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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Prove energy conservation using Noether's theorem

I wonder how you prove that energy is conserved under a time translation using Noether's theorem. I've tried myself but without success. What I've come up with so far is that I start by inducing the ...
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2answers
70 views

Newton's Cradle: why does it stay symmetric? [duplicate]

How is it that always the same number of balls leave at the other end in Newton's cradle. I understand that the momentum needs to be conserved, but as momentum is defined as p=m*v couldn't you have a ...
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60 views

Work done according to Newton's Principia

Actually I was going through the principia, reading Newton's derivations of the properties of ellipses. Suddenly I had this question which got stuck in my mind - Throughout Principia Newton uses the ...
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2answers
107 views

Do conservation laws contradict quantum mechanics? [closed]

Take for example the double-slit experiment interpreted in the Copenhagen sense. The particle leaves as an object with mass, yet passes through the slits as a massless wave, only to collapse again as ...
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Feynman's paradox : “conservation of angular momentum or Lenz's law ?” [duplicate]

When I was in high school, out of my interest I read electro-magnetism topic from Feynman's lecture on physics. There was a Paradox question in that topic, answer of which i am seeking here... Below ...
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1answer
40 views

Velocity Related To Gravitation [closed]

Which Equation is correct $v^2=\large{\frac{2GM}r}$ OR $v^2=\large{\frac{GM}r}$? I am getting confused here.
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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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3answers
58 views

Is conservation of energy only for isolated systems?

I am thinking in the mechanical context. Everywhere I research (e.g. Wikipedia) the law of conservation of energy is stated only for the special case of an isolated system. I am wondering if ...
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3answers
37 views

Finding an object's final velocity with distance dependant force [duplicate]

I am (just for fun) trying to calculate the velocity with which the moon would impact the earth if it stopped orbiting and simply headed towards earth. I know the force is $\frac{G m_1 m_2}{r^2}$. ...
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1answer
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Can we produce energy from pure substance having considerable mass? [closed]

According to Einstein's formula, $E=mc^2$. Any substance which has mass can be converted into energy or energy is also existing in the form of mass. So I am very much interested to build a machine ...
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49 views

Change in linear momentum [closed]

Let A and B be to rigid blocks. A is stationary while B is moving with a velocity $u$. So if B collides with A. (These blocks are on a frictionless surface.) What would happen after B collides with ...
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1answer
35 views

In this energy conservation problem, why are the answers different with different units? [closed]

Really basic question, but basically I'm given a change in height in centimeters (that's how I measured it). From that, I'm supposed to find the initial kinetic energy. $KE_i = PE_f$ After doing ...
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1answer
55 views

Understanding potential energy

I am self-studying the classical mechanics using the book by Taylor, and I have a question about the potential energy. The book (pg 111) says: If all forces on an object are conservative, we can ...
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0answers
40 views

What are the conditions for the string of a pendulum to become slack? [closed]

Suppose the bob of a stationary pendulum is given a sharp hit to impart it some horizontal speed. Now what are the conditions for the string to become slack after rotating a particular angle? ...
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2answers
37 views

Why do people say that 'power flows' into or out of a component in an electric circuit?

Currently, I am learning about electric circuits, and how to apply the basic concepts of physics, like work and energy. But I keep running into the same phrase--on Wikipedia, my textbooks, ...
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1answer
35 views

Are all heaters exactly as energy efficient as each other?

I many applications, efficiency is measured in terms of how much of the input energy is wasted generating heat instead of the actual type of energy you want. For example, lightbulbs generating heat ...
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1answer
44 views

Free fall in non-uniform field

Imagine I'm a space-diver, with mass $m_1 $, 500 miles above the Earth's surface at $x_i$. I want to calculate my position, velocity, and acceleration as a function of time, accounting for the Earth's ...
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1answer
47 views

Energy conservation around a black hole

In the Schwarzschild black hole, the Killing vector "time translation" $k^a$, so that the following quantity is conserved along a geodesic: $$E = -g_{ab}k^au^b = (1 - ...
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1answer
101 views

Is Phone-Charging Plant Product Even Possible?

Bioo, a new company making the rounds in the press, claims to produce "electricity from plants' photosynthesis." Specifically, their website claims that a single potted plant with their integrated ...
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14 views

Derivation of Poynting theorem in matter

In most textbooks I have read they derive the Poynting theorem using the Maxwell's Equation in vacuum and the fact that the force density $f=\pmb{E} \cdot \pmb{J}$. Then they just generalize it ...
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1answer
43 views

What are the assumptions behind the Lagrangian derivation of energy?

What are the assumptions behind the Lagrangian derivation of energy? I understand that we're searching for a function $L$ that describes a set of physics so that solving the energy minimization ...
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0answers
34 views

Is expectation value of the Hamiltonian always the energy? [duplicate]

There are time dependent & space dependent systems (magnetic fields) and time independent (particle in a box or harmonic oscillator). In the latter the expectation value is the 'average' energy ...
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1answer
40 views

Conservation of energy and realm of possibility

The law of conservation of energy states that energy cannot be created or destroyed. Based on this principle, you can safely conclude that any effect resulting from a cause must somehow keep all ...
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1answer
36 views

Expansion of the Universe and Conservation of Energy [duplicate]

How the expansion of the universe doesn't violate the principle of the conservation of energy? If we would put a spring between two objects, the distance would get bigger between them, so would the ...
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0answers
26 views

Collision time of two particles [duplicate]

The question I am trying to answer is the following. Consider two particles of mass $m_1,m_2$, initially at rest at a distance $2a$ apart. They are attracted to each other via Newton's law with ...
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3answers
77 views

Where does the energy of a photon trying to escape a black hole go?

I've heard "light cannot escape a black hole" explained several ways. One is that if a photon inside the event horizon tries to escape a black hole it loses energy to gravity. As it loses energy its ...
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1answer
79 views

How does the uncertainty principle relate to quantum fluctuations?

I found a webpage that just kind of mentions the uncertainty principle lightly but doesn't really go into detail as to why we need it in the first place when considering quantum fluctuations and ...
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1answer
27 views

Conservation of energy in a sound wave

I have two ultrasonic transducers, an emitter and a receiver, and I'd like to know how the energy of the spherical wave is conserved. I guess the energy is proportional to its amplitude and it ...
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1answer
37 views

Why can't an ocean liner be powered by an engine that takes heat from the ocean water and eject ice cubes?

The following excerpt comes from the physics textbook Understanding Physics, by David Cassidy, Gerald Holton, and James Rutherford: A ball lying on the floor will not somehow gather energy from ...
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1answer
104 views

Can $E=mc^2$ be derived from the Minkowski spacetime metric? $s^2=x^2+y^2+z^2-(ct)^2$?

Can $E=mc^2$ be derived from the Minkowski spacetime metric? $s^2=x^2+y^2+z^2-(ct)^2$? If so, how? Can the equivalence of mass and energy be derived from the Minkowski spacetime metric? Has someone ...
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2answers
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If I spinned a fan in a vacuum it will keep spinning forever. Why can we not make energy out of it? [duplicate]

Suppose we created a vacuum and spinned a turbine inside it with some amount of force. According to newton's second law it will keep spinning as there is no air resistance, so why can we not make ...
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2answers
41 views

Why is Kinetic Energy = (-) Total Energy and Potential Energy = 2 $\times$ Total Energy?

I came across this relation while reading on the Bohr atomic model. Are there any other forces for which these relations hold good?
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1answer
19 views

Spheres collide with merry-go-round [closed]

Four spheres, with uniform densities $\rho_1, \rho_2, \rho_3, \rho_4$ and radii $r_1, r_2, r_3, r_4$, respectively, roll without slipping with constant velocities $v_1, v_2, v_3, v_4$ along tracks ...
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Why does positive work done by internal conservative forces $\implies$ decrease of potential energy?

Potential energy can be thought as the amount of work that the force can potentially do on the point because of its position. $$W=-\Delta U=U_{initial}-U_{final}$$ A positive work done by a force ...
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34 views

When should gravitational potential energy be included in the law of conservation of energy

I have a problem that says: A block of mass 0.249 kg is placed on top of a light, vertical spring of force constant 4 975 N/m and pushed downward so that the spring is compressed by 0.090 m. ...
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1answer
55 views

Interference of light waves question

We were recently asked to solve a question in class which goes as follows: In a modified Young's double slit experiment, a monochromatic uniform and parallel light beam of wavelength $6000$ ...
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2answers
58 views

How does conservation of energy work?

I understand that the law of conservation of energy states that energy cannot be created or destroyed, it can only change from one form to another. This means that the total energy before a certain ...
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3answers
505 views

Poynting's theorem - energy conservation

Poynting's theorem: $$\int_V\left(\vec{E}\cdot\vec{J}\right)\,\mathrm dV = -\dfrac{\partial}{\partial t}\int_V\dfrac{1}{2}\left(\epsilon_0 E^2 + \dfrac{1}{\mu_0}B^2\right)\,\mathrm dV - ...
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1answer
18 views

Max theoretical efficiency of a weight powering a car [closed]

Say we had a 700 gram car. There is a 2 kg weight on a launcher that is supposed to power it. What is the maximum theoretical speed it takes to travel 8 meters? Its a little hard to describe so ...
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33 views

Find the Energy release in this Nuclear Reaction (Fission)

Plutonium undergoes nuclear fission according to the equation below: The value of $x=3$. The question is to estimate the energy released in this reaction. I know the Binding Energy per nucleon ...
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22 views

Intensity of interfering light waves

I had a conceptual question above light wave interference. Suppose that two light beams, each of an irradiance $I$ interfering on an area $A$ of a screen, such that all of the light from each beam ...
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1answer
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why laser beams don't get reflected (or knocked away) when they intersect with each other?

laser beams are photons with the same frequency and the same direction, but according to the wave-particle duality, photons have mass. but if we shoot two masses and they intersect at some point ...
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How is energy conserved in a Crookes tube?

In a Crookes tube electrons are accelerated by an electric field and gain kinetic energy. If the electric field is provided by a capacitor the capacitor would be discharged by the anode current when ...
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1answer
61 views

Work-energy theorem for a system

While studying energy conservation on Morin I found this explanation about the work-energy theorem for a system. The work–energy theorem stated before is relevant to one particle. What if we are ...
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1answer
18 views

About Electric potential

when we bring a unit positive charge from infinity to a point in the electric field EF does work on the charge and external work is also done on the charge in same amount but in opposite sign. then ...
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3answers
3k views

Why do travelling waves continue after amplitude sum = 0?

My professor asked an interesting question at the end of the last class, but I can't figure out the answer. The question is this (recalled from memory): There are two travelling wave pulses moving in ...
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3answers
99 views

Car Crash Question… (Conservation of Momentum?) [duplicate]

Dumb car crash question. I think this is a conservation of momentum problem. In one scenario, two 4000 lb cars (car A and car B) each traveling at exactly 35 MPH in opposite directions have a ...
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Could we challenge the imposibility of the second law of thermodynamic by the Adiabatic Process of the first law of thermodynamic?

by this question I mean in adiabatic process there is no heat exchange, so in efficiency formula if we substitute zero then the efficiency would be 100%.
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1answer
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Mechanical energy $E<0 \implies $ the system is in a bound state?

In a gravitational field if a body has negative mechanical energy, then the system body-planet (or star) is in a gravitationally bound state. The explanation that I have in mind for that is: since ...
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0answers
25 views

Why don't big bang photons conserve mass and energy? [duplicate]

A photon from the big bang has lost most of its momentum and energy. What does it push against? Does it break the 'laws' of conservation of energy and momentum? Is there any possibility that momentum ...