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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174 views

Why don't we have free energy generators? [closed]

Why can't you create energy out of nowhere For me it's not enough that a smart guy for hundreds of years said so. Some scientists and religious people said that the earth was flat - untill someone ...
3
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1answer
56 views

Problem - Demonstration of electromagnetic energy conservation

I want to solve the following problem from my Classical Electrodynamics' book: Consider a pontual mass body, $m$, with charge $q$, which moves with an uniform velocity $v$ in a region where an ...
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0answers
27 views

Power exerted by the contact force on a wheel slipping without rolling?

I was taking an old (2008) Mechanics of materials examination, and the system is a sphere on a half circular motionless rail. We first study the motion of the sphere in the case of rolling without ...
0
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1answer
50 views

Where is the energy coming from in a see saw?

In this video: https://www.facebook.com/mytopfm/videos/10152482869686147/ two people are on either side of a see saw going quite high. As they continue to fall they keep seeming to go just as high. ...
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1answer
663 views

How do I derive the formula of coefficient of restitution?

I have learnt in collision mechanics about the term Coefficient of restitution, $\mathrm{e=\frac{v_{2f}-v_{1f}}{v_{1i}-v_{2i}}}$. But the sad part is that, in my book only the formula is there. My ...
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2answers
93 views

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 ...
2
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1answer
158 views

Vector Potential Oscillating E Field of the “Null” Field of a Hertzian Dipole?

The vector potential of a Hertzian dipole falls off spherically as $1/r$. The polar axis of the dipole is a "Null" field -- meaning no electric and magnetic field. The absence of magnetic field is ...
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1answer
261 views

Increase in kinetic energy of a system when there is no external force

If a man starts running on a boat with an acceleration $a$ with respect to the boat, there is no external force that acts on the Boat+Man system (assuming friction due to water to be zero and ...
3
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2answers
142 views

Can light absorb energy?

Everybody knows that all physics systems absorb light quanta, any material thing can absorb light quanta. But the reverse question is , can light absorb energy and therefore changes its frequency? If ...
0
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1answer
86 views

Is Voyager 2 slowing down?

Voyager 2 will leave the dominating gravitational influence of the Sun for a region where other stars have comparable influence on the craft. By the time it gets that far away, how much will it have ...
0
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2answers
161 views

Work done by reaction forces between objects

Assume that there are no friction forces. If we had a particle sliding down a wedge that is free to move on a smooth surface, why do we ignore the work done by the reaction forces on both the particle ...
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1answer
61 views

How Is Universal Energy Conserved Here? (Cosmology)

There are three scenarios I will like to discuss here. 1- Jupiter and other "failed star" planets. These are gas giant planets which are much much bigger than earth (100 times or more, by mass) but ...
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4answers
183 views

if a body is stopped by means other than friction, where does energy go?

Suppose we have an object in space. say gravity is negligible, and the thing has a rocket on both sides. the thing fires one rocket for, say, 5 seconds. now it's moving. The object has kinetic energy. ...
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1answer
119 views

Do particles lose energy when they emit gravitons?

Is everything in the universe very slowly decaying due to gravity, do particles lose energy when they emit a graviton and gain energy if they happen to absorb a graviton?
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2answers
186 views

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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2answers
3k views

Do black holes violate the first law of thermodynamics?

When a black hole absorbs matter is it destroying that mass, thereby destroying energy, therefore violating the first law of thermodynamics?
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2answers
71 views

Electricity from vacuum [closed]

Can I generate electricity in complete vacuum without using any conventional sources? Using Casimir theory, according to him, he kept two plates nanometers apart in a vacuum. According to the law of ...
2
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2answers
155 views

(Local) Conservation of Energy in Quantum Mechanics

Generally, we say that conservation of energy is a local law; the change in energy in some small region of space is equal to the energy flux out of that region. However, in quantum mechanics, we can ...
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2answers
263 views

Temperature rise during friction

Why does the temperature increase when the load increases during friction? Why does the temperature increase when the contact time increases during friction?
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4answers
431 views

Is work done by a pseudo force?

If a body is viewed from the frame of another body which is itself accelerating, will work be done by the pseudo force acting on the first body in the frame of second body ( provided the first body is ...
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4answers
114 views

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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1answer
90 views

Where extra stored energy of Capacitor go ? [duplicate]

Suppose We have a capacitor with capacitance $\ C $ and charge $\ Q $ . So total stored energy is $$ E=\frac{Q^2}{2C} $$ Now if I connect a capacitor with same capacitance parallel with it then ...
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5answers
168 views

Does work done require interaction between system and surrounding?

There's no work done for a person climbing upstairs because the energy is converted to PE within system only. The person is the system. How true is the above statement?
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2answers
326 views

Vertical circular motion: minimum velocity

In vertical circular motion while finding the minimum velocity at the bottom-most point for looping the loop, why do we take the tension at the topmost point to be zero?
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3answers
2k views

Impossibility of time travel due to energy conservation?

I just watched the movie Terminator Genisys (It wasn't as bad as people say) and started pondering time travel. While pontificating and stretching my limited remembrance of AP Physics, science ...
3
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1answer
100 views

4 momentum in particle physics, collision of positron and electron

If I have a positron striking an electron at rest to create 2 pions( + and -) and I want to calculate the minimum kinnetic energy that the electrons can possess to create these pions, then the created ...
2
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1answer
80 views

What is minimum total energy?

I thought that the energy for a system is constant. But in a question in my physics book we have been asked to determine the minimum total energy from a graph of potential energy vs position? What ...
0
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1answer
109 views

Does energy exist? [closed]

None of this is going to be rigorous or have a right answer, so please don't take it as though I'm demanding one. I'm just interested in everyone's thoughts. It feels to me (and possibly you) that ...
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2answers
108 views

If energy isn't globally conserved, can we extract useful “free” work?

Previously, we discussed why energy is not globally conserved under general relativity. It seems counterintuitive to me, however. Does this mean we can extract useful work from this "free" energy? ...
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2answers
180 views

Conservation of Energy vs Conservation of Momentum in Rotational Dynamics

It is clear to me why angular momentum is always conserved, and how in some cases energy is not necessarily conserved within the system (in those cases where bodies deform, or friction is involved). ...
0
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1answer
659 views

Convective boundary condition

Consider a fluid over a sheet which is placed horizontally along the x-axis. The lower face of the sheet is in contact with another fluid at temperature $T_f$ ( it is heating the sheet). The sheet is ...
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1answer
99 views

Acceleration orthogonal to movement direction

I've been told that acceleration orthogonal to an objects movement direction doesn't require energy. Thus when a satellite goes around the earth, the change in direction caused by gravity doesn't use ...
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2answers
176 views

How did people discover the conservation of mechanical energy?

I want to write about the history of energy, and am focusing on mechanical energy at the moment. My question is: Can I attribute the conservation of mechanical energy to the power of maths, ...
0
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2answers
119 views

Can matter be formed from nothing? [closed]

Well, I wanted to know whether matter can be created from nothing? Could matter be created in pure vacuum or does it require some energy? If energy is required, How is energy converted to matter? If ...
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1answer
162 views

Finding the V(speed) of a 12KG bucket in pulley with the law of conversation of energy [closed]

A system of two paint buckets connected by a lightweight rope is released from rest with 12kg bucket 2m above the ground floor. Use the principle of conservation of energy to find the speed with which ...
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3answers
334 views

Infinite acceleration

Let's say we have two planets at a stand still within reasonable distance of each other. They will accelerate towards each other and subsequently collide. If instead we give them a sufficient (but ...
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1answer
102 views

Slingshot Energy into traveled distance [closed]

If I have a slingshot and I am told that when pulled it has about $E$ joules of energy, how do I compute the height at which an object of mass $m$ would travel if the slingshot is released in the ...
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1answer
81 views

How can we account for a friction force changing sign in a conservation of energy equation?

I want to solve a simple mechanical problem whereby three forces act upon an object. $F_1 = (10 - x)$ N $F_2 = -3 N$ (friction force) $F_3 = -10x$ N So at $x$ = 0 we see that $F_1$ + $F_2$ + $F_3$ ...
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2answers
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seperate engines working vs single engine

What is more fuel efficient: Two identical cars running at 50mph or one being tied to the other and only the first dragging both at 50mph? Or what requires less fuel: Two cars to start from stop and ...
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2answers
420 views

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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5answers
266 views

What happens to the kinetic energy of a dropped ball when it comes to rest on the ground?

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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2answers
1k 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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3answers
87 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 ...
7
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1answer
250 views

Lorentz invariance, energy-momentum conservation & the locality of interactions

I have been reading these notes ("Minkowski Spacetime: A Hundred Years Later", by Vesselin Petkov) 1, in which the author states (in the middle of the text on page 137) that "The only Lorentz ...
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1answer
170 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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1answer
86 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
24 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
110 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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5answers
289 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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4answers
696 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 ...