# 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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3answers
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### Mechanical energy in a body moving upwards

Why is it that mechanical energy is always conserved, I mean when an object is thrown in air, why does the kinetic energy convert to potential energy and not any other form of energy?
5answers
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### When I walk down the stairs where does my potential energy go?

When I leave my room I walk down three flights of stairs releasing about 7kJ of potential energy. Where does it go? Is it all getting dispersed into heat and sound? Is that heat being generated at the ...
0answers
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### I need to know whether these videos are fake or not [closed]

video 1 - https://youtu.be/4CiJKk5kWTo video 2 - https://youtu.be/a0sRo4ew5qA These videos are pretty convincing, but from the physics point of view... idk
1answer
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### Explain heat-pump analogy of kinetic energy transfer

Many questions on this site revolve around the seeming paradox of different reference frames for kinetic energy. A good example is this one about how walking on a moving platform contributes tons of ...
1answer
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### Pendulum with Oscillatory Support - A question on Lagrangian Mechanics

Recently I have been attempting Morin's Introduction to Classical Mechanics (2008) but I got rather stuck on question 6.3 on the topic of Langrangian Mechanics. Attached are the problem and the ...
3answers
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### Are these motion equations equivalent in Newtonian mechanics?

Let's say we're working with a particle being subjected to a conservative force $\vec{F}$. Using Newtonian mechanics, we know that the equation of motion $r(t)$ for the particle is found using Newton'...
1answer
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### What minimum force would you need to apply in order to do work against gravity without accelerating an object? [closed]

In this example, the object is placed on a frictionless surface. After letting go, the object travels back and forth on a U-shaped tack. Without a mass or angle to the incline, how am I supposed to ...
2answers
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### Is it possible to increase the efficiency of a propelled vehicle by extracting and using the energy from its own suspension system?

If there is a propelled vehicle and it has a passive suspension system (Springs for example): Is it possible to increase the efficiency of the system "propelled vehicle" by extracting energy ...
0answers
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### How to calculate total energy of rocket launch?

I'm trying to simulate a rocket launch and I want to check that the total energy of the system is conserved the rocket and the fuel that was burnt. I know that for the rocket the energy is the kinetic ...
2answers
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### Could energy conservation really be violated by virtual particles? [duplicate]

I was wondering about whether quantum fluctuations can really violate energy conservation. For example, as far as I know, virtual particles are a result of the Heisenberg uncertainty principle, and ...
0answers
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### Work power and energy in the case of chain [closed]

Please check my work. And my solution here. But my answer does not match with the actual answer. Where I am wrong here?
1answer
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### What does the Hermiticity have to do with the conservation of energy?

Naomichi Hatano's Non-Hermitian quantum mechanics (link here) published in PTEP 12, 2020 says that Hermitian operators are used when the energy is conserved but when it isn't then non-Hermitian ...
1answer
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### Energy conservation and Continuity for water travelling up a tube

Suppose I have a tube bent at right angle such that one end points upwards and the other end is immersed in a river where water is flowing with some velocity v. The vertical part of the tube is having ...
1answer
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### Discrepancy of the Keplerian speed by a factor when calculated with different methods

I needed to derive today the Keplerian speed for a circular motion in a gravitational field. Just for the hell of it I decided to derive it once via the forces and once via the energies and strangely ...
0answers
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### Symmetries of nature and the conservation laws [closed]

My book states that conservation laws have a deep connection with symmetries of nature. I initially found this statement meaningless. Some research taught me that the Law of Conservation of Energy is ...
1answer
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### How is energy in the universe constant? [duplicate]

We have been taught that the universe has a constant amount of energy but if energy can be converted into matter, how is it constant?
2answers
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### The so called “Energy Approach” vs “Force Approach”, when and how should they be used?

I'm new to physics and I've been trying to solve a few high school olympiad questions. I've figured that I approach the questions by analyzing the forces acting on objects and trying to induce ...
1answer
51 views

### If mass and energy can neither be created nor be destroyed then how in nuclear fusion mass is converted to energy with the formula $E = mc^2$

It is said by law of conservation of mass and energy that: Mass and energy can neither be created nor be destroyed. But if it is true then during a nuclear fusion or fission reaction how is mass ...
3answers
51 views

### Conservation of energy vs. force equations in circular motion [closed]

when studying for a mechanics exam I got really confused with one of the questions in my textbook. At first glance it seemed like an obvious conservation of energy problem, however that led me to the ...
1answer
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### Doubt in getting the different result from invariant quantity in Energy-momentum four vector and energy-momentum conservation

We have an antiproton moving with kinetic energy $\frac{2}{3}GeV$ and collides with a stationary proton. The rest mass energy of both are $1GeV$. I am trying to find the momentum of the antiproton. ...
1answer
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### Energy conservation and calculating second cosmic velocity

I am curious about such a thing. While calculating second cosmic speed we consider this equation: $$E = \frac{mv^2}{2} - G\frac{M_E m}{r_E}$$ which we say must be $0$, when the satellite escapes from ...
4answers
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### Why does the light of this infrared laser become visible after reflection?

We use a 780 nm laser in our lab, and that makes it in the near infrared (IR) range. The majority of people can not see this wavelength of light. However, when the beam reflects off of an object (see ...
1answer
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### Why would a rock thrown upwards, a rock thrown sideways and a rock thrown downwards have the same final speed? [closed]

This is the full question: A girl throws a stone from a bridge. Consider the following ways she might throw the stone. The speed of the stone as it leaves her hand is the same in each case. Case A: ...
4answers
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### Rollercoaster physics! [closed]

I'm completing an assignment about roller coaster physics, however, I'm having difficulty understanding a concept. The image depicts a rollercoaster with a motor, however, I am not sure what it means ...
4answers
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### Why is energy not conserved for an impulsive force?

Why is energy conservation not valid in the case of an impulsive force? Let me give an example of a problem that concerns my question. Suppose a horizontally flying bullet of mass m gets stuck in a ...
1answer
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### How does a transfer of chemical potential energy occur in a circuit?

I am aware that simple DC circuits use a battery which is connected to several elements. In the case that the element is a resistor, I read that the circuit as a whole is an isolated system for energy ...
1answer
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### Preventing the release of potential energy

Suppose I pull on the string of a bow without releasing it. My muscles are converting chemical energy (from food) to let the bow string stay at this position but this energy is not stored in the bow, ...
1answer
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### Difference in answers when using thrust force and energy conservation

The problem I was trying to solve was this one. Using the conservation of energy, I obtained that the velocity $v$ equals $$\sqrt{\frac{(2lx-x^2)g}{l-x}}$$ In the left segment, the momentum doesn't ...
1answer
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### Is energy conserved in a geodesic?

In old school physics we know that Work done (resulf of energy) = Force × displacement. but according to Einstein, free-fall is not a result of a force but just the result of objects following the ...
0answers
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### Photons travelling in parallel and colliding (Thought Experiment)

Suppose two photons displaced by a distance $\delta$ are travelling with momentum vectors that are parallel to each other, and further assume that no other particles exist such that the photons can ...
2answers
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### If heat is supplied to a system consisting of water and ice at 0 degrees Celsius, which will absorb heat first and what is the underlying physics?

I am supposed to answer this question in terms of the Zeroth and First Law of Thermodynamics. So, I wrote this: “If heat is supplied to a system, the system will absorb the heat first. In this case, ...
0answers
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### Can one prove the conservation of all forms of energy from Newton's laws? [duplicate]

I've learnt using the Work-Energy Theorem one can define potential energy and prove the conservation of mechanical energy. However, this only works for conservative forces while some forces in nature ...
0answers
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### What mechanisms could keep Universe at 0 energy?

In my understanding is right, there is no obvious guarantee of the conservation of energy on the cosmological scale. The expansion of space-time causes the red-shift of radiation and therefore the ...
2answers
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### How can you calculate the delta-v for a Hohmann transfer using potential gravitational energy?

I've been trying to calculate the delta-v required for a Hohmann transfer. I simply assume that, as no loss of energy happens on a vacuum, all you need to calculate is the sum of potential $(-GMm/r)$ +...
1answer
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### Relation between conservation of energy and conservation of mass of a fluid flow

Let us consider the following hypotheses for fluid flow: Incompressible flow Steady flow That being stated, let us consider a horizontal pipe with constant diameter, thus constant cross-sectional ...
2answers
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### How does gravitational potential energy pertain to a single gas particle escaping the atmosphere?

What's the effective difference between a helium molecule moving at 11.18 km/s and one moving at 11.2 km/s at the edge of the atmosphere? Is the idea that, with a particle moving just below the ...
2answers
127 views

### Does this research paper prove that warp drives are impossible?

Does this preprint prove that warp drives are impossible? J. Santiago, S. Schuster and M. Visser, "Generic warp drives violate the null energy condition" It states that the NEC (Null Energy ...
2answers
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### How do photovoltaic panels obey the law of conservation of energy?

The sun shines on a photovoltaic panel and it makes electrical energy, presumably by absorbing the photons in the incident light. But where does the energy transferred into electricity come from? Does ...
1answer
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### Does kinetic energy qualify as negative energy in the wormhole of Kip Thorne and Mike Morris?

Referring to Lagrangian methods if potential energy of gravity is considered as positive energy, then kinetic energy should be considered negative. Recent publications on frame dragging could suggest ...
1answer
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### Can we get information of time using energy methods?

I faced up the following problem today: Two uniform cylinders of the same size but different masses roll down an incline, without slipping, starting from rest. Cylinder A has a greater mass. Which ...
1answer
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### Do Friedmann's equations really result in energy conservation?

I was looking into Friedmann's equations for the evolution of the universe and encountered that if one combines them they result in $$dE + dpV = 0$$ as noted here in equ. 29.11. and the author claims ...
1answer
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### How is energy conserved in case of gravitational acceleration? [closed]

If two uniform moving bodies in space are at some point accelerated towards each other due to gravity then their kinetic energies increase and their potential energies decrease and when they collide ...
3answers
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### Is it possible to get the SHO approximation of a pendulum without using energy conservation?

I tried to get the approximation for small angle of a simple pendulum using only $\sum \mathbf F = m\mathbf a$ and cartesian coordinates (that means only $x$'s and $y$'s, without $\theta$). After some ...
7answers
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### How does the energy mass equivalence not contradict conservation of energy? [closed]

As we all know energy and mass are equivalent. When I burn wood a small amount of mass is converted to energy. In theory, I could convert energy to mass. This is not practically possible, but, if I ...
1answer
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### Momentum and conservation of energy [closed]

In this problem, I am confused about whether to use energy conservation or momentum conservation? I found this in a book by Resnick and tried every possible method I could think of but could not reach ...
1answer
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### Sources of magnetic fields and violation of conservation of energy

We know that current is a source of magnetic fields, for example when there is a current flowing through an infinitely long wire, this current creates a magnetic field, now I know that this magnetic ...
4answers
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### Homework: Acceleration and momentum

A massless pulley can rotate freely around a horizontal axis. Around the pulley is attached a non-stretching string connected to 2 identical blocks that are at rest. A ball of clay is dropped from ...