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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Rolling on a track in vertical plane

A sphere of mass m and radius r is released from rest at point A on a track in vertical plane. The track is rough enough to support rolling between A and B and from B onwards it is smooth. My ...
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Potential Energy in conservation of energy [closed]

In energy conservation theorem, everywhere $PE=mgh$ is used. What I found is that change in PE= mgh. According to me, while calculating total energy, instead of change in PE, total PE should be used ...
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Question about Helmholtz's paper “ON THE CONSERVATION OF FORCE”

Below follows the exact extract from Helmholtz's paper "On the conservation of force". Let us now imagine, instead of the system $A$, a single material point $a$, it follows from what has been just ...
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Confusion regarding the conservation of mechanical energy

If it's always the case that---when all forces are derivable from a potential energy---$$\Delta T = W = -\Delta V$$ so that $$\Delta \left(T + V\right) = 0,$$ why is energy not conserved when $V$ is ...
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Is energy conserved in a Van der Pol oscillator?

The Van der Pol Oscillator is governed by a 2nd order ODE with nonlinear damping. The 'position' of the oscillator is the solution to $$x''(t) = \mu (1 - x^2(t)) x'(t) - x(t)$$ Here $\mu$ controls ...
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Is this statement true for all conservative forces or just attractive conservative forces?

'Change in potential energy is always equal to the negative of the work done by a conservative force.'
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How to calculate velocity of an object at the bottom of a circular hill if there is an energy loss due to friction?

Object slides down a circular hill and loses some of its initial mechanical energy due to friction. As an object slides down, friction increases and force in direction of motion decreases until the ...
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Mechanics Question - Bullet fired into Block

I am studying for the GRE and came across this question (Question 93 from Sample Exam 2 in Kahn and Anderson's Conquering the GRE): A bullet of mass 5 g is fired at a block of wood of mass 1 kg, ...
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How is energy conserved in this case?

Consider a permanent magnet introducing a magnetic field at some fixed angle to a loop of wire on a spindle. If a fixed current is allowed to run through the wire, the lorentz force introduces a net ...
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How does $dS \ge 0$ imply a system cannot receive more energy than work supplied?

I am reading a book that asserts the following: The second law arises from an equation of the form $$dS = \mu\,dE + \mu\,dW \tag{1}$$ where $dS$ is the change in entropy, $dE$ the change ...
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Energy in inertial frame of reference

If a body A is moving with a constant velocity v and an observer on that body A observes another body B to be at rest then the kinetic energy of B is zero. So is energy dependent on the frame of ...
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How does flow work arises in enegy balance using Reynolds Transport theorem?

I will use the below mentioned form of Reynolds Transport theorem(usually derived in Fluid Mechanics context) to give a relation between a Control mass system(no mass in or out) and Control Volume(...
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What happens to the energy of an absorbed photon?

Photons are electromagnetic wave fields that travel at the speed of light. When they are absorbed by an object, their momentum and energy is transferred to the absorbing object. What is that ...
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How should energy loss in a hydraulic jump be calculated?

The energy loss in a hydraulic jump is still calculated with the old equation of Bresse from year 1860; (I.e., equation 7 in this paper from 2017) $$ \frac{\Delta E}{E_1} = \frac{(\sqrt{1+8Fr^2}-3)^3}...
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Can this sum be done without integrating?

As by using conservation of mechanical energy, we can just be concerned about the final and initial position and the velocity could be found out. Am I not right? But I am not getting the correct ...
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Why does energy conservation give me the wrong answer in this inelastic collision problem?

Suppose there are two objects. The first object is stationary and has the next parameters: v1=0 m/s, m1= 4kg. The second object is heading towards the first one with speed v2= 8m/s and mass m2= 60 kg. ...
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Energy conservation when exchanging meson

In Prob 1.2 of Griffith's "Introduction to Elementary Particles" 2nd ed., it says that: However, I don't understand why it states that "they must temporarily violate the conservation of energy by ...
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How does conservation of momentum work here? [closed]

Imagine 2 oppositely charged particles attracted to each other, say an electron and a nucleus, assume their force of attraction is very large compared with any external applied force we're about to ...
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Do black holes violate the conservation of mass?

This question has confused me for quite some time now. I have searched it up online, and the basic answer is: 'Mass is a form of energy. When black holes die they release the amount of energy that ...
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Conceptual question regarding velocity in projectile motion vs. up a ramp

I have a question regarding conservation of energy in regards to two different scenarios: In the first case I was told that a ball with an initial velocity, V, was launched at an angle, $\theta$ and ...
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Is that reasonable to use kinetic energy and potential energy of a fluid system to define the laminar, transition and turbulent region?

This is my assumption ( I don't know whether it exists or not ): I suppose a fluid system always contains two types of energy, i.e., kinetic energy and potential energy. The kinetic energy of a ...
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Potential, Kinetic, Thermal Energy?

When things move, potential energy turns into Kinetic energy, right? Then how does Kinetic energy convert to thermal energy?
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Why can't we make a perpetual motion machine by using a magnet to pull up a piece of metal, then letting it fall back down? [duplicate]

Obviously, a perpetuum mobile isn't possible by any law in physics, because energy can't be "created" or "destroyed", only transformed. This said, I've had an idea for a perpetuum mobile and can't ...
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Do conservative forces obey Newton's laws of motion?

Do conservative forces obey Newton's laws of motion? Is the force between two charge particles a counterexample?
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How is energy conserved during simultaneous refraction and reflection of light?

My question is regarding the partial reflection and refraction of light when incident on a refracting surface. Here, the energy of Incident Ray is E=hv of Reflected Ray is E=hv and that of Refracted ...
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Help understanding a spring-mass system where mass is not attached

This is the scenario: We have a spring resting on a surface with no mass on it. Let the $y$ position of the top of the spring be $l_0$. When we put an object with some mass on top of the spring, the ...
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Could the flex in a skyscraper due to wind be used to produce power?

While browsing 'Hot Questions' on Stack Exchange I came across this question: Can we use the stored gravitational potential energy of a building to produce power? It got me thinking about the use of ...
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Conserved and invariant quantities in a nuclear reaction

For a reaction $$a+X\rightarrow Y+b$$ in the book, Introductory Nuclear Physics by Krane, he has written that the total energy and the total momentum both in RHS and LHS remains conserved in Lab frame,...
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Can we use the stored gravitational potential energy of a building to produce power? [closed]

There are skyscrapers sitting and pushing on the ground with tremendous weight. Is it possible to convert this weight/force to harness energy to power the building? Maybe build the building on top ...
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Concerning the energy stored in electromagnetic fields

How do we know that $$u = \frac{1}{2}\left(\epsilon_0E^2 + \frac{1}{\mu_0}B^2\right)$$ gives the energy density of electromagnetic fields? Is it a postulate of classical electrodynamics? Griffith ...
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Does it take energy to move something in a circle?

Work can be calculated by $W = Fd\cos\theta$ where $\theta$ is the angle between the force $F$ and the displacement $d$ Let's say there's a ball and a (physically ideal - no friction etc.) robotic ...
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Do orbiting planets have infinite energy?

I know that planets can't have infinite energy, due to the law of conservation of energy. However, I'm confused because I see a contradiction and it would be great if someone could explain it. ...
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Radiation pressure and conservation of energy

TL;DR: How can radiation pressure conserve energy, if we consider the case where the atom absorbs all the Energy of the incident photon via its newly excited electron, and stills gains additional ...
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An energy/model based controller to minimize overshoot and response time of a mass spring system

Consider a very simple system: $$m a + k x = F \, , \tag{1}$$ where $m$ is mass, $a = \ddot{x}$ is acceleration, $k$ is the spring's elasticity, $x$ is position of the mass and $F$ is the force from ...
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How does Destructive Interference redistribute energy?

I have tried looking at the many other questions on a similar topic, however, they don't provide a definitive answer to my question. What I managed to gather so far, is that if a light wave is super ...
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Applying energy and momentum conservation to the problem of pulling a bent carpet at a constant speed

Consider this system. A long, thin, pliable carpet is laid on a floor. One end of the carpet is bent back and then pulled backwards with constant speed $v$, just above the part of the carpet ...
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Harnessing molecule vibration energy

Disclaimer: I am learning physics for fun, please dont kill me but explain where I am wrong. As I understand all molecules have a lowest energy state in which they posses some amount of momentum even ...
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Why friction causes energy to be lost in terms of heat when it appears to be an energy transfer mechanicsm?

For example, when we move/walk, we apply a force (via friction) on earth, and the earth in turn on us. So essentially I see it as an energy transfer as follows: Suppose I move in same direction as ...
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Why can't a particle interaction $A + B \rightarrow C$ conserve energy and momentum at the same time?

I learned that in the Feynman diagram for electron photon scattering, the intermediate particle represented by the middle solid line must be a virtual particle because it violates energy-momentum ...
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On inserting a thin capillary tube into a large tray of water, does the total surface energy and the total potential energy of the system increase?

In both cases, the system is the liquid in the (tray+tube). I have made reasonably accurate calculations for the gravitational potential energy part. It follows from the calculations that the ...
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Energy in Many-Worlds Interpretation

A few answers on here seem to suggest to something like the following: In the Many Worlds Interpretation, the amplitude of a branch exponentially decreases over time and so the energy of that branch ...
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Unclear assumption in deriving fluid energy conservation laws

I am currently working through Alexandre Chorin's Mathematical Introduction to Fluid Mechanics. In the first chapter, he treats the change in Kinetic energy of a fluid region $W\subset D$ subject to ...
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How do we define the quantity $Q$, in the conservation of energy? And what does it rely on?

Noether's theorem to me explains how a certain defined quantity (Q) is conserved (locally) in time due to the time translation symmetry, and to be more specific; if we had a ball that is placed in a ...
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How to calculate energy (KWh) from Temperature (T),Air quality (Co2), and illumination (lux)?

I have data of temperature, air quality, and illumination.I want to calculate energy consumption (KWh) from these parameters.My data is shown in the image . Also, examples are also shown in image 2 .
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Energy can be created

Consider a device which converts kinetic energy into electrical energy ( just like piezoelectric generator). Now I install this on two trains of very high mass approaching each other (moving in ...
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Does the force on magnet depend on coil, when it is being moved relative to the coil?

I am new to electo-magnetism, so i prefer simple language, the problem is: When we move a magnet relative to a coil such that one of its face is towards or at the coil, an EMF is induced in the coil ...
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Proving that gravitational potential is work done by the object against gravity while KE increases and PE reduces

There's a person in my class who thinks that the formula for gravitational potential (-GM/r) represents the work done by gravity to move an object from infinity to any point in the gravitational field....
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How do I derive the angular frequency of a simple pendulum through conservation of energy? [closed]

Is it possible? I'm not exactly sure what I'm doing wrong. So far I've gotten: $mgl(1-$cos$\theta) = \frac12\omega^2l^2$ Which then gives $\omega = \sqrt\frac{2g(1-cos\theta)}{l}$ which is ...
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obtaining a force equation using the time derivative of energy

i saw this trick for obtaining a force equation but i don't understand why it works lets suppose our system conserves energy then $$E=U+E_{k}=const$$ $$\frac{dE}{dt}=\dot E_{k}-F=0$$ and then to ...
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gravitation (kinetic energy and angular momentum)

When a satellite in a circular orbit around the earth enters the atmospheric region it encounters small air resistance to its motion. then (1) kinetic energy increases (2) angular momentum decreases ...