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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With two balls connected to a string find minimum upward velocity that can be given to one of the balls such that the other leaves the ground

The exact question is given below: Two identical small balls A and B each of mass m connected by a light inextensible cord of length l are placed on a frictionless horizontal floor. With what ...
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Is Many Worlds Interpretation consistent? [closed]

How can Many Worlds Interpretation (MWI) be consistent, if it claims to be deterministic, while explaining energy conservation of worlds" by using Born's probabilistic rule?
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Where does the energy when a photon hits an electron but the electron doesn't doesn't gain enough energy to change level? [duplicate]

Sorry if my question is a little wordy. I was wondering the energy given by a photon goes if the electron it collides with doesn't gain enough energy to move up an energy level?
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Destructive interference of waves [duplicate]

If 2 waves have the same frequency and amplitude moving over each other but one of them is ahead of the second so that the phase difference is pi "half wave length" so a perfect and complete ...
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2answers
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Does work expend a permanent magnet?

My understanding of a permanent magnet is that it has the potential to do work. This would seem to me to imply it has a kind of magnetic charge similar to how a battery is charged with energy. Does ...
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On energy conservation [duplicate]

According to conservation of energy,energy can neither be created nor be destroyed then where did the energy come from? Because there is no creation of energy we can't create
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How do electrical motors obey the conservation of energy?

Feynman introduces the concept of an electromagnetic motor with this diagram. I believe I understand why the wire turns when there is current through it, because $F = q(\mathbf{E} + \mathbf{v} \times ...
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Maximum speed in a spring-mass system

I am studying energy right now and I can use only gravitational potential energy, elastic energy and kinetic energy to solve some problems. My doubt is how can I prove that the maximum speed in a mass ...
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Conservation of Mechanical Energy in Collisions

The Law of conservation of Mechanical Energy states that in a closed system with no non-conservative forces acting, the energy of the system will always remain constant. This makes sense for an ...
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Problem with Noether Theorem to prove that energy is conserved

Suppose an action $S = \int _{t_1}^{t_2} L(q(t),\dot{q}(t))$ that is invariant under an infinitesimal constant time translation $t \longrightarrow t' = t + \epsilon$, of course with $\epsilon = ...
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Deriving $Q_{1}-Q_{2}=W$ for a perfect gas that had gone through a Carnot cycle

I saw multiple times that if an engine, say Carnot engine, absorbs heat $Q_{1}$ at $T_{1}$, does work $W$, and expels heat $Q_{2}$ at $T_{2}$, then we should have$$\tag{1}Q_{1}-Q_{2}=W.$$ Let us say ...
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Do Quarks violate the law of energy conservation?

Look, I am not too knowledgeable in this area, but all I know is the following. So please bare with me. Idea Since Quarks are always in pairs and are bonded together by the Strong Force and cannot ...
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Centre of mass between two objects remains at rest if it was at rest initially given that external force=0

My question is If there was a superdense particle of mass $M/2$ kept at some distance $h$ from a planet of mass $M$ and radius $R$. If the superdense material is suddenly dropped and if we assume ...
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Can Increment / maximization of Entropy be the “Cause” behind any phenomena?

I understand that the increment/maximization of Entropy (of the universe) is "Accompanied" with all "Natural" phenomena we see. In many of the questions, I and others have asked on Stack Exchange, ...
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Conservation of energy in collision if A hits B but fails to move B

If body A hits body B but fails to move B what happens to its kinetic energy? There is no increase in potential energy. So what is accounting for this loss?
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How is mass-energy conserved during false vacuum decay?

If we live in the false vacuum, there is a probability that spontaneous vacuum decay could occur: quantum fluctuations could result in a bubble of true vacuum that expands at the speed of light. In ...
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3answers
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How can the energy be conserved, as negative work done on a moving body also wastes energy stored in my muscle? [closed]

Suppose a particle like object is moving on a frictionless surface. If I push it back to reduce its kinetic energy, it also wastes energy stored in my body. Is energy really conserved here? Again, ...
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What gives an object its gravitational energy?

To apply some force we need to spend some energy, like to move an object distance $\mathbf s$ with a force $ \mathbf F$ we must spend $ \mathbf F \cdot \mathbf s $ energy. Now, every object has a ...
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A doubt in the concept of potential energy [duplicate]

The concept of potential energy is best understood with examples, so let me give an example: There lies a mass of $1~\mathrm{kg}$ on ground, I lift the mass to a height of $1~\mathrm m$. Since I ...
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2answers
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Where does the potential energy come from for a charge?

The potential energy of a charge which is brought near another charge (of the same sign) from infinity by applying an external force which is equal to the electrostatic force of repulsion. The ...
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1answer
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Questions about a pendulum

A pendulum is swinging. I am reading about the way it moves (regards the law of conservation of energy), and if I understand correctly, the max speed is reached when the pendulum passes through the ...
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Projectiles on a horizontal plane [closed]

If I projected any thing with angle $\theta$ with the horizontal on a horizontal plane, will the initial speed be equal to the final speed (neglect air resistance)?
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Is conservation of energy violated when two bodies move closer due to gravitational pull? [closed]

Suppose 2 objects are placed not too far away. These objects start moving towards each other due to gravitational pull but there is no application of external force. So why do These objects move? ...
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What happens to the energy when two waves completely interfere destructively? [duplicate]

this question has been asked many times and none seem to completely answer or i am unable to understand the answers So let us consider two mechanical transverse waves traveling in opposite directions ...
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Slingshot around a black hole, dipping into event horizon

Suppose we have an object swinging by a black hole. My understanding is that if it remains outside the black hole, it just swings by and doesn't lose any energy (except maybe gravitational waves?). ...
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Which is more fundamental - a system's tendency to minimize it's energy or a system's tendency to maximize it's entropy?

The idea of the Minimum Energy Principle is the driving force behind Maximizing the Entropy or to have Maximum Entropy is a driving force behind a system's tendency towards Minimum Energy? Both seem ...
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What does a universe look like where light cannot lose energy and is emitted all the time?

In a universe where light cannot lose energy, and is emitted all the time, what is the result?
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Is it possible to conserve the total kinetic energy of a system, but not its momentum?

It is possible to conserve momentum without conserving kinetic energy, as in inelastic collisions. Is it possible to conserve the total kinetic energy of a system, but not its momentum? How? To ...
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When two waves undergo complete destructive interference then what happens to the energy of the two waves? [closed]

https://physics.stackexchange.com/q/23953 Here the author of the answer mentions that complete destructive interference acts as a mirror . As the potential energy of the ...
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1answer
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Pendulum speed at angle $\phi $, expressing $d\vec{l}$ in terms of angle [closed]

Problem statement: Pendulum swings at angle $\phi = \phi_{0}$ , and $v_{\phi_0} = 0$. We want to find the velocity when it swings at an angle $\phi$. Attempt at problem: By work energy theorem $$ \...
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Problem: find power required to keep metal attached to electromagnet

Suppose I need to find out the efficiency of electromagnet (solenoid) mounted on the table. I energize it so that $1\text{lb}$ steel object gets attached to it. I energize it with DC current from a ...
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What is a reason for the energy to not conserve in QM perturbation theory?

Consider the transition rate of the evolution of the state $|i\rangle$ to a state $|f\rangle$ - the Fermi golden rule: $$ d \omega_{if} = 2\pi |\mathcal{M}_{fi}|^{2} \delta(E_{f} - E_{i})d\nu, \quad \...
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Why doesn't the hydraulic lift violate the law of conservation of energy

Suppose a force $F_1$ is applied to the left side of the piston of a hydraulic lift and the displacement is $d_1$. So the force on the right side is $F_2$ and the displacement is $d_2$. So $$F_1 d_1=...
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Why is it called “Conservative” field?

I understand -mathematically- what a conservative field is; it is a vector field that can be expressed a gradient of some scalar function OR it is a vector field whose line integral is path ...
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1answer
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Find the potential energy of this object when it is on the ground of the Earth [closed]

A mass $M=1kg$ has potential energy $U=0$ when the distance to the Earth is $\infty$. Find the potential energy of this object when It's on the ground of the Earth. (The potential energy of the object ...
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Moment of Inertia for a car

I'm in a physics class in which we calculated the moment of inertia of a ball rolling down a ramp using a conservation of energy equation: $${1 \over2}mv^2 + {1 \over2}Iω^2 = mgh$$ where $ I $ is the ...
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Where does the power delivered to car's wheels go?

Okay, so power is work/time. Most cases, when power is provided to something, energy is gained as kinetic energy or lost to friction. But in a car, the engine puts power ( torque x rpm/5252) to ...
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Question About Collisions Between Neutrons

I came across this question from AP Classroom that asked something along the lines of, Neutron A was traveling at velocity: $v$ and has wavelength $\lambda$. It then hit neutron B and after the ...
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1answer
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Does the total energy of a system equal the sum of the energy of its products when discussing a particle decay?

So say I have a particle decay $$a \rightarrow b+c$$ I am aware that by conservation of energy the initial energy and final energy should be equal. $$E_i = E_f$$ However does that also mean that ...
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1answer
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How energy is conserved in destructive interference of plane waves? [duplicate]

Let us consider two plane waves of same amplitude and frequency but having 180 degrees phase difference.Both the waves are having energy E in them which is directly proportional to the square of the ...
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2answers
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Kinetic Energy in a Conservative Field [closed]

My problem states that $\vec{F}$ is a conservative field, ie: $F = \nabla \phi$ for some scalar potential $\phi$. $$\begin{align}\vec{F}&=m\vec{a}\\ &=m\frac{\mathrm{d}^2\vec{r}}{\mathrm{d}t^...
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Is concept of entropy really indispensable? Especially when the concept of potential energy can serve the purpose?

We see that all the natural systems aspire for minimum potential energy state and we also see that all the natural systems also aspire for maximum entropy state. Now from this understanding it seems ...
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Velocitiy of the mass on the moveable spring system [closed]

Actually I am confused at the this question. Generally kinetic energy is not conservative at momentum questions if there is no elastic collision but there is no friction on the system too. Should I ...
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How energy is conserved when a body moves due to differential radiation emission?

Suppose a body losing heat by radiation in the space, a white dwarf star for example. After some time its content of energy decreases by the same amount of the radiation released. But it is possible ...
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The Feynman Lectures

Page 4-5 of The Feynman Lectures, under the topic of conversation of energy, writes this, while it tries to give a feel about conservation principle. Let us now illustrate the energy principle ...
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Why the Lagrangian doesn't have an explicit time dependence?

I have a simple question regarding an example presented by Leonard Susskind and George Hrabovsky in their book on Classical Mechanics The Theoretical Minimum. In page 151, they state: "If there is ...
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Finding the height of the slope of a rollercoster loop-de-loop with friction [closed]

I have the following diagram: I am given all the listed parameters in the diagram, plus the mass of the pink body. The red track has a friction coefficient with the body equal to $\mu$, and the loop-...
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How objects reflect green light?

I'm currently interested in energy levels and I've learned that there are energy gaps between molecular orbitals that makes things appear transparent (for visible light). Thus, objects like glass can ...
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Doesn't a commmon emitter amplifier violate law of conservation of energy?

Basically voltage and current is gained in an amplifier which ultimately results in power and energy gain? Where does the energy come from? Does it violate law of conservation of energy?
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Why is the power unchanged by a transformer?

If we have a step up transformer then the voltage at the secondary side will be more than the voltage at the primary side. Since we all know that POWER = VOLTAGE * CURRENT and because voltage at the ...