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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Conservation of energy when focusing black body radiation on another black body

This question about whether or not it is possible to focus black-body radiation to make something hotter than the radiation's source was answered mostly negative: the second law of thermodynamics ...
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17 views

Spinning, decelerating coin on a table

A coin spun on a table will eventually end up flat on the table, ignoring the cases where it remains balanced and comes to rest on its edge (or falls off etc). Before the coin comes to rest on its ...
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29 views

Inelastic Collision and Energy

How is is possible for momentum to be conserved and for KE to not be conserved? Momentum is related to velocity and velocity is related to KE, therefore if KE was lost, how can the system have the ...
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622 views

Conservation of Angular Momentum, as related to a flywheel

Trying to work out some pesky flywheel dynamics for a project I'm working on, would love some for your assistance to better understand the underlying concepts. For a given flywheel (thin-walled ...
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1answer
40 views

Alternatives to the neutrino in the beta spectrum?

It is well known that the neutrino appeared to explain the power distribution in the beta decay spectrum. (see PSE) What other explanations candidates were available? It is stated that the ...
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14 views

Explanation to; Energy loss from a vertically bouncing mass on a spring [on hold]

Basically i have been assigned this title in my physics coursework of the (IB) and im struggling to get started. I have read my way through the internet, with ought any success. I hope one of you ...
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3answers
432 views

Two dimensional elastic collisions with varying angle of incident

If in an elastic collision I know all initial values and that mass for each object remains constant throughout the collision (but different from one another) how can I determine their final velocity ...
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42 views

Is refraction of light a thermodynamic process?

Is refraction of light a thermodynamic process? Can it be explained by conservation of energy? If so, does temperature has an effect on refraction of light?
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379 views

If conservation of energy was wrong, how would we know about it?

Suppose you just started learning physics and you've been introduced to conservation of energy and kinetic energy. Apart from those concepts you know next to nothing. Then you observe an inelastic ...
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work and energy with air rifle [on hold]

A $4 \text{ g}$ air-rifle pellet is fired into a target block attached to a glider. The total mass of the block and glider is $0.5 \text{ kg}$. The target slides along the track $1.455 \text{ m}$. The ...
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2answers
59 views

What observations would be needed to falsify the law of conservation of energy? [duplicate]

I've been doing some thinking, and began to wonder: What observations have led us to the conclusion that ‘energy can neither be created nor destroyed’? Essentially, this means that the big bang ...
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Why can't I do this to get infinite energy?

I know that I cannot do this because of conservation of energy, so I am looking for an answer as to why this will not work. So by my understanding of Einstein's whole famous $E=mc^2$ thing it is ...
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161 views

The Uncertainty Principle and Energy Nonconservation

The uncertainty principle is listed in most textbooks and articles as $$ \Delta E \Delta t \geq \frac{\hbar}{2}.$$ This can be derived in many ways in many different settings, most of them involving ...
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207 views

If a bell is rung in a perfect vacuum and is suspended by opposing magnets will it ever stop ringing?

As far as I am aware a bell rung in a perfect vacuum would only lose energy and stop ringing due to friction between the bell and the hanging mechanism (due to the lack of air resistance). Therefore ...
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3answers
134 views

Does energy conservation not hold in fission and fusion processes?

I have read that during fission and fusion processes, there is some kind of equilibrium between the single nucleus and the disintegration products, so they are constantly being converted into each ...
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5answers
241 views

In an elastic collision, can we choose between cons. of energy and cons. of momentum?

Suppose we have two solid spheres with masses $m$ and $M$, respectively, and that $m$ is significantly less than $M$. The lighter sphere is placed directly on top of the heavier one, and the two are ...
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3answers
356 views

Energy Analysis of Niagara Falls in Linus Pauling's “General Chemistry”

I just started reading Linus Pauling's "General Chemistry" and the first example confuses me. He writes: Example 1-1. Niagara Falls (Horseshoe) is 160 feet high. How much warmer is the water at ...
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23 views

Bernoulli problem [closed]

water is pumped from a to b. the velocity of the water pumped is V. can you write please the energy conservation equation? and then the bernoulli equation?
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2answers
114 views

Dropping a weight onto a spring scale

Say I drop a 5kg weight from a height of 1 meters onto a spring scale like many people have in their bathrooms. On impact the scale will show a higher weight than 5kg. Question: Which quantities ...
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1answer
98 views

Weightless person in a cart on a rail [closed]

In a problem I'm trying to solve I'm having a conflict between my understanding and the proposed solution of the problem. In the problem, a cart of mass $m$ is put at rest in the start of a ...
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0answers
15 views

Drawbacks of passive energy utilization [closed]

Solar plants utilize free energy coming from the Sun. Is that possible that on some significant levels of solar energy utilization we may encounter a negative side effect? Cause energy will avoid from ...
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2answers
95 views

Where did energy come from?

So it is my understanding that before the big bang there was a very small point that held all of the matter in the universe. My question is what form did energy of the universe take on when it was in ...
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1answer
130 views

Heat Equation with In-Depth Radiation Exact Solution [closed]

I am looking to solve the heat conduction equation in a semi-infinite solid with in-depth radiation on the domain $-\infty < x < 0$. The governing equation of this problem is: $$\rho c ...
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5answers
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Where do magnets get the energy to repel?

If I separate two magnets whose opposite poles are facing, I am adding energy. If I let go of the magnets, then presumably the energy that I added is used to move the magnets together again. However, ...
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2answers
2k views

Finding maximum speed in a work-energy problem [closed]

I have the following problem: The Royal Gorge bridge over the Arkansas River is $310\text{ m}$ above the river. A $57\text{ kg}$ bungee jumper has an elastic cord with an unstressed length of ...
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16answers
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Can a car get better mileage driving over hills?

Two towns are at the same elevation and are connected by two roads of the same length. One road is flat, the other road goes up and down some hills. Will an automobile always get the best mileage ...
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5answers
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How does mechanical energy conservation work? [closed]

I am a little confused about how mechanical energy conservation operates when it comes to things like predicting velocity. I know that if conservative forces are the only forces acting on a body, then ...
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1answer
96 views

In a CMCS 2-body system, why does the speed of the particles after collision stay the same?

A particle $m_1$ is traveling with velocity $v$ toward a stationary particle $m_2$. The velocity of the center of mass is given as $v_c=\frac{m_1}{m_1+m_2}v$. Changing to a moving coordinate system, ...
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Different kinds of energies in the form of $\frac{a_1a_2^2}{2}$

It seems to me that the energy of some kind frequently takes the form $$\frac{a_1a_2^2}{2}$$ Where $a_1$ and $a_2$ are some variables. So, for example, kinetic energy has $m$ and $v$; rotational ...
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1answer
49 views

Finding the equation of motion

I am currently working on a problem where a particle of mass $m$ can move along the x-axis with no friction. The particle is moving in a gravitational field and the potential energy is given by: ...
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1answer
53 views

Why is it necessary to supply constant electricity to make a laser work?

As we know that in lasers there are excited atoms. When energy is provided in the form of light, heat or electricity to these atoms, these excited atoms after sometime go to a lower state of energy ...
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3answers
38 views

When moving from one position to another at a constant velocity, how does the conservation of energy hold?

I know that this might be a duplicate question, but I have not found any satisfactory answers that clear up my lack of understanding. Here is my question. Say a sloth hangs on a tree in the middle of ...
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345 views

Optimal speed for the water wheel

The hydroelectricity plants extract the potential energy of highly deployed massive object (water) as it falls down. Without turbine, all that energy would be converted into speed (kinetic energy) at ...
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1answer
28 views

Velocity of mass colliding with another mass under the influence of gravity

If two spherical objects of mass $m$ and mass $4m$ are in outer space with no other forces other than the gravitational attraction between them and they're separated by distance $d$, what velocity ...
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1answer
64 views

Pendulum problem where it hits a peg swings around and lands on top of it

Using energy equation from when it is released to when it reaches its highest point after hitting the peg I get change in kinetic energy = 0 and change in potential due to gravity = 0 Does the string ...
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70 views

How is energy converted when massless spring is attached to a rigid support

Say a massless spring of length l is attached to a rigid support. It is extended to length l + x. Now at this position, the ...
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4answers
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Popular depictions of electromagnetic wave: is there an error?

Here are some depictions of electromagnetic wave, similar to the depictions in other places: Isn't there an error? It is logical to presume that the electric field should have maximum when ...
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36 views

Work and energy

when a ball of mass m is brought with uniform velocity from infinity into the g field of the earth at a distance r from it, the potential energy of the ball earth system decreases from 0 to -GMm/r. ...
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55 views

Energy conversion (conceptual) question

Suppose I have a electronic car. It runs by electrical energy. So when wheels rotate, mechanical energy is produced. Then why can't we convert it back to electric energy and produce somewhat unlimited ...
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2answers
43 views

Approximating energy loss caused by drag force

We know the usual aerodynamic drag equation is given by: $$F_d = -bv^2$$ Where $b$ here is just some constants combination related to the property of the fluid and the material passing through. My ...
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Conservation of momentum but not kinetic energy in inelastic collisions

In inelastic collisions, the kinetic energy of the system is not conserved but the momentum is. Kinetic energy is: $0.5 \times \text{mass} \times \text{velocity}^2$. Momentum is: ...
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24 views

Fluid approximation for inflaton?

In cosmology there is a widely used fluid approximation which applies when the mean free path is very small compared to the scale of observation and then all the properties of stress energy tensor can ...
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2answers
74 views

Energy conservation $\iff \frac{dE}{dt} = 0\ $?

If I'm asked to prove that a system is/ isn't conservative and compare it to whether or not the Hamiltonian is conserved, does that mean I need to compute the time derivative of energy $(T+U)$? Doing ...
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1answer
47 views

Difference between speed of light and of bullet after passing through a barrier [duplicate]

I just read that, when a ray of light traveling in vacuum at $c$ strikes a glass slab, its speed decreases and then when it re-emerges it gets back to its original speed i.e $c$. If I draw a ...
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133 views

Example of law of conservation of energy

Law of conservation of energy states that "Energy of the universe remains constant. Energy can neither be created nor be destroyed, but it can be transformed from one form to another". So I ...
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1answer
115 views

Changing momentum of moving trolley

We have a trolley of mass $m$ and it is moving at a velocity $v$ along a smooth horizontal plane. It is full of water, and is leaking water at a constant rate out of the bottom of the trolley, i.e ...
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2answers
61 views

Is potential energy an energy exists or potentiality of energy? [closed]

I want to ask whether Potential Energy is truly existing Energy - though i know the energy concept is abstract and we do not know what is energy truly as Feynman said - or it is the potentiality that ...
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1answer
44 views

Doesn't the Big Bang negate the law of conservation of matter? [duplicate]

I was watching Hawking (2004 film) and in the movie, Hawking has a sudden insight that all the matter and energy observed in the known universe may, in fact, have sprung out from nothing (singularity) ...
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1answer
39 views

Ideally black plates

Let's consider $m$ ideally black, thin, infinite plates. Let $T_i$ be the temperature of the $i$-th plate. For given $T_1 > T_m$, why is the radiant flux between any two adjacent plates the same ...
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Proof of the conservation of the energy functional for the Gross-Pitaevskii equation?

From the Gross-Pitaevskii equation \begin{equation}i\hbar\frac{\partial\psi}{\partial t}=\left(-\frac{\hbar^2}{2m}\nabla^2+V+g|\psi|^2\right)\psi\end{equation} using the variational relation ...