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 the [energy-conservation] tag if it is specifically about its conservation.

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How to calculate kinetic energy on a trajectory?

I've got this weirdly formulated task, that I try to solve: Given a curve $\gamma$ parametrised by $\eta$ where $x = \dfrac{p}{2}\,(1-\eta), \quad y = p\,\eta,\quad t = \sqrt{\dfrac{m\,p^3}{a}}\,\...
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Does burning something with higher temperature make the burning faster?

Like if I burn wood at 500 C, or 1000 C, is there a relationship? Any equation that model this? Thanks
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Heating up the exterior wall of a stainless steel thermos with air-activated heat patches to keep contents hot for much longer. Sounds plausible?

Upon being exposed to oxygen, the air-activated heating patches work as oxygen-triggered exothermic chemical reactors that can generate temperatures north of 60ºC. Some even reach skin burning ...
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Motion equation for particle in potential field with const $L$

Particle with mass $m$ moves in potential $U(r) = br^3$ write the equation of motion for const $L$ What i tried: Can i just use : $$ F=-\nabla U(r) = -3br^2 \rightarrow ma = F $$ Yet im not sure that ...
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2answers
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What does potential energy really mean?

I have a lot of doubts regarding the potential energy definitions First of all,I would try to express my Understandings(they might be wrong)regarding the issue I was told that if Work done on a body ...
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Is the temperature of light affected by color filters?

This video demonstrates how varying color filters alter the energy of photoelectrons emitted from a light source: https://www.youtube.com/watch?v=kcSYV8bJox8 But I am confused by what these different ...
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1answer
31 views

Is the force vs position graph same as force vs displacement?

Will the area under force vs displacement curve and force vs position curve both give net work done on body? if yes,I can't understand why both give work? are these two graph the same?
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1answer
47 views

Does Momentum better describe what happens when an arrow strikes a target than Kinetic Energy?

I've been researching medieval archery for many years, but I have trouble in visualizing the physical forces involved as relate to penetration. PLEASE NOTE: All help appreciated but please in words as ...
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An almost self-resonance phenomenon

Consider the following oscillatory circuit, with zero/negligible resistance: Let us employ a tiny demon who will be periodically switching between the inductors back and forth, so that exactly one of ...
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3answers
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How to calculate energy loss from falling balls?

Good evening, I am doing a science investigation about "Which surface will affect the energy loss of a bouncing basketball the most?" and I am having trouble calculating the energy loss for ...
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Find internal energy in relation to number of mole of gas

I have some equation for internal energy of some gas which is function of pressure and volume, for specified number of moles in this gas, let's say $m$. So i have: $$U(p,V)_{n=m}=..$$ Question is how ...
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82 views

Why is the magnetic force not doing work on the falling current carrying wire?

Given that the loop of wire is falling down and the magnetic flux is changing, a current is induced in the counter-clockwise direction. When calculating the force on the wire, $ Ids \times B $, it ...
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How do you calculate the real "impact force" of a low mass but fast moving object?

I have a question about "sandblasters" and further more about high velocity sand grains. Sandblasters are really efficient to get rid of rust on steel. The ejection velocity of the sand ...
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Why does energy of first excited state decrease as two finite square wells move apart?

Suppose I have two finite square wells of potential $-V_0$ and lengths $a$. Let their separation (symmetric with respect to the origin) be $b$. When $b=0$, the two square wells combine to form a large ...
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4answers
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Kinetic energy depends on $\theta$ but symmetry argument says otherwise

A free particle with coordinates as shown has kinetic energy $$T = \frac{1}{2}m\left(\dot r^2 + r^2\dot\theta^2 + r^2\sin^2\theta\dot\phi^2\right)$$ So we see $T$ depends on $\theta$. Now suppose we ...
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Does the impact cause by object on other object depend on force applied by it or momentum of that object?

Does the amount of force an object experience at a high height in a waterfall is same as the amount of force the object experience at a lower height in waterfall? Does change in kinetic energy also ...
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What's the meaning of unit of moment $\mathrm{N\cdot m}$? How do you explain it? [duplicate]

I know the meaning of units with division like $\mathrm{m/s}$ or $\mathrm{m/s^2}$ etc. they make sense, like $2\ \mathrm{m/s}$ is like the car pass $2\ \mathrm m$ in $1$ second, you know what I mean? ...
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Why does the mass-energy associated with nuclear potentials obey $m = \gamma m_0$?

Here is a thought experiment that has me confused. Imagine a particle, the nonexiston. If possible, ignore quantum mechanics and let the nonexiston be a classical particle with spooky action at a ...
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Would there be a force carrier particle in the uniform field theory?

So from what I understand, each fundamental force (with the exception of gravity unless you count the hypothetical graviton) has a force carrier particle that mediates the force. Does that mean if the ...
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Central force motion and angular cyclic coordinates

(Goldstein 3rd edition pg 72) After reducing two-body problem to one-body problem We now restrict ourselves to conservative central forces, where the potential is $V(r)$ function of $r$ only, so that ...
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Work-Energy Principle Derivation

I am currently in Mechanics I and both my professor and my book have derived the work principle in this way and I even asked about its derivation during class, but it has me puzzled. I don't ...
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2answers
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Energy consumption of UAV if hovering in wind conditions

I am a computer scientist unexperienced with physics, so this might be simple. I consider a UAV (drone - quadcopter) that has a task to hover, not move from the position where it was placed above the ...
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2answers
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Why can a magnetic dipole have an energy in the magnetic field?

It is well known that the energy of a magnetic dipole in the magnetic field is given by $$ V = - \vec \mu \cdot \vec B $$ But I wonder how there can be a concept of energy at all. You can imagine the ...
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4answers
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Average value of energy in statistical mechanics

I haven't taken any classes in Statistical Mechanics, but in studying Structure of Matter I found some ideas I'm not very familiar with, related with the average value of energy ($E$). Given a $p(E)$ ...
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Fluid Mechanics Problem Derivation (Energy Transport Equation) [closed]

I have been trying to solve the second part of this problem (when the water is drained) for days now, but I cannot manage to get to a point where I am able to get an answer. The problem is supposed ...
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1answer
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Full elastic collision in two dimensions with equal mass [closed]

I am currently dealing with a physics problem. It is stated that a pool billard ball collides with another. I guess it's a fully elastic one, it's not mentioned. Furthermore, it's stated that, after ...
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Do you have a funny Physics story? [closed]

Sometimes you need a funny story to just lighten things up. Hope you enjoy this one. In high school, my physics teacher, who hated cats with a passion, would do physics problems with cats. "The ...
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1answer
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Please help me find the error in my solution of the work done by gravity in a falling chain problem [closed]

The problem is as follows: A uniform chain of length $l$ and mass $m$ overhangs a smooth table with one third of the chain hanging off the table. Find the work done by gravity when it completely falls ...
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1answer
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How much work can thermal energy accomplish alone? [closed]

Lets say we have a flywheel, approximately 30cm in diameter weighing 10kg and at 30 degrees C. It is currently spinning in a frictionless environment at 1000rpm. How cold would you have to bring the ...
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1answer
41 views

Gravitational Potential Energy Lab Confusion

I had a lab that tested the dependence of gravitational potential energy on its position and the goal out of each exercise was to see if kinetic energy equaled potential energy. A cart was on a flat ...
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1answer
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Why Each degree of freedom, (i.e. motion in $x$, $y$ or $z$) has $\frac{1}{2} k_B T$ worth of energy associated with its random motion?

I am trying to interpret the result of $\frac{3}{2}k_BT=\frac{1}{2}m\langle v^2\rangle$ I understand that temperature is equal to the average kinetic energy associated with particle motion. $T=\frac{1}...
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Thermal resistance [closed]

The problem is the following. Let a resistance be $R$, a surface be $S$, the tempeture be $C$ and the time be $T$. $S$ is a cylinder placed on the ground, vertically (stand up), and $R$ is positioned ...
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1answer
47 views

Are hydrogen spectra guaranteed to be unique?

I was shown an interesting question that goes something like this: Consider the energy released by the transition in Hydrogen when the electron falls from $n=4$ to $n=3$. Explain how we know that ...
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1answer
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Question about ball collision in 2D and conservation of momentum & kinetic energy

I have a question regarding the collision of 2 balls (of the same unit mass m=1) in 2 dimension, please. We suppose that, right before the point of collision, the velocity of ball A was [1, 3]. The ...
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2answers
47 views

Work done in a $pV$ diagram

From what I gather, work done in a $pV$ diagram is the area under the curve. I have this figure here: The solution says the quantity of work in A is greater than that in B. I am kinda confused, aren'...
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1answer
38 views

Does $U_v=U_p$ where $U_v$ and $U_p$ is change in internal energy of system at constant volume and pressure, respectively?

Suppose a chemical reaction is happening once at constant pressure and another time at constant volume. For constant pressure, we can write $\Delta H=Q_p$ and For constant volume: $\Delta U_v=Q_v$ (...
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4answers
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Why do the bubbles in a champagne accelerate?

The bubbles in champagne are familiar to most people. They form almost exclusively at the specific points in the champagne glass, and from these they rise faster and faster. Why do the bubbles in ...
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3answers
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What is meant in condensed matter physics by a “gap” and why is it so important?

I come from a HEP background and moved to condensed matter physics. I keep seeing the word “gap” being thrown around a lot: this system has a gap, this is a gapless system, the spectrum is gapped, ...
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1answer
63 views

What to make out of this.....? [closed]

Let's set up an experiment first. Suppose there is a body of rest mass $m$ at rest in an observers inertial frame and it is accelerated to almost $c$ (speed of light) at constant acceleration $a$ with ...
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Is the law of conservation of energy broken by the side-effects of heating and cooling a liquid that evaporates and condenses?

Let's say there's a puddle of water on the ground. I use a magical device to give it enough thermal energy to vaporize into water vapor. The water vapor floats up into the sky. I then use the magical ...
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If you thermally ionize cesium does the plasma have a net charge?

Cesium is a fairly easy to ionize metal. But it were thermally ionized would the ions be of any net charge?
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1answer
27 views

Difference in energy required between flying Verically vs flying in the same horizontal plane

An insect of mass 150mg is hovering. What would be the energy required to fly straight up 10cm (against gravitational force) vs forward 10cm? Weight of insect:150mg Displacement:10cm I want to know ...
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1answer
69 views

Conflict between Bra-Ket notation and Integration

Suppose, I have a wavefunction given by $\psi(x,t)$. This wavefunction, over time, becomes $\psi(\alpha x,t)$. I've been asked to compute the final kinetic energy of this new wavefunction, in terms of ...
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1answer
44 views

How do electrons absorb energy?

I'm trying to understand how lamps work. I have read that the way this happens is when voltage is applied. The free electrons move due to the electromotive force (EMF), and when they collide with the ...
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Why does the power series method result in discrete levels of energy in the case of the quantum harmonic oscillator

Working my way through Griffiths' "Introduction to Quantum Mechanics", I found the following excerpt: The result is the same as what I'd gotten using the operators' method, and yet in this ...
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1answer
37 views

Vertical ideal spring, streched by weight, different results working with forces and energy [duplicate]

Imagine we have a vertical spring with constant $k$ and load on it (like a weighing machine) a mass $m$. Initially, the energy of the system is $0$, since the spring it's on it's original position and ...
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Work done by force due to internal pressure of liquid drop

I am extremely puzzled in the situation mentioned below, which is a problem of Excessive Pressure in Liquid Drop. This is my question: A liquid drop exerts a pressure on walls of the drop, and the ...
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1answer
70 views

Why the dispersion law have the form $\omega(\mathbf{k}_{1}+\mathbf{k}_{2})=\omega(\mathbf{k}_{1})+\omega(\mathbf{k}_{2})$?

In the book Mathematical Foundations of Quantum Field Theory by Albert Schwarz they define a particle corresponding to the Hamiltonian $H$, as a generalized vector function $\Phi(\mathrm{k})$, obeying ...
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2answers
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Confusion with Impulse and Work [duplicate]

I get Momentum and Impulse as well as Work and energy but struggle when it comes to connecting the two ideas. I understand that an objects KE can change without its momentum changing, Like in a ...
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2answers
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How to illustrate that the energy density of EM waves is NOT frequency dependent?

I can technically follow the derivation of the EM energy density expression from Maxwell's equations, as here for example. But it seems really strange that, for waves, it does not depend on frequency....

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