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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Any reason why local spacetime translation invariance sources perturbations that breaks global spacetime translation invariance?

In general relativity (GR), metric perturbations are sourced by the energy-momentum tensor. Given that energy and momentum are defined as the scalar and vector quantity conserved by time and spatial ...
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Violation of conservation of energy [duplicate]

Sand runs from a hopper at a constant rate $\frac{dm}{dt}$ onto a horizontal conveyer belt driven at a constant speed $v$ by a motor. The power needed to drive the belt can be calculated as follows: ...
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Confusion regarding work done by external force

Suppose a ball from outer space is moving towards earth. The force of gravity attracts the ball more and more strongly as it moves closer and closer to the earth. Now, let me apply an external force ...
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Water tower hydro generator? [closed]

$500$ gallons, $100$ feet high, at tank $10$ inch pipe, every $4$ feet a reduction of $2$ inch pipe diameter, ending in $1/4$ inch tubing to jet nozzles, each nozzle would spin an independent ...
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What would be the stopping time if there was friction when a body rotates around a cylinder by a string? [closed]

[ How would we solve this question if friction was also present between string and cylinder?
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Energy from spacetime expansion?

Are there any processes or mechanisms in the universe involving spacetime expansion where energy is produced somehow out of this expansion? I ask this in part due to this article by Sean Carroll 1 ...
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Would a warm cup with water and ice inside cool the water faster than if the cup was normal temperature?

Imagine you pour water and ice inside a glass cup (assume that the water and cup are at ambient temperature). Now, suppose that you get another cup but heat it up before pouring the water and ice. ...
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Energy and momentum conservation laws [closed]

I'm a highschooler, trying to solve this problem: A little rock of mass $100 g$ is thrown at a velocity of $20 ms^{-1}$, with an horizontal direction, and loadges into a little polystyrene box with ...
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Stopping potential and EM radiation

I'm a little confused about something We know from the photoelectric effect, that when a beam of light falls on the surface of a metal with very low ionization energy, electrons get ejected, one way ...
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Can we use a rotating fan to generate electricity and then use that electricity to run that same fan forever?

Can we use a rotating fan to generate electricity and then use that electricity to rotate that same fan forever? A rotating fan has kinetic energy. That can be converted into electricity using ...
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What happens to the muon's energy during its decay?

wikipedia (Muon - Muon decay) says that: The dominant muon decay mode (sometimes called the Michel decay after Louis Michel) is the simplest possible: the muon decays to an electron, an electron ...
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4 votes
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Two permanent magnets are allowed to attract each other. The field energy MUST decrease as they speed up. But Griffiths implies it INCREASES. How?

This is a relatively long question however I think I hit on a fundamental issue regarding the conservation of energy during magnetic attraction and how it is taught in standard texts. I'd really ...
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Substituting the conservation of angular momentum into the Binet formula results in contradiction [duplicate]

Background Information The lagrangian of a particle in a central force field $V(r)$ is $$ L=\frac12m(\dot r^2+r^2\dot\theta^2+r^2\sin^2\theta\dot\varphi^2)-V(r). $$ The particle must move in a plane, ...
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What is the maximum height a pendulum can reach? [closed]

I've been trying to solve this problem: The sphere of mass 𝑚 of a pendulum of length 𝐿 is initially held vertically. When a wind blows with a constant force 𝐹 non-conservative, show that if the ...
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Impulse and momentum (max velocity condition) [closed]

Why maximum subsequent velocity is attained when the block comes to rest relative to top of the hill? Please refer to the diagram below.
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Speed at the bottom of slopes

Let us consider two hills of the same height, with one drastically steeper than the other. Ignoring friction and air resistance, is it true that if we release a ball at the top of the hills, their ...
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Conservation of energy while swinging on a swing without slack

So I have an issue with the following classical problem: Let's say you are on a swing of length $l$ hanging downwards, and you get pushed in a horizontal direction with speed $v_0$. How large should ...
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How does conservation of energy relate to cosmic microwave background radiation?

I've seen posters on a number of forums, including this one, telling us that conservation of energy doesn't apply to cosmic microwave background radiation. This is of some concern because, for ...
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Will a spinning magnet slow down or lose its magnetism? [duplicate]

In an otherwise empty universe, a bar magnet spins pole-over-pole, emiting a magnetic wave (and also an electric wave (electromagnetism)?). Since emitting an electromagnetic wave requires energy, it ...
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Does gravitational potential energy exist?

If an object is taken at a height $h$ above the ground, its potential energy is $mgh$. When it is then released from there, it falls freely due to the force of gravity and its kinetic energy increases....
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Why does a rocket engine that produces a constant thrust over a set period of time have less energy if it has more mass? (Zero-$g$) [closed]

A rocket engine with the thrust of 1N working for 10 seconds will add more kinetic energy to the rocket if it is attached to a 10kg rocket and less if it is attached to a 20kg rocket. The rocket ...
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Energy conservation or work-energy theorem? [closed]

Let's assume a block of mass $m$ is allowed to go down from rest on an inclined plane. There is a spring at the bottom on the inclined plane with inclination $\theta$ as shown in the figure. We want ...
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Conditions of work-energy theorem

The work-energy theorem states that the net work done by all forces on a particle is equal to the change in kinetic energy of the particle. I have a few queries on this theorem: Do the timing of the ...
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What the black body radiation stand for?

i'm having hard time to figure out what is happening in a black body in thermodynamic equilibrium. Let me tell what i know so far: by definition the black body emits and absorb all radiation from and ...
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How to get $v=\sqrt{2gh}$ using Lagrangian mechanics? [closed]

If I have a ball that is height $h$ and mass $m$ then its gravitational potential energy $= mgh$ where $g$ is the gravitational constant. It's kinetic energy is $= \frac{1}{2}mv^2$. I wish to use ...
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Conservation of energy with metastable pure states and doubts about energy-time uncertainty relation

Experimentally we know there are metastable states in nuclear physics and molecular physics. For example, the isotope Tantalum-180m ${}^{180m}\text{Ta}$ is long-lived metastable excited state, with a ...
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Where to use conservation of energy? [closed]

I have seen many problems in physics where the solution starts wih "using conservation of energy, we get" without telling anything about why do we start with like this, My concern is how do ...
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3 votes
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Proving the time derivative of Hamiltonian is zero [duplicate]

So I know that in Noether's theorem, it is stated that $$\frac{d}{dt}\left(\frac{\partial L}{\partial \dot x}\dot x - L\right)=0$$ where the Hamiltonian $H$ is defined as $$H = \frac{\partial L}{\...
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Cosmic strings increasing internal energy as the Universe expands?

I was reading an article by Edward Harrison, which tackles the problems of conservation of energy at cosmological scales. At some part (point 2.4) he cites several article, including one by Rees and ...
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Gravity train with the help of batteries?

Imagine a gravity train (https://www.math.purdue.edu/~eremenko/train.html) that goes through Earth's crust from point A to B. The train could advance through the ride just by the force of gravity and ...
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Energy in $RC$ Circuit

In circuit A, the total energy dissipated in the resistor is $\frac{Q_i^2}{2C}$ which equals the initial energy, meaning that all the energy was dissipated in the resistor, and lost as heat. Here's my ...
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Why energy conservation implied $\partial_{z} T_{z\bar z}=-\partial_{\bar z} T_{zz}$?

The conformal in variance in 2d CFT required $T_{z\bar z}=0$ (the vanishing of the trace $T_a^a=0$) and the conservation equation were $\partial_{\bar z} T_{zz}=\partial_{ z} T_{\bar z\bar z}=0$, ...
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Closest distance of approach [closed]

Suppose a charged body $q_0$ is at rest. Another charged body $q_1$ at infinite distance starts moving towards $q_0$ with velocity $v_1$. Gradually the velocity of $q_1$ will decrease and that of $q_0$...
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Does laser trap atom cooling change the electron shell levels?

Do the electron orbits change during the cooling of the atom in a laser trap? When cooling down, do the electrons come closer to the nucleus of the atom or do they move away? Or do the electrons leave ...
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Total rotational energy of oval-shaped object [closed]

I am analysing the rotational energy of an oval-shaped ball (after taking a series of videos of a rugby ball bouncing), through the following 3 axes of rotation...: ...I calculated the angular ...
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How did Noether use the total time derivation to get her conservation of energy? [duplicate]

I was informed by @hft that by combining the total time derivation: $$\frac{dL}{dt} = \frac{\partial L}{\partial x}\dot{x} + \frac{\partial L}{\partial \dot{x}}\ddot{x} + \frac{\partial L}{\partial t}$...
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Electromagnetic stress-energy-momentum tensor and stress tensor

The purely spatial components of the energy-momentum-stress tensor for a perfect fluid are clearly related the components of the ordinary stress tensor inside the fluid. My question if this case is ...
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Yet another perpetual motion machine: does this imply large selective membranes are not possible? [closed]

So I was just thinking about the buoyant force and came up with what seemed like a simple perpetual motion based on it. Obviously such things are not physically possible so I'm trying to figure out ...
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Does a water heater consume more energy if constantly kept on than if it is made to heat the water on demand?

Suppose I have a water heater, an insulated tank of water that when turned on heats water to a predefined higher than ambient temperature and keeps it at that temperature. Suppose I don't need the ...
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Where does the excess energy go in this problem?

Two identical plastelin balls were suspended on a non-extensible weightless string with the same length L, which are fixed at the same point. One of the balls was deflected 90 degrees from the ...
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How conservation of energy looks like in the moving frame?

It is obvious that a car accelerates by converting its chemical energy in the fuel to produce kinetic energy to accelerate itself. If the energy is lost into friction and heat, the car will slow down. ...
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Why did Noether use the Lagrangian for her conservation of energy theorem?

So I know that for Noether's conservation of energy theorem, the Lagrangian is used. However, I know that the Lagrangian doesn't always equal energy. So why did she use the Lagrangian and not other ...
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How is a motorcycle whip possible?

For clarification on what I'm talking about, please see this video link at 0:40 https://youtu.be/YAUOFy1QyiE?t=40 The riders appear to be breaking the laws of physics by rotating one way, and then ...
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On energy conservation and time-ordered interaction diagrams

I've found the following lectures given by professor Mark Thomson (here's his particle physics web page); in the third one, Interaction by particle exchange and QED, it's stated that time-ordered QM ...
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Is the total gravitational energy in the sun greater than the energy that is produced by the sum total of the nuclear fusion contained therein?

I think this is off topic, maybe an astronomy question. Unlike a chemical chain reaction or a fission chain reaction I believe fusion cannot sustain inself. Is more energy going out than is going in ...
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Why do we give so much importance to energy, i.e., the conserved quantity under time symmetry?

In almost all equations—from GR to QFT—energy conservation is a tool for solving those equations, but we know that energy on large scales is not conserved. Why do we still use this (not) conserved ...
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Are one-dimensional bifurcations dissipative or conservative?

I have elementary questions about one dimensional bifurcations. It should be remarked that I have also searched and read previous posts of Physics stack exchange. However, these did not help me with ...
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Piano on the moon

Imagine a piano on the moon. You strike a chord. Since there is no atmosphere, there is no medium for the sound to travel. so where does the sound energy go? Does it just dissipate as another form of ...
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Energy conservation in a system of two bodies

I have no idea why my previous post was marked as a homework question where the solution was clearly visible. I was asking to clear a concept of mine regarding system of bodies. However,i will keep ...
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Where is the excess energy going to when a solar inverter is operating a photovoltaic (PV) panel away from the maximum power point (MPP)?

The MPP is the maximum power point of a solar panel and a PV inverter will typically try to find this MPP to yield the maximum energy ouput. But sometimes it might be neccessary to operate the panel ...
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