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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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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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2answers
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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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1answer
63 views

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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2answers
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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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1answer
40 views

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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1answer
38 views

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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1answer
42 views

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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1answer
27 views

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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1answer
79 views

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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2answers
49 views

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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1answer
48 views

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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1answer
34 views

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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5answers
270 views

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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1answer
53 views

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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1answer
37 views

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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2answers
41 views

Change of specific internal energy due to temperature and volume

The change of internal energy due to temperature and volume may be written as $$ dU=C_V dT + \left( T\left(\frac{\partial S}{\partial V}\right)_T-P\right) dV$$ where $$\left(\frac{\partial S}{\partial ...
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Vacuum energy in continuous phase transitions

In a first-order cosmological phase transition, the tunneling from the false to the true vacuum (see left picture below) releases false vacuum energy corresponding to the potential difference $\Delta ...
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1answer
48 views

Is it possible to create usable light from magma?

Say you drilled a hole down to the magma layer. Hot lava emits light. From what I've seen lava is red(?) hot. Then if you put some fibre optic cables down into the magma layer maybe protected at the ...
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2answers
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What is the justification for going from $\frac{d(v^2)}{dt}dt$ to $d(v^2)$ in this derivation regarding work in the presence of kinetic friction?

I do not understand what is being done in the last line of the quote, where we manipulate one integral to get another. Incorporating Newton's second law $\Sigma\mathbf{\vec{F}}=m\mathbf{\vec{a}}$ ...
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3answers
59 views

What happens if the gravitational force mg is greater than the normal force?

We know that the earth exerts a normal force equal to our weight on us . Now if somehow we harness the normal force and use it to do some other work (for example to produce electricity) then what ...
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1answer
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Does $L=T-V$ still hold when $L$ is NOT time-dependent?

I am aware that the Lagrangian $L=T-V$ where $T$ is the kinetic energy and $V$ is the potential energy when $L$ depends on, for example, $r, \dot{r}, t$. My question is, does this still hold when the ...
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1answer
49 views

Energy-frequency dependency of a photon and its medium of propagation

Is the equation relating a photon's energy to its frequency the same regardless of its medium of propagation? And for the equation relating its momentum to wavelength as well?
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1answer
47 views

Does a hot object in space/vacuum gets cold? how? [duplicate]

I'm trying to understand QM and many stuff gets in conflict in my head. Black body radiation means a hot object produces light in certain frequencies. Meaning it's losing energy. Yet they talk about ...
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Work done in the presence of gravity

Consider a system where a block is to be raised from the ground to a height of 1 meter above the ground. It is given that when the force acting on the block and its displacement act in the same sense, ...
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8answers
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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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1answer
33 views

Sun's energy and grand unification

According to Wikipedia, The approximate grand unification energy value is equal to $1×10^{25}$ eV or $10^{16}$ GeV This is equal to $1602176$ J. Now, The Earth receives 174 petawatts (PW) ...
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4answers
150 views

On Work and Energy [duplicate]

I saw in my class that work done on an object is the energy transferred by a force. Energy is the ability to do work. question 1) Why is this definition so roundabout? question 2) How exactly does a ...
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3answers
157 views

What exactly is temperature? [duplicate]

I'm trying to have a really deep understanding of quantum physics and so I went back to what is blackbody radiation until suddenly I realized that my understanding of temperature is that of... well, ...
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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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1answer
38 views

Heat flow for an ideal gas in Isothermal Expansion

I'm having a bit of difficulty grasping the concept of heat flow in isothermal expansion -- most likely because I don't fully grasp the concepts yet. We were given a problem that the professor ...
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1answer
38 views

Bond graph, how to use it? [closed]

Yesterday I heard the word "bond graph" and I felt like searching a bit about it. From what I understood, it is a graph that links the flow of energy between "states" in multiphysics systems. I am ...
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1answer
79 views

Energy formula for finite potential well [closed]

The energy formula for infinite potential well is $$E=\frac{n^2h^2}{8ma^2},$$where $m$ is the mass of the particle, $a$ is the width of the well but in the case of finite potential well, I actually ...
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2answers
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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 ...
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3answers
87 views

Thermal energy turning into mass?

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 ...
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1answer
44 views

Free energy along a reaction coordinate

I've come into this issue when trying to understand biased sampling methods, in particular, umbrella sampling, but I think the question is more general. A recurring argument is that, along a reaction ...
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3answers
63 views

Could pressure differentials be used to produce power?

There is potential energy between something like a vacuum in a container and a gas on the outside of it, or vice-versa. Could this energy be harvested in a vacuum, or close to vacuum situation like on ...
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1answer
46 views

Centripetal force related problem [closed]

If a bike rider constantly keeps rotating on a vertical circular path, what is the required minimum velocity on the highest point of the circle to keep him on the circular path without falling?
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1answer
79 views

Eigenvalues of the Hamiltonian

Is every eigenvalue of the Hamiltonian a form of energy? If not are there values of the Hamiltonian that do not correspond to the energy of the system?
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2answers
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Falling from a high cliff [closed]

I've always wondered about the reason why falling from a high cliff could actually cause death, is it the normal force exerted back from the ground? or is it related to energy? or what? thnx.
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2answers
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Interpretation of the energy-time uncertainty [duplicate]

From the uncertainty relation it follows that: $$\Delta E \ \Delta t = \hbar$$ $\Delta E$ is the energy uncertainty of a state, $\Delta t$ should be the uncertainty of the lifetime $\tau_b$ of the ...
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2answers
154 views

Should I walk or run to my towel when naked and wet? [closed]

This morning when I got out of the shower, I realised I had left my towel in the bedroom. Naked and wet I ran as fast as I could to get the towel. On my way back I didn't run, but rather walked, and ...
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1answer
68 views

energy storage and flux produced by an inductor

The unit of magnetic flux is the Weber and is proportional to L x I or Henrys X Amps. Flux is also proportional to J/I or energy per Amp. Energy stored in an inductor is given as 1/2 LxIxI. When I ...
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1answer
45 views

“Plushness” of a simple mechanical damper?

What heuristics generally define the "plushness" of a mechanical damping or braking system? For example, if a person or some mass is to collide with a fixed wall or the ground (e.g., after a vertical ...
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2answers
52 views

How does kinetic energy change into different forms of energy?

Is the kinetic energy the only thing that is generating heat when you rub your hands together? If so, exactly how does it do this? How does kinetic energy change forms? If not, what other energy forms ...
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4answers
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How much energy is in a 75 kg human being? Including atomic and electrical (charge) [closed]

To calculate the energy in 75 kg of mass is easy with E = mc2. But, how to calculate how much charge are in all our atoms? It is, too, a gargantuan amount of energy.
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4answers
61 views

Bicycle Fuel equivalent on l/100km

Is there a way I can calculate the equivalent how much a Human powered bicycle consumes in the equivalent l/100 Km of Diesel fuel? And also can I calculate the same for Gasoline fuel? And for ...
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3answers
140 views

Classical Mechanics: work

This a a fragment from my Theoretical Physics (- Classical Mechanics) notes: $$W_{1 \to 2} = \int_{t_1}^{t_2} \boldsymbol{F}\cdot\boldsymbol{v}\,dt = m \int_{t_1}^{t_2} \dot{\bf{v}}\cdot{\bf{v}}\,...