Questions tagged [energy]

Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding [tag:energy-conservation] if it is specifically about its conservation.

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How to understand the process of the kinetic energy formula? [duplicate]

$$\frac{1}{2} mv^2 = K.E.$$ What is the purpose of $v^2$; why can't use $v$ instead? Why this question is arising because of that I learned the whole process of speed= distance/time. (I know How it ...
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Include other degrees of freedom in Maxwell-Boltzmann Distribution possible?

According to Boltzmann statistics the number of particles $n$ with a certain energy level $E_j$ in case of discrete energy levels is: $$n(E_j) = \frac{N}{Z} \cdot e^{-\frac{E_j}{k_B T}} \cdot g_j$$ ...
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What is a Hamiltonian of a System?

What is a Hamiltonian of a System? When learning about Hamiltonian for first time it is an object introduced as Legendre Dual Transform of Lagrangian of the same system. And we learn further that it ...
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Why a Box moving at Constant Velocity isn't doing Work?

This was a question from an A-Level past paper and the question is pretty straightforward. I was in a hurry so I couldn't think much on this problem and chose $C$ as my answer, checked it on the ...
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Which puck has the greatest kinetic energy at the finish line?

The question is: "Two pucks one with mass 3kg and the other with mass 1kg are pushed along a frictionless table with the same horizontal force from rest from the same starting point to the same ...
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How do I understand Kinetic energy formula? [duplicate]

$$\frac{mv^2}{2}= Kinetic Energy$$ Can you explain me? What is purpose of $v^2$, $mv^2$, I am trying to understand the formula.
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A Spontaneous Reaction with greater ordering

I wanted to ask a question regarding entropy in a reaction. I was given the reaction $2H_{2}$ (g) + $O_{2}$ (g) $\rightarrow$ $2H_{2}O$ (l) and asked: the overall process is spontaneous but ...
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Max. Entropy = Min. Energy?

If a closed system has a submaximum entropy, then we can assume that in theory we can extract energy and thus the system goes to the maximum entropy state and no free energy is available. On the other ...
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Equipartition theorem - concerning the square dependence of energy

So the equipartition theorem states that if the energy dependence is square ($\langle\,E\,\rangle= as^2$ + ...(something not dependent on $s$)) then each variable (degree of freedom) contributes ...
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Derivation for relationship between rest mass, kinetic energy and momentum

I am trying to find a particle's rest mass in terms of its kinetic energy and momentum. Is this following derivation correct? $(E_{0}+K)^2=E_{0}^2+(pc)^2$ $2E_{0}K=p^2c^2-K^2$ $2m_{0}c^2K=p^2c^2-K^...
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Work-kinetic energy theorem for time-dependent force

Does the work-kinetic energy theorem hold in all the cases? Specifically is true for a time-dependent force?
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Energy stored in system or field?

I am having difficulty in understanding whether fields store momentum and energy or particles store them or both fields and particles store them? System of potential energy In the above source it ...
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Energy extraction from the layers of sea water?

Could it be possible/reasonable to extract Peltier thermoelectric energy by using a long rod of suitable material that connects the cold bottom of the ocean and the warm surface layer?
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Can non-conservative field store energy when work is done?

Taking a non-conservative field like magnetic field (in presence of current density), when work is done can it store energy? If so how?
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A doubt regarding Planck's quantum theory [duplicate]

I am a beginner and I have a problem regarding Planck's Quantum Theory The following is one of the postulate of Planck's Quantum Theory 1) The energy of each quantum is directly proportional to ...
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4answers
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Does freezing point depression also affect the speed at which something melts?

If you have a pure water frozen, then it should begin to melt if you apply 33°F air to it. If you have impure water that's frozen, it's melting temperature might have dropped to 30°F. If you apply ...
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Can you predict how much fuel you need to launch a rocket based on conservation of energy?

I was just wondering if there was a way to calculate how much fuel you need for launch, possibly to do with chemical energy to kinetic energy. Thanks
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Are conservation of mass and conservation of energy *independent* constraints for physical systems?

I'm modeling fluid systems and want to use conservation of mass (aka 'continuity') and conservation of energy as constraints to help solve for certain system parameters. But it occurred to me that the ...
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1answer
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Why is it easier to ride a bicycle than to run? [duplicate]

While it is very hard and energy-consuming to run a marathon, covering the same distance by bike is doable by most people. Where exactly does the gain come from? Assuming for simplicity that we are ...
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1answer
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Kinetic energy always time independent?! Where is my mistake? [closed]

I have some problems understanding the Lagrangian and the Hamiltonian formalism. Those can be condensed in the following "derivation" of $\frac{\partial T}{\partial t} = 0$ from the equation $\frac{\...
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Confusion on kinetic energy quadratic forms and eigenfrequencies

I am new to the idea of expressing kinetic energy in terms of the quadratic form. I noticed that online, people often express the kinetic energy as: $$T = \frac{1}{2} \dot q^T M \dot q \tag{1}$$ ...
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1answer
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Mass required to accelerate to relativistic speed

What would the rest mass of a rocket need to be if it carries a payload of 1kg and is accelerated to 1/2 c and then decelerated to a stop. Assume: 1 - energy required is obtained via fusion of ...
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A spacecraft orbiting the sun charges a battery which becomes a tiny bit more massive. How does this effect the velocity of the spacecraft

According to E=mc² the spacecraft which stores the Energy from the sun becomes a tiny bit more massive. how does this effect the impulse of this spacecraft. Would it grow because P = (m + extra mass) *...
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In the tunnel through the Earth thought experiment, how does total energy of a particle vary when it goes from surface of the Earth to its center?

In the typical tunnel through the center of the Earth thought experiment, a particle at the surface would oscillate between the two diametrically opposite points ignoring the rotation of the Earth. ...
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Energy consumption [closed]

If the yearly worldwide energy consumption is 80,000 TWh, and a particular earthquake generates 1.6x10^13 Joules of energy. If we could harness all of the energy from the earthquake, for how long ...
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1answer
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Projectile coming to a full stop at impact versus breaking through and continuing on

In the following scenario neglect any deformation forces within materials and assume the collision is inelastic. There are no instances of rebounding of the projectile at impact. There is no flying ...
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4answers
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Does energy exist, or is it just a relationship between other fundamental things?

Could anyone tell me what energy really is? I searched for it, and some people said that energy doesn't exist physically and it is not fundamental, but it is a relationship between other fundamental ...
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Why is the entropy production inside the system zero for a reversible process?

For a reversible process, why is the production of entropy within the system equal to zero?
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What is the relativistic energy of a bounded static particle?

Premise: The speed of light is set $c = 1$. Let's consider an electron in an external electromagnetic field. Its four-momentum will be $$p^{\mu} = (E, \bar p) = (\gamma m_e, \gamma m_e \bar v),$$ ...
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What happens when amplitude of light increases?

As $E=hf$ and $\mathrm{amplitude}^2 = \mathrm{energy}$ of an EM wave, if I increase the amplitude does that then affect wavelength and frequency to keep $c$ constant? I assume so since high energy ...
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139 views

Do calculations require energy?

A typical computer takes some energy "from the wall" while operating. I know that energy is spent on: Heating various components (psu/vrm/cpu/ram/ssd, so on) Fan/air motion to remove that heat (also ...
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1answer
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How can internal energy be expressed as a function of any two of $p, v, T$?

In the book of Irey, Theormodynamics, the author states that (while talking about single phase substances) For a simple compressible media, we may choose our measurable independent variables any ...
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Whats the next step after commercial fusion reactors? [closed]

I'm trying to write some science fiction where they've had nuclear fusion working for quite sometime, interplanetary travel is no problem, and they're debating making the next step to a radical new, ...
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A hypothesis based on the principle of conservation of momentum

When a ball hits a wall (ground), some quantity of its momentum must be transferred to the wall at rest. If else the ball would bounce back with the exact same speed as it did when it collided with ...
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3answers
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Kinetic energy of an object

We know that motion is relative. So let an object(o) be moving with a certain velocity with respect to an object (A).If another object (B) is moving with respect to (A) at a given moment and they ...
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Value of $\beta$ in Boltzmann statistics when degeneracy of quantum states is taken into account

The relationship between entropy $S$, the total number of particles $N$, the total energy $U(\beta)$, the partition function $Z(\beta)$ and a yet to be defined constant $\beta$ is: $$S(\beta)=k_BN \...
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Elastic collision

In an elastic collision, 2 processes can be said to take place as mentioned here in the accepted answer: Why does a billiard ball stop when it hits another billiard ball head on? First by the ...
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Do delocalized electrons have different energies?

Consider a molecule with alternating single and double bonds between carbon atoms, such that multiple resonant configurations are possible, e.g. beta-carotene. While using UToko's edx course about ...
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1answer
23 views

Polar form of Newton's Second Law

Can someone please show the steps for deriving the total energy of a particle from the Polar form of Newton's Second Law?
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1answer
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Contradiction between conservation of momentum and conservation of energy [closed]

Let's say that a $2\text{ kg}$ cart is rolling (frictionless) along a surface at $2\text{ ms}^{-1}$. A suspended $2\text{ kg}$ weight is then dropped onto it from above. If we apply the law of ...
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1answer
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Work-Kinetic energy theorem derivation

So I came across this derivation in the book Classical Mechanics by Herbert Goldstein. I don't follow from the second step onwards. I understand that there's a dot product, but how do you compute it? ...
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1answer
42 views

KE in photoelectric effect

I photoelectric effect when a single photon's energy is absorbed, why don't all get the same Max $KE$. Because for an electron, there can be no loss in heat or any friction and all. Why do most of the ...
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1answer
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Clarification on temperature's relationship to container volume

My physics textbook says that if a container that contains some amount of gas is expanded then the temperature of the gas will decrease But isn't this only if the container is increased in volume ...
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1answer
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Interpretation of change in entropy and how to formulate an expression for how much energy required to mix gases.?

From statistical mechanics one cab derive the change of entropy of the mixing of two ideal gases with the result $$\Delta S_\text{mix} = - n R \Sigma_{i} x_i \ln(x_i)$$ where $n$ is the total amount ...
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Why expression of energy occurs in triplet?

I have basically two questions in mind,which are Why expression of energy occurs in triplet? Why the expressions are somewhat symmetrical? Coming to elaborated form of 1st part, we have $$U=\frac{...
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Why is photon quantized in vacuum? [duplicate]

Apparition of quantized energy levels in quantum physics is usually explained in analogy with sound waves in a box (like in music instruments): the wave has to satisfy the boundary conditions and ...
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1answer
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Vibrations in Stones and Crystals [closed]

I have read studies done that indicate certain sounds can either disrupt or reinforce the molecular structure (I think this is the terminology used) of living organisms. This is based on the idea that ...
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$\Delta K=\Delta U$ vs $\Delta K = -\Delta U$

While doing physics homework, I noticed that in some problems, the change in kinetic energy(KE) is equal to the change in potential energy(PE), even though I learned that conservation of energy shows ...
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2answers
103 views

Physical concept of imaginary energy [closed]

I want to know that what's the physical description of complex energy? The energies have the real and imaginary parts.
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1answer
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Oscillation of 2 identical pendulums connected by a rubber band

While solving A level past papers, I came across the following question. For reference, this is the Edexcel GCE A2 Physics Paper 2 from June 2018. What does not make sense in my mind is the fact that ...