# 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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### Why does energy flow downhill/entropy increase?

I'm not sure if this is asked anywhere else before, but so far my search isn't fruitful. I'm in my second bachelor of physics and mathematics, and currently taking a thermodynamics course. We just ...
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### Will a wheel keep rolling in perfect vacuum? [duplicate]

I have a query which I cannot resolve by finding enough arguments. A perfectly round wheel that also doesn't have any contractions due to gravity is placed in a perfect vacuum, so no drag is present. ...
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### How to find acoustical output of a horn?

If I had a horn or cone that could be written as a function $f(x)$ rotated around the x-axis, how would I calculate the output, knowing the input (intensity, frequency, everything)? I want a way to ...
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### How much kinetic energy is there in the observable universe at any given time?

I know that it fluctuates, but, what would be a good estimate for how much kinetic energy is in the universe right now? I know that things cancel it out, but, how much is there?
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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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### Energy Density with a Linear Dielectric Inconsistency

The energy density of an electromagnetic field with a linear dielectric is often expressed as $0.5 E \cdot D$. It is also known that energy can be found by $0.5\int_{V} \rho V dV$. Using the latter, ...
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### Energy balance of this machine is positive. What is wrong? [duplicate]

Machine on this picture is inspired by video: https://www.youtube.com/watch?v=rxFXsoqbfrk There must be error in the computation of energy balance or there is another negative energy. Can you help ...
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### can we use the power of ice to float to make free electricity?

ice and water have a volume to weight ratio of 12/11 so 11 tons of ice will support 1 ton of weight 1 ton of ice takes 1000 watts or 12000 b.t.u. of energy to be produced question is , at what ...
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### Why is the mechanical energy lost whenever an object undergoes plastic deformation?

I have read here that "Mechanical energy is lost whenever an object undergoes plastic deformation." Why is that? I mean the object would be changing its shape, and while it's changing its shape it ...
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### Potential Energy equation is wrong as $mg$ force acting upwards (by us) and gravitational force ($-mg$) get cancelled and no movement is possible [duplicate]

In physics, the potential energy of an object is mgh. We say that the potential energy of an object is equal to the work done on it to make it reach that height. So the work done = mg h. Mg is the ...
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### Question about quanta and $E=h\nu$

It is my understanding that Planck's major breakthrough was that energy comes in the forms of 'packets' called quanta, each of which has the energy of $6.62e-34$ of one Joule. By looking at Planck's ...
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### Human vs. machine energy expenditures

I've read that a typical human-powered bicycle power generator generates around 100W. For the same amount of effort, the bike travels probably around 20k/h on a road. Does that mean it requires 100Wh ...
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### Manual jack up of Maxwell-Boltzmann scale parameter?

I first noticed this phenomenon when i was a child, about 4 decades ago. I needed a piece of wire but could not find a pair of pliers with which to cut the wire. So i used a technique i once saw my ...
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### Energy balance in a thermodynamic system

I think I've missed this point in class but I was wondering how to determine for example if I have a turbine, the Energy balance would look something like this: $W$ as work $h_1$ as intake and $h_2$ ...
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### Minimum energy of proton converting into a neutron

I'm given the isotope sodium 22 (which is positron emitter) and I'm stuck with calculating the minimum energy that a proton needs to convert into a neutron and a positron. It got to do with $E=MC^2$ ...
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### How to convert MeV to velocity? [closed]

For example, in the fission process: $${}^{235}\mathrm{U} + n \rightarrow {}^{140}\mathrm{Xe} + {}^{94}\mathrm{Sr} + 2n$$ the masses in atomic mass units are: \begin{align} M\left({}^{235}\mathrm{...
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### Direction of work done in gravitational field

For an object that is at a distance $R$ from the earth core, if we want to move it further away to a distance $R+A$, we will do positive work on it since the direction of displacement is in the same ...
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### Potential energy of ice cube is container dependent?

The potential energy of an ice cube floating at sea can be measured from the surface of the water, which remains at a constant height as you drop the ice. However, what about when you drop it into a ...
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### What is the “special time dependence” that develops in an Ostrogradskian instability?

I've been reading papers that deal with Lagrangians containing second- and higher- order derivatives of field variables. In this paper in Section 3.1, I found this very interesting quote: The ...
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### Why does conservation of energy give a wrong result?

Suppose you have two massless springs, both of length $d$ and spring constant $k$, in the following arrangement: As indicated by the gray springs, the two inital springs both get streched by a ...
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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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### How can non-polarised light exist? [duplicate]

Typically unpolarised light is shown like this: Basically, from what I've seen, unpolarised light is just a lot of overlapping individual light waves. However, if they truly overlapped, would their ...
When you heat an object it's mass increases $E=mc^2$. What I don't understand is that this energy can be converted to mass and still be thermal energy too (object still hot). When a radioactive ...