy. Is there some way to get electrical energy out of the equation by itself (i.e. somehow convert the ...