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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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Does a rocket moving in a circle expel exhaust at a greater velocity?

Consider a rocket undergoing constant acceleration in a rectilinear path. Assuming the rocket is ideal (ignoring fuel mass and energy loss such as sound, radiation, vibration, etc.) a portion of the ...
Trever Thompson's user avatar
0 votes
1 answer
29 views

Why in two block system, both block tends to have same velocity at maximum potential energy when no external force is applied? [closed]

Consider a hemispherical bowl and a block placed inside it if the bowl is given velocity $v$ and given that there is no friction, the block will gain kinetic energy and then will rise to a maximum ...
Guess's user avatar
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0 votes
1 answer
21 views

Energy Transfer and Work in Action-Reaction Forces [closed]

simple but confusing for me. I hope this question doesn't close or disappear. Questions continue to arise about work and energy. Situation Description A and B exert forces on each other and do work. ...
NOH WHIREA's user avatar
0 votes
1 answer
80 views

Is it possible to lower the energy of the vacuum?

The energy of the vacuum is given by $$\sum_k \frac{1}{2}\hbar\omega_k.$$ However the frequency $\omega_k$ depends on the wavevector $k$ and some constants like the speed of light $c$, which in turn ...
eeqesri's user avatar
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-1 votes
2 answers
121 views

Is it possible there can be a non-Fourier model of string vibration? Is there an exact solution?

I am looking for a model of string vibration that does not assume the string is Fourier. Is there a Hamiltonian? The equation of motion must be a function of length and tension, not time, and it must ...
Terence Allen's user avatar
2 votes
2 answers
59 views

Do off-centre forces create additional energy?

When a force is applied to a rigid body at its centre of mass, it accelerates. Over time, that acceleration becomes velocity, which gives the body a certain amount of kinetic energy. However, if that ...
oak's user avatar
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0 votes
2 answers
63 views

Energy of a flowing fluid

A pump transfers mechanical energy to a fluid by raising its pressure. The pressure of a flowing fluid is associated with its mechanical energy. $\mathrm{Pa}=\mathrm{N}/\mathrm{m}^2=\mathrm{N}.\mathrm{...
Kakashi's user avatar
0 votes
1 answer
32 views

"Friction at a contact point", rubber-band experiment

I was working on some practice problems in my book, and the question was to describe all the energy conversions that happens when a person pulls on a rubber band and hits it on a board (any type of ...
SMK's user avatar
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6 votes
2 answers
291 views

Does life erode Earth's mass over time?

Since life converts matter to energy, and there's no natural process that does the opposite (aside from supernovas), does this mean that the mass of our planet is gradually diminishing? I asked ...
Lynx's user avatar
  • 61
3 votes
5 answers
945 views

Is the work I do on the object always equal in magnitude but opposite in sign to the work the object does on me?

According to the definition of work, the work I do on an object is equal to the force I apply to the object multiplied by the distance the object moves. By Newton's third law, the reaction force (F21) ...
NOH WHIREA's user avatar
2 votes
0 answers
53 views

If a slice of a 4 dimensional metric violates an energy condition, does the 4 dim metric violate it aswell?

I 'm currently studying analogue gravity see this paper for a review. Here a 2+1 dimensional metric is derived: $$ ds^2 = -dt^2 + (dr - \frac{A}{r} dt)^2 + (r d\theta - \frac{B}{r} dt)^2 $$ Now it ...
DifferentialgeometryCrusher123's user avatar
1 vote
0 answers
33 views

Does moving the idler in a wheel and disk CVT conserve energy or momentum? [closed]

Consider the wheel and disk CVTs (continuous variable transmissions) below. Configuration A comprises a CVT disk coupled to the "system under control" 's', whereby a control wheel 'cw' ...
Mr. Haelscheir's user avatar
0 votes
1 answer
33 views

Energy loss on increasing moment of inertia

A body connected to a inextensible, massless string which is being spun around by a man in a circular path. The rope is then slowly released by the man from his fingers and stopped at a certain radius....
Kyathallous's user avatar
2 votes
1 answer
48 views

Does gravitational redshift conserve energy?

It is claimed that redshift due to cosmological expansion doesn't conserve energy. Does this exception also apply to gravitational redshift? Why or why not?
Derek Seabrooke's user avatar
2 votes
0 answers
48 views

Measurability of the energy difference of two states with different mole numbers

In H.B. Callen's thermodynamics (2nd ed. p. 18), after expaining why "the methods of mechanics permit us to measure the energy difference of any two states with equal mole numbers", the ...
Yao Wang's user avatar
0 votes
1 answer
115 views

Why is the Poynting flux not conserved across the interface between two conducting materials?

This question is part of my attempt to use an answer to another question I've posted on this site. If I have two materials with complex indices of refraction, $\widetilde{n}_0$ and $\widetilde{n}_1$, ...
Roy Smart's user avatar
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0 answers
49 views

Dark energy and conservation of energy in General relativity [duplicate]

i know that conservation of energy in general relativity has been discussed multiple times here at PE, a popular explanation on the topic is Sean Carroll's blog "Energy is not conserved" ...
FACald's user avatar
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0 votes
1 answer
46 views

How does a mass that's dropped when strung by an inelastic, slack string continue its motion? [closed]

Say a mass is connected to a light, inextensible string of length $l$. The other end of the string is fixed to a point O. If initially, the mass is kept at a horizontal distance of $\frac {l}{2}$to ...
Shakthi Weerawansa's user avatar
1 vote
1 answer
76 views

Why is the First Law of Thermodynamic related to Fluid Equation?

In Cosmology, there is a equation called Fluid Equation: $$\dot{{\varepsilon}}+3\frac{\dot{a}}{a}(\varepsilon+P)=0.$$ It is derived by taking time derivative of the First Law of Thermodynamic: $\dot{E}...
Polaris5744's user avatar
2 votes
0 answers
62 views

The $\alpha$ particle's energy inside a nucleus is lesser than the Coulomb barrier height. Justify

The $\alpha$-decay is usually explained via quantum tunnelling. This is because the $\alpha$ particles do not have sufficient energy to climb over the Coulomb barrier. But how do we know this? We can ...
Solidification's user avatar
0 votes
2 answers
64 views

How can a free electron and an ion in a plasma recombine without violating conservation of energy and momentum?

This question is the inverse of How does ionization of gas molecule affect the translational kinetic energy of the molecule? The answer to that question is that a molecule cannot spontaneously ionize: ...
Hugh Perkins's user avatar
2 votes
0 answers
49 views

Modified Joule-Thomson expansion

I was trying to solve a thermodynamics problem about the Joule-Thomson (Joule-Kelvin) expansion and I can't reach the answer. The book has the solution, but there's a term which I don't get where it ...
Facundo's user avatar
  • 29
0 votes
1 answer
48 views

Do all nonlinear systems store energy?

I would like to clarify, this question comes from my own curiosity while solving for nonlinear differential equations. I have noticed that I lack the fundamental understanding of linearity/...
Evank800's user avatar
0 votes
2 answers
48 views

Does a block's velocity becomes zero when it hits a spring?

It says the velocity of the block becomes zero at the instant of maximum compression. But does it not go with a constant velocity as it's a smooth part? It should hit the spring and then oscillate for ...
android's user avatar
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0 votes
0 answers
20 views

Energy and momentum conservation for light reflection [duplicate]

There is a radiation pressure which means that when light reflects from a surface, the surface will get some momentum/energy. If so, what is the balancing change on the light/radiation side if light ...
Damir Tenishev's user avatar
0 votes
2 answers
109 views

Are calories an appropriate measurement of energy for biological systems? [closed]

According to this source, a calorie is an inappropriate unit to quantify the energy contained in food because a "calorie is heat energy" and "humans don't absorb photons" but ...
Jon Behnken's user avatar
2 votes
0 answers
37 views

When are two object guaranteed to keep getting further and further away?

In a two-body problem, it is known (if I understand correctly) that if the specific orbital energy of the system is $\varepsilon \geq0$, then the objects must eventually escape each other. My question ...
Remeraze's user avatar
  • 125
7 votes
7 answers
4k views

Is it impossible to construct a Faraday cage that can block a *static* electric field?

I think the answer is yes. My reasoning is this: Imagine for argument's sake, we could have a charged negative source that has its field blocked by a Faraday cage. We can transport a positive charge ...
KDP's user avatar
  • 5,687
0 votes
1 answer
37 views

Origin of ideal gas fluid dynamics equations including the adiabatic index

I was given a system of equations that supposedly describes the fluid dynamics of an ideal gas. The equations are: $$ \begin{align} &\frac{\rho}{m} = \frac{p}{T} &\text{(ideal gas law)} \\\\ &...
Nico G.'s user avatar
  • 241
2 votes
2 answers
233 views

It doesn't seem like energy is conserved for "input-output theory" of a cavity

For a long time I have struggled to make sense of "input-output theory." And unfortunately, I still cannot make any intuitive sense out of it. The key equation (found in this famous resource)...
Steven Sagona's user avatar
0 votes
1 answer
62 views

What is the total mass-energy of baryonic matter?

From what I understand, conservation of energy does not apply to the total energy of the Universe, because it constantly expands and the new, created space has a constant non-zero energy of its own. ...
Quantum Wonder's user avatar
0 votes
2 answers
113 views

Paradox? Why does energy conservation appear to be violated in this simple geared system?

While answering a question about gears and levers, I found I could not get the work in to equal the work out in this simple gear system illustrated below: The gears are set up to replicate a lever, ...
KDP's user avatar
  • 5,687
0 votes
1 answer
37 views

Does the energy of a photon in comoving space change?

Assuming a flat FLRW universe that is expanding: In comoving space, does the energy of a photon decrease or stay constant? A physical argument for this would be nice.
Matrix23's user avatar
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1 vote
1 answer
19 views

When an element near the star surface absorbs the discrete frequency of star's em radiation do this absorbed photons ought to accumulate?

We all know about emittion lines that are empty from a discrete em freqency depending on the element that absorbs that frequency band near the star which we observe when looking at these astrophysical ...
Krešimir Bradvica's user avatar
0 votes
0 answers
51 views

Despite tides the Moon, as far as I know, doesn't slow down. Where does the energy come from? [duplicate]

Hy, I don't have anything to do with physics, so please forgive me if I explain my question in a weird way ;) As the Moon rotates around the earth it is creating tides and waves on the earth. IE it's ...
Jakob S.'s user avatar
0 votes
1 answer
42 views

How can we measure the kinetic and potential energy of a horizontal projectile?

Say a baseball is shot from a batting machine in a straight line (I don't know much about baseball, but hopefully this makes sense). Normally, when demonstrations on kinetic and potential energy are ...
user386598's user avatar
2 votes
4 answers
149 views

Why exactly does time translation symmetry lead to conservation of energy? [duplicate]

As far as I know (and I don't know much), Noether's theorem claims that time translation invariance of the laws of physics leads to the conservation of energy. The way I understand it is that if we ...
Parzh from Ukraine's user avatar
6 votes
3 answers
1k views

Conservation of momentum in Newton's cradle

Imagine a Newton's cradle with 5 balls with mass of each ball is $m$. In a case where two balls are dropped against three balls, if we write an equation considering that momentum is conserved, $$ 2mu=...
Yara Try's user avatar
  • 145
0 votes
3 answers
83 views

Why does my potential energy decrease when lifting a book?

I was reading morin's intro to mechanics book, and i reached the point where he was discussing work potential energy equivalence, which i find very confusing right now... basically we are analysing me ...
Aditya_math's user avatar
0 votes
1 answer
139 views

How do I calculate change in flux in an augmented rail accelerator without violating conservation of energy?

Suppose I have two rails and a wire connecting them I'd like to accelerate. Then $\frac{d\phi}{dt}=Bvl$, where $B$ is the magnetic field strength where the wire is, $v$ is the velocity, and $l$ is the ...
Joseph Summerhays's user avatar
-1 votes
3 answers
106 views

Suppose a shell of a sphere formed by matter and with large radius, start afterwards to diminish the radius, give $E=mc^2$ the system stops

Thinking in classical mechanics terms but with the knowledge that $E=mc^2$ let's make the below thought: Suppose you have a shell of a sphere formed by a mass uniformly distributed over the surface of ...
George Kourtis's user avatar
0 votes
2 answers
47 views

Does all electrical energy in circuits convert to heat?

Electric power is defined as "the rate of transfer of electrical energy within a circuit" as per wiki. Its formula is given as $P=VI$. But then heat dissipated by current in a circuit is ...
SHINU_MADE's user avatar
1 vote
2 answers
75 views

Energy created by gravity

So just a thought experiment: I take my rocket and fly through space. Meanwhile I pick up some piece of debris that experiences no (big) gravitational pull. I attach it with an infinitely long rope to ...
TheBest_Kappa's user avatar
1 vote
0 answers
64 views

Are there structures or systems that can have a high angular momentun that are not made by protons and/or neutrons?

Cosmic structures such as neutron stars, white dwarfs or black holes can have high amounts of angular momentum (high spin velocities). However, these are all made by protons and neutrons (black holes ...
vengaq's user avatar
  • 2,466
0 votes
1 answer
38 views

How do I associate signs with energies when using the law of conservation of energy?

When I was studying vertical spring systems, I came across this question: A spring that has a 1.25 kg mass at one end is attached to a fixed support and left to oscillate freely. When oscillating, the ...
Rohan Rajasekar's user avatar
0 votes
2 answers
52 views

Is energy conserved on a ball sliding without friction?

If a ball is sliding without friction (down an inclined surface) is energy conserved? I can’t think of why it wouldn’t be, but my intuition on this stuff is really bad.
edster101's user avatar
-1 votes
2 answers
76 views

How can maximum kinetic energy not equal to total energy in SHM$?$ [closed]

A linear harmonic oscillator of force constant $2×10^6$$ \,\text{N}\,\text{m}^{-1}$ and amplitude $0.01 \,\text{m}$ has a total mechanical energy of $160 \,\text{J}$. Find ratio of maximum potential ...
MathStackexchangeIsMarvellous's user avatar
2 votes
3 answers
162 views

Why do objects roll down slopes at the same speed?

I had to do an online simulation in class, and for some reason an object with twice as much as another rolled down a slope at the same speed of the other object. In other words, objects with a large ...
user386598's user avatar
3 votes
0 answers
80 views

Noether's Theorem in relativistic cosmologies [duplicate]

Is Noether's Theorem valid within the context of relativistic cosmology? If not, does this mean that the universe does not conserve energy on cosmological time and distance scales?
niels nielsen's user avatar
1 vote
2 answers
158 views

Does the positive energy particle emitted from Hawking radiation directly equate to the amount of mass contained within the black hole?

With virtual particle-antiparticle pairs in Hawking radiation, one member of the pair falls into the black hole, typically with negative energy, while the other escapes, typically with positive energy....
Pinto's user avatar
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