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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Energy conservation in induced emf in open loop

We have a closed loop in the influence of magnetic fields of electromagnets somewhat like this. (Kindly ignore the currents and forces shown - the coil is stationary and initially no current flows) ...
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The work due to friction given an influential force

Statement: I am trying to work through Chapter 8, Problem #70 of OpenStax University Physics Volume 1 and I cannot figure out what I'm doing wrong. The problem statement is as follows: A particle of ...
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3answers
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Are 40 Kelvin interstellar gas clouds evidence of virtual positron and real electron annihilation

Given that Hawking radiation includes separation of a virtual electron-positron pair, then why cannot it be the case that in the vicinity of a real electron it annihilates with the virtual positron? ...
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Energy convertion

Why cannot light energy and sound energy be interchanged? Why is energy converted always in kinetic? Why cannot potential energy be converted from one form to the other when energy is in mechanical ...
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Work done and change in mechanical energy

I have a hard time understanding the equation $W_{Driving} +W_{Friction} = \Delta E_{mechanical}$ Is there any derivation of it?
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Does gravity follow law of conservation of energy? [duplicate]

Law of conservation of energy states that energy cant be created nor destroyed.I dont understand how it is applicable for a object moving under influence of gravity.If some force has been applied on ...
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Is the energy of an electromagnetic wave, in electron volts, an 'average' or a 'peak' value?

Since EM waves oscillate, is the energy given in eV an average or maximum value? In a related question, can a detector (of whatever sort) 'miss' an EM wave if the amplitude is at a minimum when it ...
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Discrete energy rounding

If energy is only found in discrete amounts what happens when the amount of energy falls below the minimum individual unit? does it have a floor or ceiling function or does it matter what type it is?
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Is there violation of energy conservation when a particle transforms to its anti-particle, for example $B^0\to\bar{B}^0$, or $K^0\to\bar{K}^0$

additive information : $B^0$ and $\bar{B}^0$ don't have the same mass, $K^0$ and $\bar{K}^0$ don't have the same mass. is there energy violation? if yes, is the key explanation of this coming from the ...
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Are there any real ways at least in theory to manipulate quantum vacuum for propulsion or energy generation? [duplicate]

https://patents.google.com/patent/US10144532B2/en US Navy was granted a patent for polarised vacuum propulsion using microwave radiation and high em energy resonating cavities. It seems to be a ...
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3answers
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Work and energy - explanation of definition [duplicate]

I have a question about the definition of energy. the definition is "ability to do work". what does it mean? what is the ability that the definition talks about? what is the meaning of ability in ...
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When an electron around an atom drops to a lower state, is 100% of the energy converted to a photon?

Let's say we have an electron around atom. Let's say the electron drops into a lower electron shell. Is 100% of the energy difference converted to a photon? Does the atom recoil at all? Is the any of ...
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1answer
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Atomic model - absorption [duplicate]

I have a question regarding the absorption of light in e.g. hydrogen atoms: I didn't quite understand how the electron knows that it can only absorb photons of the right energy. I thought that when ...
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Frictional losses in flexible tubing with multiple branches opening up to a large tank

I am working on designing an experiment where I will be creating a wall-jet flow where the wall is a rotating wing. After some thought I realized I need to estimate the pressure losses due to the pipe ...
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Determining Whether a Given Hamiltonian is Conserved

in simple terms I'm looking to understand how we can tell whether a given Hamiltonian (or one that we've deduced) is conserved or not? I've tried looking at other similar questions but am not sure I ...
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Not able to clearly understand the text of a problem [closed]

I've not clearly understood what it's meant by the text of this problem. To me, it seems that I have to show why an object travels AB in less time than ACB. However, if an object travels from A to C, ...
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Find the resultant force in a motion problem with a loop

In the approximate diagram, the block with mass $m$ starts without movement in the position $P$ in the top of a hill with height $5R$. After that, the block falls down through the hill, and reaches a ...
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How is speed affected (or not affected) by acceleration by gravity?

I am a new physics student and I was reading a question posted on here earlier: Which Sphere is Fastest? It posits that three spheres thrown with the same speed at the same point (one thrown ...
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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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2answers
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What if we set the exact equality $u^2=\frac{2GM}{R}$? Escape or not?

Let an object of mass $m$ is thrown up with an initial velocity $u$ from a spherical gravitating body $A$ of mass $M$ and radius $R$. Let us assume that when it reaches a height $h$ its velocity ...
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1answer
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Why does setting kinetic energy equal not work in inelastic collision?

From my understanding, in an inelastic collision there is no loss of kinetic energy. So if we take the following problem: A 7700kg car is traveling 18 m/s and strikes a second car. The two stick ...
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1answer
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$E = mc^2$ and Binding Energy Confusion [closed]

I have been introduced to $E =mc^2$ and binding energy recently while studying nuclear physics and I have a hard time understanding the following: Mass-Energy what does $E=mc^2$ really mean, as I ...
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2answers
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Inconsistency in spring compression under falling weight

I think this is a simple enough question but I am not able to see the consistency in two different math of the same problem. Here is the problem, One has to determine the compression of the spring. $...
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1answer
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What is wrong with the energy of a particle, of which the mass increase with time?

Let a particle move in space with constant velocity $v$. Its mass is directly proportional to time: $m=\mu t$, where $\mu$ is a constant with dimension kg $\text{s}^{-1}$. A single force acts on the ...
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What makes it more likely that the universe expands instead of saying the non-expanding parts actually contract?

I have always wondered why we consider that galaxies are drifting away and not just contracting at a particle level so that locally there is no visible contraction. I mean, if gravity consumes energy, ...
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Is the universe a closed system?

Most places say it is. But according to the multiverse hypothesis, gravitons and other quantum particles could travel from one universe to another. Wouldn't matter-energy then be transferred and thus ...
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At infinite separation do charges get kinetic energy?

When charges are infinitely separated, the potential energy is reduced to zero. As we know that loss in potential energy is nothing but the gain in kinetic energy . So do charges start flowing at ...
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1answer
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Where does the kinetic energy of an object annihilated in a matter antimatter reaction go?

A bullet is moving at $0.99 c$. Now we annihilate it by having it collide with an equal mass of antimatter. (assume instant and perfect reaction) This will turn both bullet and antimatter into photons....
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Tritium decay energy too small for quark model

So $^3$H turns into $^3$He through $\beta^-$ decay, right? The energy of this reaction is only 18.6 keV. However, according to the constituent quarks model, the only thing different between $^3$H ...
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Question on energy conservation from the stress tensor of a classical scalar field

I am struggling to answer an old general relativity exam question, which is as follows: "Consider a scalar field $\phi(t,x^i)$ with potential $V(\phi)$ on a general spacetime. Its stress tensor is ...
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2answers
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Momentum of solar panel after photon is absorbed [closed]

A solar panel installed on a car of mass 1 kg lying on a friction free road. Let the efficiency of the solar panel be 100%.let the momentum of photons be $m_o$ and and the car be at rest before ...
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1answer
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Perpetual magnetic attraction

There are two magnets, M1 and M2. Let's say that there is an attractive force existing between M1 and M2. M1 is attached to a surface, while M2 is free to move. Now slow this motion down, and M1 is in ...
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Mechanical Energy Concept Problem

When an object is at rest on a smooth horizontal "surface" (floor) its potential energy $P_E$ equals zero and, as the object is at rest, its kinetic energy $K_E$ is also zero. When the same object is ...
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1answer
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How is energy conserved in an elastic collision between two unequal masses?

In an elastic collision between two masses, if one mass is much heavier than the other, then the heavier mass will continue to move with same velocity while the lighter mass doubles its velocity. How ...
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2answers
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What is the significance of maximum compression in spring?

A block of mass $m$ sliding on a smooth horizontal surface with a velocity $v$ meets horizontal spring of spring constant $k$ fixed with wall. Find maximum compression of spring. Why is the word ...
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Basic Question on Black Holes and the Potential Loss of Conservation of Energy/Mass [duplicate]

I am reading Kip Thorne's "Black Holes and Time Warps" and I have a basic question on a topic he is discussing while talking about Einstein and Eddington's disbelief in black holes. So the general ...
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Why doesn't a hydraulic lever violate conservation of energy?

Suppose I apply some force on one side of Hydraulic lift where area is less, and the fluid in the lift raises some heavier object on the other side where area is more, Now work done is $Force\times ...
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1answer
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Kinetic Energy Comparison Marble Question

A uniform marble of mass M rolls down a symmetric bowl, starting from rest at the top of the left side. The top of each side is a distance h above the bottom of the bowl. The bowl is rough enough to ...
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A problem of energy conservation in GR

The following discussion with the Minkowskian space-time. It is known that the conservation is described by $P_{\xi}=\int_{\Sigma} T_{a b} n^{a} \xi^{b}$, where $\Sigma$ is a Cauchy surface and $n^a$ ...
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5answers
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Destructive interference of light and destroying energy?

I've had a hard time with destructive interference of light, and the possibility of destroying energy. I've read countless articles here and elsewhere, leaving me with the answer of something to the ...
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1answer
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Is a rocket engine most efficient when its velocity is equal to the exhaust velocity?

I was reading Dale's answer on this question and from the answer I concluded the following: 1- When the rocket is traveling at a velocity much less than the exhaust speed, most of the chemical energy ...
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2answers
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Where does the energy for osmosis come from? [duplicate]

A thought experiment: A U-shaped tube with semi-permeable membrane at the base. The tube is completely thermally isolated from its surroundings. The liquid (solvent) is at some temperature $T$. ...
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Kinetic Energy change in particle to wall collision

The Question A particle of mass m is moving in the +x direction with a speed v. It is approaching a massive wall which is also moving in the +x direction, but with a speed u < v (see figure) The ...
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Angular Momentum vs Kinetic Energy [closed]

Suppose we have a spinning top, with angular velocity $\omega$, its speed is null and there is no force applied to it. Let $J$ be its moment of inertia. We can say $E_k = \frac{1}{2}J\omega^2$ and $L ...
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Does energy conversion happen when an electromagnet attracts an object?

I am doing science with my 6th grade son, and there is a circuit with an electromagnet (as below) When asked to describe the energy conversion when the switch is turned on, part of the answer ...
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Getting a Conserved Quantity from a Lagrangian [duplicate]

So I've been messing around with the implications of Noether's theorem, and though I conceptually get what it's saying, I'm having a hard time actually using it to retrieve a conserved quantity from a ...
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Why do I get negative height?

A baseball is thrown from the roof $50.1^\circ$ above the horizontal. The initial velocity is $11.5\ \mathrm{m/s}$. I'm trying to find how high the ball goes. I choose positive direction upwards. $$...
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Chemical energy

A physics problem I am solving has a rocket which launches and releases a certain chemical energy $u$. What is chemical energy? How would I incoporate it into the equations of conservation of energy? ...
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Perpetual Motion Machine 1: Magnetic Superconductor Loop Generating Energy with Piezoelectrics [closed]

I know there are a lot of perpetual motion machine designs out there, but I haven't found one that looks like this one. I will use this diagram. Assume the entire ring is made of a superconducting "...
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1answer
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Destructive interference in vacuum (energy conservation) [duplicate]

The following question was proposed by a student durante a lecture for a grad course in EM. What happens to the energy of an EM wave during destructive interference in vacuum, in regards to the ...