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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3answers
101 views

Using Gravity for Infinite energy?

Can gravity be used as an infinite energy source? What if you throw objects down, say a hill, and had a panel on the bottom that would use the force of the fall and convert it to energy? Would this ...
0
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2answers
51 views

Why can we find forces like this using energy methods?

Often to calculate forces in complex situations (usually involving complicated mathematics), I have seen various people employ energy methods. The answer comes out correct, but I can't understand why ...
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1answer
17 views

Solving a first order non linear differential equation in the specific case of the orbit of a particle moving under a central conservative force

Hello it's my first time posting in here and I hope someone in here can help me as my teachers quite dislike questions that aren't specifically in the curriculum. I'm reading About classical mechanics ...
2
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3answers
71 views

How to evaluate energy conservation for magnetization?

A permanent magnet can attract a certain magnetic material mass, giving a certain amount of energy $U_{mag}$. If we first use this magnet to magnetize $n$ ferromagnetic pieces, we would have $n$ ...
2
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1answer
48 views

Potential Energy in a System and its Relation to the Total Energy

In this answer it was stated that potential energy is a property of a system and not an individual particle. If we have two particles (1 and 2) interacting via a conservative force, we can write an ...
0
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2answers
101 views

Bernoulli equation as an energy conservation equation

I don’t want to learn and apply Bernoulli's equation. It is complicated and you need to remember a lot. I instead want to apply energy conservation. But I don't always find it easy. For example, when ...
1
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1answer
78 views

Is throwing a ball upwards an isolated or non-isolated system?

We have to write a lab report on an experiment where a ball is thrown upwards and it's motion is detected by a motion sensor, i am just confused whether the system is isolated or non-isolated
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1answer
52 views

Gravitational PE lost v Elastic PE gained in mass - spring

If a spring has a load $m$ added to it (and so is extended by $x$), the gravitational potential energy lost by the mass will be $mgx$. The elastic potential energy gained by the spring is ...
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1answer
68 views

Lever Mechanical Advantage

In the attached image the force output of the plank is calculated to be the work done (FDf) divided by the height the block rises (Dw). So the force the plank applies on the block is (FDf)/(Dw). But ...
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1answer
80 views

When to use a whole vector approach vs an energy approach?

My professor has just introduced two new ways to solve projectile motions. One approach involve using trigonometry and vectors and the other involves using the idea of conservation of energy. My ...
0
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2answers
156 views

In a nuclear reaction, where does the energy go?

Lets say two hydrogen fuse together, where does the energy released go? Is it carried away as momentum imparted on the helium atom? Is it carried away in neutrinos? Is it carried away as gamma rays? ...
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3answers
202 views

What happens to a particle and antiparticle that collide?

Matter can never be destroyed, so what happens to those particles? Do they just disappear? Where does the mass go?
2
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2answers
103 views

Why is an $LC$ oscillator lossless, but $C V^2 / 2$ energy is lost to a capacitor connected to an ideal voltage source?

It is mathematically proven that in an $LC$ oscillation that all the energy gets transferred from the inductor to the capacitor and vice versa. There is no energy loss as there is no load in the ...
0
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1answer
100 views

Quantum field theory with constraint: energy-momentum conservation?

Suppose I have a 2-form field $B$ and a Lagrange multiplier field $\lambda$, then the Lagrangian $S = \int (B \wedge \delta B + \lambda \delta B \wedge \delta B)$ with a Lie derivative operator ...
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2answers
169 views

Energy loss in Capacitors

We have this formula in our textbook for loss of energy when two capacitors are connected together. They mention that it is due to heat dissipation. However, we have not considered any such term in ...
2
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2answers
52 views

Does $v^2=v_0^2-2gh$ work if the positive axis is up and the initial velocity is down?

In the situation where the positive axis is up, the acceleration due to gravity is $g$, and the velocity is represented by the equation $v^2=v_0^2 - 2gh$. This works great if the initial velocity is ...
0
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2answers
656 views

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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1answer
151 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
54 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
40 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
560 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 ...
4
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2answers
89 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
141 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
231 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
141 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 ...
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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 ...
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2answers
144 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 ...
3
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4answers
179 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
111 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?
5
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2answers
159 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
70 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
151 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
222 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?
8
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4answers
407 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 ...
5
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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
82 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
139 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
310 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?
7
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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
93 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
75 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
96 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
101 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
164 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). ...
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1answer
556 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 ...