# 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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### If conservation of energy was wrong, how would we know about it?

Suppose you just started learning physics and you've been introduced to conservation of energy and kinetic energy. Apart from those concepts you know next to nothing. Then you observe an inelastic ...
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### Can a hybrid vehicle ever be more efficient than a hydrocarbon-only vehicle built with the same parts?

Based on the laws of thermodynamics, shouldn't it be theoretically impossible for a non plug-in hybrid vehicle to ever be more fuel-efficient than a vehicle that connects the same engine directly to ...
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### Does unitarity imply conservation of energy?

Not too long ago, someone began to discuss the thinking and motivation behind the Lagrangian and its formalism for the Newtonian framework and an intuitive understanding of such formalism. Somehow, it ...
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### Perfectly centered break of a perfectly aligned pool ball rack

Imagine the beginning of a game of pool, you have 16 balls, 15 of them in a triangle <| and 1 of them being the cue ball off to the left of that triangle. Imagine that the rack (the 15 balls in a ...
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### Why does destructive interference not stop a wave?

In this picture, the two waves keep moving even after undergoing destructive interference.
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### Conservation of energy in Lenz's law

When we bring opposite poles of two magnets together, they attract each other (or vice versa). Now, we can say that the kinetic energy gained by the magnets is due to the attractive force. Similarly, ...
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### Change of kinetic energy of a rock lifted by a force greater than its weight

A rock is lifted for a certain time by a force $F$ that is greater in magnitude than the rock’s weight $W$. The change in kinetic energy of the rock during this time is equal to the A. work ...
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### Why is all of the energy from a battery stored on an inductor but only 50% on a capacitor?

I am learning about inductors and capacitors and we derived the energy stored on a capacitor to be 50% of that delivered by the battery. We did this considering a circuit of a capacitor connected to a ...
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### Why doesn't this capillary action generator work?

So I was doing a bit of reading. Apparently the obstacle to generating energy from the forces driving capillary action is breaking the surface tension at the top of a capillary tube. It is just ...
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### How to tell if the collision is elastic or inelastic?

I'm a programmer and a game developer, not a mathematician or a physicist. So please go easy on the math :) I know two things: How to find the new velocities of two objects after an elastic ...
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### In wave motion of a string both kinetic energy and potential energy are minimum at $y=y_\text{max}$ then why does the string comes down again?

In wave motion of a string both kinetic energy and potential energy are minimum at $y=y_\text{max}$ then why does the string comes down again? As everything in tries to attain lowest energy possible ...
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### What is happening to rotational kinetic energy when moment of inertia is changed?

I know this question is asked here a lot, but I just had to ask this to finalise the concept. When a system lets say a rod of length $L$ and mass $M$ is rotating with angular speed $omega_1$ its ...
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### Relationship between height and velocity in conservation of mechnical energy

I'm a high school physics student, and we recently did a lab on the conservation of energy where we measured the speed of a marble at varying heights on a rollercoaster track. We were supposed to ...
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### Recovering energy from a modern, magnetic-levitated flywheel

A modern flywheel rotor is suspended in a vacuum by magnetic bearings. This means that nothing touches the rotor as it spins. When time comes that we need to recover that stored kinetic energy, how do ...
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### Does a planet/star itself lose mass when it 'emits' gravitational waves?

Orbiting planets and stars can create gravitational waves, as seen with the LIGO. But are they also losing mass-energy. Beside that, the kinetic energy associated with their orbital angular momentum ...
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### Conservation of Energy and Momentum Regarding Forces - clarification needed

The other day, my teacher stated something along the lines of, "Conservation of momentum is not violated by the actions of internal forces, but the conservation of energy is violated. Energy is ...
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### Would a thinner atmosphere (such as high altitude) yield better solar panel energy generation?

I am curious. If you were to put a solar panel up in high altitude or on the surface of the moon, both of which have much less atmosphere to reflect/refract light, would that solar panel produce more ...
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### What causes more damage, a head-on car crash at 30km/h or one car at 60km/h crashing into a stationary one?

Which will cause a greater impact? Two identical cars colliding head on (in opposite directions), each going at $30\;\mathrm{km/h}$ OR One car travelling at $60\;\mathrm{km/h}$ colliding into a ...
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### What evidence is there for additional dark energy coming into existence when space increases?

As I understand since cosmological constant is a 'constant' - increasing the space must generate additional dark energy that fills that space This does sound counter-intuitive to me, but I am sure ...
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### Is any energy required to deflect an asteroid, with force always perpendicular to its trajectory?

Let's assume there is an asteroid traveling on a straight line (far from any gravitational source), and we need to deflect it from its actual trajectory, so we build a rocket motor on the surface and ...
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### How much power would it take to stop a bullet with a magnet?

If a 7.5 gram copper-jacketed lead bullet (say, a 9x19mm Parabellum) was travelling at 360 m/s, how much power would it take to diamagnetically stop it in the space of one meter? This question comes ...
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### Vacuum energy and perpetual motion

The part of the Einstein equations of general relativity referred to vacuum energy, introduce a repulsive term in gravity. This means that as the space become bigger and bigger, vacuum part become ...
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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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### Conservation of energy and wavefunctions

I'd appreciate a bit of clarification on how conservation of energy works in QM. The infinite square well has a set of stationary states, each corresponding to one of the discrete energy levels of ...
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### Maximum reading after dropping a mass onto a scale

If one drops an object onto a scale, the needle will temporarily spike to a high reading. Given a mass $m$ dropped from a height $h$ above a spring scale, is there a good way to approximate what the ...
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### What specifically is incorrect about the Dirac Sea interpretation?

So taking the square root of $E^2 = (m_oc^2)^2 + p^2c^2$ yields two solutions. The Dirac Sea treats the negative solution as an infinite space of electrons with negative energy. All the observable ...
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### How does a flywheel generate electricity at a constant voltage?

I believe I am missing something simple here. My question concerns flywheel energy storage. Say we have stored some amount of energy in a spinning flywheel. The flywheel is attached to a generator. ...
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### Does energy conservation apply to Casimir effect?

If you cancel out some quantum field modes using two 'Casimir' plates you decrease the average energy density in the region and gain potential energy in Casimir force approximately proportional to the ...
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### How is it possible that the earth has the concept of energy equilibrium: the same amount of energy comes in and out?

I was reading this article on the Earth's Energy Budget, as well as a few other articles that basically contained the same information: That there must be an equilibrium of energy coming into the ...
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### Velocity of an satellite in an elliptical orbit

Consider some some satellite of mass $m$ around earth, traveling in an elliptical orbit about a focus (earth). Let's suppose at one point $A$ the satellite is a distance $a$ units away the focus at ...
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### Virtual particles/quantum tunneling - conservation of energy?

I'm confused as to how the above phenomena can take place since arent they breaking the law of conservation of energy (even, if temporarily)?
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### What would happen if energy was conserved but phase space volume wasn't? (and vice-versa)

I'm trying to understand the relationship between the two conservation laws. As I understand, Liouville's result is a weaker condition: it relies merely on the particular form assumed by Hamilton's ...
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### Total Energy of the Universe? [duplicate]

I've heard the total energy is zero, but I've also heard it cannot be said to be zero since there's so much unknown stuff in the universe. Is that true?
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### what happens when I roll a gyroscope along its axis of spin

Say: I have a gyroscope that is spinning in the xy plane along the z axis. I then roll its spinning axis by some angle theta Now I know the gyroscope will resist my attempting to change its axis of ...
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### Conservation Laws in Photon Parametric Down-Conversion

As Wikipedia explains, one photon passing through a crystal sometimes down-converts to two photons. Wikipedia says total energy and momentum are conserved by just considering the three photon states; ...
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### Energy conservation in radioactive decay

To show my current understanding, I'll use alpha decay as an example and list my questions at the end. Could you please correct me if I'm wrong. An alpha particle forms in the parent nucleus. It's ...
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### Is ADM energy (momentum) conserved?

Energy is a slippery concept in GR, but in an asymptotically flat spacetime there is a perfectly reasonable concept of energy (indeed, several) which goes by the name the ADM energy:  E_\mathrm{ADM} ...
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### Will extracting power from ocean tides affect the moon's orbit in the LONG run?

Conservation of energy says that any energy extracted from our ocean's tides would have to come from somewhere. Since these tides are a gravitational effect due to the moon, it would seem that the ...
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### How can we explain the difference in change of kinetic energy, due to different frames of reference?

Imagine a ball ($m= 1\,{\rm kg}$) moving at a velocity $2\,{\rm m}/{\rm s}$ towards a wall. When it hits the wall, it suddenly stops, thereby liberating all its ${\rm KE}$ as heat. Here, the initial ...
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### I need help figuring out what is wrong in this aspiring perpetuum mobile

Credits: My question is motivated from a question from another user (https://physics.stackexchange.com/q/143377/), I just reformulated what I think he tried to ask into, what seem to me, simpler terms....
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### How to model energy loss in a rotating body?

I recently asked a question about modeling instability in a rotating rigid body. I now realize that I was mentally confounding two different effects: The "Dzhanibekov effect" in which a rigid object ...
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### Do particles keep on emitting energy?

Accelerated electrons emit electromagnetic waves, but those at rest do not. So accelerated electrons emit energy and those at rest do not. (At rest hypothetically, I know it is not permissible ...
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### Conservation of energy in quantum teleportation

Consider the quantum state teleportation protocol of Bennett et. al. How does one prove that this protocol would never violate the conservation of energy? At the face of it, it doesn't seem to be ...
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### Is this short paper correct?: “Heating the coffee by looking at it. Or why quantum measurements are physical processes”

I've just found the following paper in arXiv written by a Spanish politician who is also a physicist. In it he claims that you can setup a sequential Stern-Gerlach-type experiment in such a way that ...
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### A paradox to Lenz's law

I have read that in simple words, Lenz's law states that: The direction of current induced in a conductor is in such a fashion, that it opposes its cause. This validates law of conservation of ...