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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Shawyer EM Drive — Momentum Conservation Violation

I have been following the developments around the Shawyer EM drive for about 2 years now as NASA and other parties test it out in various environments. To preface, I would very much like to see the EM ...
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2answers
1k views

Having trouble understanding the work energy principle intuitively

I'm having trouble understanding the work energy principle intuitively. This is what I'm solid on so far: If you have a ball rolling down a hill, it loses potential energy and gains kinetic energy. ...
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74 views

How to solve this pendulum problem using kinematics, not the principles of conservation of energy?

I have this question, because typically problems that can be solve using conservation of energy or just energy-related principles, can usually be solved sing kinematic equations. (At least is what I ...
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38 views

What happens to that energy?

If we want to drop a ball from a height, we calculated that potential energy at bottom is zero and we say it is converted into kinetic energy. At that movement, if it is a kind of sand, we find it ...
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0answers
46 views

Does Only Flat universe have zero energy? [on hold]

Hello I am very new to cosmology and quantum physics. I need some basic understanding (in LAYMANs term )of the following: I read that only closed universe has zero energy. But I heard Astrophysicist ...
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49 views

Is this idea practically possible? [duplicate]

I have to make a working model for my science exhibition. I have planned to make a wind turbine with some innovations. My model of wind turbine will consist of magnet it at the tip of every fan blade. ...
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5answers
156 views

Where does particle borrow energy from to tunnel?

Where does particle borrow energy from to tunnel? It is implied that particle can borrow energy and leaped over to the other side wherever that is, the shorter the gap the more energy it borrows my ...
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2answers
308 views

Why is a washing machine unable to spin-dry a duvet?

Background story: I put my duvet (synthetic filling) into a washing machine, and the machine was unable to spin-dry it. My mom told me: "It's because it is full of air". I was not satisfied with the ...
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3answers
393 views

Optimal speed for the water wheel

The hydroelectricity plants extract the potential energy of highly deployed massive object (water) as it falls down. Without turbine, all that energy would be converted into speed (kinetic energy) at ...
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15 views

Work-energy theorem? [duplicate]

The initial kinetic energy of an object moving on a horizontal surface is K.Friction between the object and the surface causes the velocity of the object to decrease uniformly to zero in time,t.How ...
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0answers
9 views

Work energy theorem problems [duplicate]

The initial kinetic energy of an object moving on a horizontal surface is K.Friction between the object and the surface causes the velocity of the object to decrease uniformly to zero in time,t.How do ...
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0answers
34 views

Solving kinetic energy without numbers [closed]

The initial kinetic energy of an object moving on a horizontal surface is K.Friction between the object and the surface causes the velocity of the object to decrease uniformly to zero in time,t.How ...
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1answer
408 views

Sufficient conditions for the energy to be not conserved?

I'm almost embarrased to ask this question because I thought I was by now very confident with classical mechanics. Someone has stated that given a mechanical system with a Lagrangian $L$ s.t. ...
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3answers
59 views

About energy… created, destroyed?:/

Consider a dam, in that first the hydraulic energy due to gravitational force gets converted into mechanical energy by using turbine. Then those mechanical energy is converted into electrical energy ...
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4answers
4k views

Proof of conservation of energy?

How is it proved to be always true? It's a fundamental principle in Physics, that is based on all of our currents observations of multiple systems in the universe, is it always true to all systems? ...
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1answer
77 views
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1answer
100 views

Example where Hamiltonian $H \neq T+V=E$, but $E=T+V$ is conserved

I'm looking for an example of a Hamiltonian $H$, where $H\neq T+V$, but the total energy in the system, $E=T+V$, is still conserved. While I'm at it, I might as well add that I'd be most interested ...
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2answers
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How is information related to energy in physics?

I recently attended a talk by Dr. Ravi Gomatam on 'quantum reality', where the speaker suggested, that conservation of energy is not a fundamental law, and is conditional, but the conservation of ...
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1answer
42 views

What is the energy-conserving delta function

I am reading about the S-matrix in QFT (Standard Model book by Burgess and Moore) and I came across the energy-conserving delta function, which is factored out of the S-matrix. I would greatly ...
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0answers
20 views

Cancelling waves and preservation of energy

In quantum physics, a particle is "defined" by a wavefunction. If you would take 2 particles with the same wavefunction, and negate one of them. They would cancel each other other out. Take for ...
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1answer
29 views

exercise involving conservation laws [closed]

Well the question says- There is a plank of mass $5kg$ and its placed on a friction-less horizontal plane and a block of mass $1kg$ is kept over it and a massless spring of natural length $2m$ is ...
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3answers
437 views

Example of law of conservation of energy

Law of conservation of energy states that "Energy of the universe remains constant. Energy can neither be created nor be destroyed, but it can be transformed from one form to another". So I ...
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4answers
382 views

A confusion regarding an example in The Feynman Lectures

In The Feynman Lectures, In the chapter entitled Work and potential energy, Feynman states: The work done in going around any path in a gravitational field is zero. This is a very remarkable ...
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1answer
63 views

How do spacecraft gain kinetic energy by passing near planets? [duplicate]

When a a spacecraft like Voyager travels close to Jupiter, it somehow gains kinetic energy. I'm not sure how this energy isn't simply lost again as it moves away from Jupiter. I'm also hoping this ...
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4answers
3k views

Why doesn't fusion contradict the 1st law of thermodynamics?

I was reading up on the 1st law of thermodynamics for my Chemistry exam and I was wondering why doesn't fusion contradict the 1st law of thermodynamics? The 1st law states that The energy of an ...
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1answer
59 views

Current on both sides of a capacitor

When a capacitor is connected to a DC circuit, what ensures that the current on both sides of the capacitor is the same? When charges arrive at one end of the capacitor they stop moving; presumably ...
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1answer
105 views

Pendulum problem where it hits a peg swings around and lands on top of it

Using energy equation from when it is released to when it reaches its highest point after hitting the peg I get change in kinetic energy = 0 and change in potential due to gravity = 0 Does the string ...
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2answers
437 views

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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1answer
43 views

first law of thermodynamics and conservation of energy

The first law of thermodynamics is referred to as a reformulation of the law of conservation of energy. I am not sure to fully understand this relationship. My way of picturing it is the following. ...
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2answers
120 views

Why does this capillary action generator not work?

So I was doing a bit of reading. Apparently the 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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2answers
59 views

Why does mechanical energy have to equal zero to find escape velocity?

A object orbiting the earth has total mechanical energy equal to \begin{align*} E^{mech} = \frac{1}{2} m v^2 - \frac{GMm}{r} \end{align*} with $M$ the mass of the earth and $r$ the distance. My course ...
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1answer
208 views

Conservation of energy when focusing black body radiation on another black body

This question about whether or not it is possible to focus black-body radiation to make something hotter than the radiation's source was answered mostly negative: the second law of thermodynamics ...
0
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1answer
130 views

Changing momentum of moving trolley

We have a trolley of mass $m$ and it is moving at a velocity $v$ along a smooth horizontal plane. It is full of water, and is leaking water at a constant rate out of the bottom of the trolley, i.e ...
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3answers
190 views

Was Joule's experiment able to show: thermal energy = $mgh$

Is my understanding of this experiment correct? There is the quantity $MC\delta \theta$ associated to the heat which I think is not an absolute value to be compared with $mgh$ since the $\delta ...
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2answers
88 views

Approximating energy loss caused by drag force

We know the usual aerodynamic drag equation is given by: $$F_d = -bv^2$$ Where $b$ here is just some constants combination related to the property of the fluid and the material passing through. My ...
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3answers
692 views

Conservation of Angular Momentum, as related to a flywheel

Trying to work out some pesky flywheel dynamics for a project I'm working on, would love some for your assistance to better understand the underlying concepts. For a given flywheel (thin-walled ...
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3answers
185 views

Does energy conservation not hold in fission and fusion processes?

I have read that during fission and fusion processes, there is some kind of equilibrium between the single nucleus and the disintegration products, so they are constantly being converted into each ...
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2answers
49 views

How does a Venturi Injector comply with energy conservation?

How does a Venturi Injector comply with energy conservation? When the water flow enters the narrower cross section its velocity increases and its pressure decreases. When the pressure gets lower ...
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33 views

What is the kinetic energy of a proton as it is falling towards the core of the star?

Given: Star's mass = $1.99e10^{30}kg$ Type II supernova occurs (in a simple form: the outer layers of the star fall into the now inactive core at speeds of 126000km/s) $10^{44}$ joules of energy is ...
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2answers
111 views

Proof of conservation of energy with the relativistic definition of force

Professor Susskind proved conservation of energy in one of his lectures by taking the classical definition of force ($F = ma$) and by showing that its time derivative is zero. How can we do that with ...
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Can the Betz limit be overcome?

The Betz Limit states that the energy extracted from a turbine can be no more than $\frac{16}{27}^{th}$ of the energy present in the wind flowing through it. Extracting more energy would make make the ...
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0answers
219 views

How do I calculate the energy balance of a trompe?

I was quite fascinated by the concept of an ancient type of air compressor, called a trompe. It entrains air bubbles into a falling stream of water via the Venturi effect, and extracts the air at a ...
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1answer
34 views

Work done by friction on a body?

I know that when a body slides over a surface, the work done by friction is not stored as potential energy in the body. It is dissipated in the form of heat. But why is it not stored as potential ...
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4answers
290 views

Do virtual particles actually physically exist?

I have heard virtual particles pop in and out of existence all the time, most notable being the pairs that pop out beside black holes and while one gets pulled away. But wouldn't this actually violate ...
2
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1answer
46 views

Spinning rubber ball with equatorial ridge

I have this rubber ball with something like a very slight equatorial ridge (sort of like Saturn's moon Iapetus) which I often spin around on my desk. I keep noticing that no matter the inclination of ...
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3answers
90 views

How is energy converted when massless spring is attached to a rigid support

Say a massless spring of length l is attached to a rigid support. It is extended to length l + x. Now at this position, the ...
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1answer
44 views

Classical Mechanics — Sign of work done

It seems that work has two possible ways to decide it's sign: Whether you take the perspective of the system or the surrounding (whether you consider work done on the system as positive, or work done ...
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1answer
74 views

A confusion regarding a paragraph in The Feynman Lectures

Under the chapter entitled Conservation of energy, under the section of Gravitational potential energy, there is this following paragraph : A very simple weight-lifting machine is shown in Fig. ...
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1answer
120 views

In a CMCS 2-body system, why does the speed of the particles after collision stay the same?

A particle $m_1$ is traveling with velocity $v$ toward a stationary particle $m_2$. The velocity of the center of mass is given as $v_c=\frac{m_1}{m_1+m_2}v$. Changing to a moving coordinate system, ...
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1answer
98 views

Isolation of a Physical System and Energy

I'm trying to grasp basic concepts of energy and i have a little doubt regarding conservation of energy. According to Feynman book and Wikipedia, the Law of Conservation of Energy states that ...