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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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Force needed to pull an end of a bended carpet lying on a floor with constant speed [on hold]

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 which is ...
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Why friction causes energy to be lost in terms of heat when it appears to be an energy transfer mechanicsm?

For example, when we move/walk, we apply a force (via friction) on earth, and the earth in turn on us. So essentially I see it as an energy transfer as follows: Suppose I move in same direction as ...
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Why can't a particle interaction $A + B \rightarrow C$ conserve energy and momentum at the same time?

I learned that in the Feynman diagram for electron photon scattering, the intermediate particle represented by the middle solid line must be a virtual particle because it violates energy-momentum ...
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On inserting a thin capillary tube into a large tray of water, does the total surface energy and the total potential energy of the system increase?

In both cases, the system is the liquid in the (tray+tube). I have made reasonably accurate calculations for the gravitational potential energy part. It follows from the calculations that the ...
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Energy in Many-Worlds Interpretation

A few answers on here seem to suggest to something like the following: In the Many Worlds Interpretation, the amplitude of a branch exponentially decreases over time and so the energy of that branch ...
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1answer
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Unclear assumption in deriving fluid energy conservation laws

I am currently working through Alexandre Chorin's Mathematical Introduction to Fluid Mechanics. In the first chapter, he treats the change in Kinetic energy of a fluid region $W\subset D$ subject to ...
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1answer
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How do we define the quantity $Q$, in the conservation of energy? And what does it rely on?

Noether's theorem to me explains how a certain defined quantity (Q) is conserved (locally) in time due to the time translation symmetry, and to be more specific; if we had a ball that is placed in a ...
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How to calculate energy (KWh) from Temperature (T),Air quality (Co2), and illumination (lux)?

I have data of temperature, air quality, and illumination.I want to calculate energy consumption (KWh) from these parameters.My data is shown in the image . Also, examples are also shown in image 2 .
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Rpm of disc calculations [closed]

I shoot a bullet of velocity Vb and mass Mb towards a disc fixed on a car. The disc mass is Md and car mass is Mc. The bullet hits tangentially on the disc and gets stuck to it. The disc starts ...
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1answer
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Energy can be created

Consider a device which converts kinetic energy into electrical energy ( just like piezoelectric generator). Now I install this on two trains of very high mass approaching each other (moving in ...
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1answer
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Does the force on magnet depend on coil, when it is being moved relative to the coil?

I am new to electo-magnetism, so i prefer simple language, the problem is: When we move a magnet relative to a coil such that one of its face is towards or at the coil, an EMF is induced in the coil ...
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Proving that gravitational potential is work done by the object against gravity while KE increases and PE reduces

There's a person in my class who thinks that the formula for gravitational potential (-GM/r) represents the work done by gravity to move an object from infinity to any point in the gravitational field....
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How do I derive the angular frequency of a simple pendulum through conservation of energy? [closed]

Is it possible? I'm not exactly sure what I'm doing wrong. So far I've gotten: $mgl(1-$cos$\theta) = \frac12\omega^2l^2$ Which then gives $\omega = \sqrt\frac{2g(1-cos\theta)}{l}$ which is ...
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1answer
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obtaining a force equation using the time derivative of energy

i saw this trick for obtaining a force equation but i don't understand why it works lets suppose our system conserves energy then $$E=U+E_{k}=const$$ $$\frac{dE}{dt}=\dot E_{k}-F=0$$ and then to ...
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gravitation (kinetic energy and angular momentum)

When a satellite in a circular orbit around the earth enters the atmospheric region it encounters small air resistance to its motion. then (1) kinetic energy increases (2) angular momentum decreases ...
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Acceleration downhill, fastest trajectory for a ball

Given 3 ways of going downhill, like in this image: Would a ball behave like that in real life? Intuitively, it makes no sense. The shortest path here is not the fastest. Any hints to the math ...
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2answers
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Is Poynting vector conserved or Intensity conserved for reflection and transmission of electromagnetic wave?

When an electromagnetic wave meets a boundary, does energy conservation mean Poynting vector of reflected + Poynting vector of the transmitted wave is equal to Poynting vector of incident wave or just ...
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Transformation of energy [duplicate]

Consider a man rotating on a frictionless turntable , initially with his hands folded . When he opens his hands the moment of inertia increases and as the angular momentum is conserved the kinetic ...
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Dissipation of energy in inductors

Please explain the dissipation of energy in a circuit containing a resistance and inductor after the switched in the circuit is opened.I mean how do you explain that the energy that is across inductor ...
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1answer
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Why does the potential energy of the system decreases when two charged particles are attracted to each other?

I've been reading about lattice energy, it says that during the formation of ionic compound the ions formed due to electrostatic force are attracted to each other and libration of energy takes place ...
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2answers
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Confusion on a phrasing of the First Law of Thermodynamics

According to Finn's Thermal Physics Third Edition, the First Law of Thermodynamics is described as: If a thermally isolated system is brought from one equilibrium state to another, the work ...
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1answer
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From the perspective of QM, how can a thermodynamically isolated system can have a constant internal energy $U$?

In thermodynamics, we assume that an isolated system has an internal energy $U$, and if there is no heat given, or work done by/to the the system, this energy is constant. However, in QM, we do know ...
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Can a neutron decay into a proton + muon (+ muon-neutrino)

Can a neutron, if it has enough kinetic energy, decay like: $ \hspace{0.3cm} n \rightarrow p + \mu^- + \bar{\nu}_\mu $ I assume it's not allowed since it's not on the possible decay modes on the ...
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Why is electric power needed at all when elevator is descending?

If the ropes are doing negative work on lift, why is electric energy needed? And why is there a limit on number of passengers in a lift in this case?? Please explain in terms of energy conservation ...
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In Schwarzchild solution what happens between $r=2M$ and $r=3M$?

Numerical Relativity by Tomas Baumagrte and Stuart Shapiro page 10. By adapting $h=c=1$, so in schwarzchild solution the areal radius $r=2M$ is the event horizon, and $r=3M$ is the photon orbit. But ...
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1answer
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Is a photon emitted from Earth less energetic that a photon emitted from the Moon

Two similar looking photons (locally) are emitted, one from Earth and the other from the Moon, and they are observed at some point X out in space. On Earth, time is slightly more dilated due to ...
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1answer
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Pion decaying energy

The question is: A pion decays into a muon and a neutrino (which is almost massless, so we’ll take it to have a mass of zero). If the pion is initially at rest, find the energy of the outgoing ...
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5answers
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Derivation of the work-energy theorem

We state the following version of work-energy theorem : $$ K_2-K_1=Fd=W $$ Where acceleration is assumed to be constant, so is the force $F$. Then the physicists proceed by writing $K_2-K_1=F[x(t_2)...
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1answer
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Explain the equation describing total energy of the electrons in HET

I have this equation describing total energy of the electrons in HET (Hall-effect thruster): $$\frac{\partial (\frac{1}{2}mn(v_{ex}+v_{e\vartheta})^2 + \frac{3}{2}nT_e)}{\partial t}+\frac{\partial (\...
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2answers
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Basics of energy

I am not able to understand the second paragraph well. Please elaborate me anybody
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1answer
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Two black holes colliding (classical mechanics)

I had this question come up in my exam where two identical black holes are in orbit around each other. There is a loss of energy via gravitational waves : $$\frac{d E}{d t} = kr^4\omega^6$$ where $k$ ...
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Why is the motion described by $F = -kx^{2n+1}$ periodic?

I understand the motion when: $$ F=-kx $$ which leads to the an ODE that then has solutions expressed as: $$ x(t)=A\cos(\omega t)+B\sin(\omega t) $$ This is periodic since a cosine function is ...
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Where does the universe gets its energy from? [duplicate]

Well let's just end everything like this. All the laws made are just theories. We don't even know how the 'big bang' happen. If space has a limited space but it keeps expanding what's outside the ...
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Stuck trying to prove conservation of energy

Some notes. I've been playing around a bit with "Classical Mechanics Lite" to get a better understanding of exactly what's happening in terms of conservation laws, symmetries, and the seemingly ...
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1answer
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Conservation of energy and issue with work-energy theorem [closed]

I have a question about conservation of energy in the event that we have a force that is position dependent. Assuming we can define a potential for it, the law of conservation of energy can be derived ...
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6answers
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Conservation of momentum if kinetic energy is converted to mass

There is a moving object. Through an unspecified (science fiction) mechanism its kinetic energy is converted to mass and the object comes to rest. The mechanism is fictional but in good scifi it ...
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1answer
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3D perfectly elastic collision between two points

There is a high probability, I think, that this question is a duplicate of some other question ... but to may knowledge, it hasn't been posed in this exact manner: Assume we have 2 points, $P_1$ and $...
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Why do energy transfers always result some heat loss?

explain that energy transfers and transformations in mechanical systems always result in some heat loss to the environment. Like why is it necessary for a heat loss?
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Max velocity before toppling risk

I want to find out the maximum velocity a machine can travel before there is a risk of toppling. Say, worst case, is that the castors hit a dead stop and the machine is brought to rest '...
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1answer
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Mass and energy in special relativity [conservation of energy problem] [closed]

A certain quantity of ice at $0$ celsius melts into water at $0$ celsius and in doing so gains $1.00$ kg of mass. What was its initial mass? Now the problem with my solution is that it of course ...
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Perpetual Motion and Thermodynamics [duplicate]

How does the second law of thermodynamics disprove all perpetual motion machines?
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1answer
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Zero-energy universe - What is nothing?

I am a layman, so excuse me in advance for the stupidity of my questions, and I hope you can answer them in a way that I can understand. I have read, here and there, that the Universe might have a ...
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Can an Opposing Current Create another Opposing Current?

In Inductors when current increases it's magnetic field induces a voltage which causes an opposing current that slows down the rise of the current that initially creates it, but can this opposing ...
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1answer
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Self-induction and conservation of energy [closed]

back EMF when switch is open is greater than back EMF when it is on; is that opposes the law of conservation of energy?
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4answers
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Water mixture at different temperatures

Let's say that we mix homegeneously and instantly cold water at $t^\circ $ C and hot water at $T^\circ$ C (like in a water tap) in ratio $p:1$. My question is the following: What is the instant ...
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5answers
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Work done in lifting a mass vs work done by mass as it falls to starting position

There are variations of this question on here but I was unable to find exactly what I was looking for so please bear with me if this is old hat. If I start with a mass m at rest at height H1 and ...
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4answers
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How does an ideal transformer satisfy law of conservation?

I have read in books and articles that, keeping the power as is transformers increase the voltage and therefore reduce the current as per the formula $$ P = IV $$ and this results to reduction in ...
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3answers
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Does potential energy get transferred from my body to a wall when I push a wall and fail to move it?

When I push a rock (and fail to move it), I do not do any work and therefore there should be no energy transfer. But my teacher says that, when one pushes a rock, energy is transferred to the rock ...
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1answer
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Can I theoretically completely convert the kinetic energy of a bullet to rotational energy of a disc, when the bullet hits it tangentially?

Will the kinetic energy of bullet be converted to rotational energy of disc(assume the bullet gets stuck to the disc). Let me assume that disc is mounted on a car standing on a frictionless surface. ...
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1answer
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How is Inflation able to create an infinite amount of energy?

Follow up to this question here: If the universe is flat, does that imply that the Big Bang produced an infinite amount of energy? As I understand Inflation theory, some time after the Big Bang, the ...