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.

learn more… | top users | synonyms (1)

1
vote
1answer
585 views

Does a rotating plank only acquire rotational kinetic energy?

I have another doubt with a Kleppner problem :(. A thin plank of mass M and length l is pivoted at one end. The plank is released at 60$^{\circ}$ from the vertical. What is the magnitude and ...
1
vote
2answers
206 views

Does distance traveled by a vehicle after its engine has been switched off depend on its mass at all?

A vehicle moving with some velocity on a rough horizontal road finally comes to rest after its engine has been turned off. Intuitively, it seems a vehicle with greater mass would stop first because it ...
1
vote
1answer
107 views

How to calculate required energy to displace a pendulum?

How can one calculate the amount of energy needed to displace pendulum with given mass m and string length L to $\alpha$ degrees from resting position when acceleration due to gravity is known?
1
vote
1answer
96 views

Analogues of mass, space, or time? [closed]

I began wondering about this when working through this question: is there anything analogous to spacetime or matter? Both spacetime (note: most of what I know of about the concept of spacetime comes ...
1
vote
1answer
96 views

Kinetic energy in Lagrangian formalism

In reading Goldstein's Classical Mechanics (2nd edition) I came across a confusing derivation. Goldstein (Eq. 1-71) derives the total kinetic energy of a system of (classical) particles as: $$ T = \...
1
vote
1answer
298 views

Kinetic energy of a body rotating on another rotating body

Consider a body which can freely rotate with respect to the inertial frame, and a rotating disk whose axis is fixed in body frame. When applying the lagrangian method (does that make a difference?), ...
1
vote
4answers
293 views

what property of steam makes it the preferable motive fluid in jet ejectors? why not air or nitrogen?

I want to know what properties of steam make it the fluid of choice in steam turbines jet ejectors. I want to understand the mechanism of energy conversion in these equipment and hence understand ...
1
vote
1answer
55 views

Why is work not related to velocity?

A very simple question; why does it cost me more energy to very slowly lift a mass $m$ over my head compared to very fast? The definition of work does not state anything about velocity, only the ...
1
vote
1answer
211 views

Conservation of energy and the 'crazy ball' product

Well I'm not sure how many people remember the crazy ball - a small ball made of rubber which bounced like crazy. What I noticed is that the ball seemed to bounce higher than the point from which it ...
1
vote
2answers
82 views

Energy of an EM Wave and its temperature and amplitude

I'm trying to understand why classical physics fails to explain black body radiation. I'm confused. According to Boltzmann, energy calculation for em wave is based on temperature. According to ...
1
vote
1answer
327 views

How would one compute the angle of deflection, in a relativistic collision - underspecified system?

Consider the simplistic case of two identical mass particles colliding elastically with the second particle initially stationary and the first particle travelling with energy $E$. By conservation of 4-...
1
vote
1answer
59 views

Energy in nuclear decays

After a nuclear decay is it a necessity that the total energy of the products is more than the energy of the original particle before decaying? (NB: by 'energy' I don't intend to include mass-energy ...
1
vote
1answer
381 views

Nonzero ground state energy of the quantum harmonic oscillator [duplicate]

Since $\frac{1}{2}\hbar \omega$ is the zero point energy of the ground state of the harmonic oscillator, then there is no way to extract this energy. Therefore, in what way is this value different ...
1
vote
1answer
108 views

Orbital angular momentum and multielectron atoms energies

Explain why states with different orbital angular momentum quantum number have different energies for multielectron atoms but are degenerate in energy for hydrogen Can anyone explain?
1
vote
1answer
1k views

Does time travel violate conservation of mass/energy? [closed]

Imagine I exist at time $t_1$ and my mass is $m$. At time $t_2$ I time travel back to $t_1$. At time $t_1$ there is now a net increase of mass/energy in the universe by $m$. At time $t_3 = t_2 - x$ ...
1
vote
1answer
371 views

Escape velocity problem

I was given a problem at school: How much Energy do we need to make a rocket of mass $m$ faster than the escape velocity so that it can travel in outer space? Here's how I worked: I know that ...
1
vote
1answer
119 views

Could an orbiting mass generate power?

If an heavy object (e.g. 10 tons) orbiting around Earth at 370 miles high, is connected with a cable back to Earth, we assumed either Earth is going to pull the mass or vice versa (or it will fall ...
1
vote
1answer
80 views

What is Work? What does the quantity suggest intuitively? [duplicate]

The mathematical formula for work says that work is force into displacement, but what is the philosophy behind it? I mean what does the quantity suggest?
1
vote
1answer
46 views

Excited Energy levels of Hydrogen vs Solids

My question has to do with Excited Energy Levels. I keep reading and learning that all objects/most solids emit infrared radiation. But to emit radiation, an object's particles must first enter a ...
1
vote
1answer
2k views

The relation between the movement of electrons and energy

So, I've been enjoying reading a lot of helpful posts, but now, I found myself in the need of asking something. I have a hard time grasping the general concept of electricity / how the relation ...
1
vote
1answer
91 views

Resource(s) for developing a good understanding of surface tension?

I have read through several junior undergraduate level explanations of surface tension. Here is a typical presentation at that level: Molecules at the surface of a fluid experience approximately ...
1
vote
2answers
220 views

Derivation of rest energy in Landau & Lifshitz

In Landau & Lifshitz The Classical Theory Of Fields there's a statement: $$\mathscr E=\frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}\tag{9.4}$$ This very important formula shows, in particular, that ...
1
vote
1answer
242 views

Wave Function of Particle in Nuclear Reaction

I was thinking and came up with the question of what happens to the wave function of a particle that decays into energy, say a neutron in a nuclear reaction. I know that conservation of probability ...
1
vote
2answers
249 views

How long should I leave the light on?

I remember my old physics teacher at school saying that if you switch a light on in a room and you're planning to return relatively quickly, it uses less electricity if you leave the light on until ...
1
vote
1answer
1k views

Solve a problem of work and energy

A man pulling sled of his daughter by a massless rope, climbing a snowy hill whose slope is equal to 15 °. Considering that the mass of the sled is $4Kg$, the girl's $26Kg$ and $\mu _c = 0,25$, ...
1
vote
2answers
2k views

Why does the amount of energy transferred depend on distance rather than time?

The change in energy of an object can be determined by the work equation, where work is the change in energy: $$ W = F \cdot d $$ I conceptualize the transfer of energy as simply a series of small "...
1
vote
1answer
265 views

stimulated and spontaneous emission

I have a question regarding the dependence on energy density of the probability rate of emission/absorption while studying lasers in particular. It is given that,the probability rate of absorption as ...
1
vote
2answers
569 views

Elastic potential energy of compressed spring? [duplicate]

If a compressed spring is dissolved in acid what happens to the elastic potential energy of the spring?
1
vote
1answer
476 views

How is the energy distributed in a proton-antiprotion annihilation?

I know the products of the annihilation, but I don't know how much energy each particle has or gets. For example I know that 1876 MeV is released for each annihilation. Now, this energy is distributed ...
1
vote
1answer
433 views

Why doesn't intensity of light affect the emission of electrons?

So electrons of specific atoms have a minimum amount of energy needed to escape the atom, called the work function, W. Now let's say that you emit a certain frequency of light, and $hf<W$. However, ...
1
vote
1answer
276 views

Maximum separation when 2 unlike charges shot apart

I want to compute the maximum separation when 2 unlike charges are shot apart from each other ...
1
vote
1answer
244 views

Basic energy calculation for N identical spin system

We have a system that has N identical spins $n_i$, and each spin can be in state 1 or 0. The overall energy for the system is $\epsilon\sum_{i=1}^{N}n_i$. My understanding: There is only one ...
1
vote
3answers
2k views

Integration by parts to derive relativistic kinetic energy

I have come across a weird integration during derivation of relativistic kinetic energy. Our professor states that i can get RHS out of LHS using integration by parts: $$ \int\limits_0^x \! \frac{d}{...
1
vote
2answers
137 views

energy of Russian meteor, does it matter whether it explodes or not?

The kinetic energy calculation for any meteor would be the traditional one half times the mass times the velocity squared. In terms of the events that occur on the ground, does it matter whether the ...
1
vote
4answers
237 views

Another question about energy(Improved)

I read this thread about energy What Is Energy? Where did it come from? EDIT: after some sleep I tried to read the answers again and found better questions to ask. so here you go As most guys have ...
1
vote
1answer
392 views

Specific electron energy gap values $E_{i+1}-E_i$ vs. photons with arbitrary energy $\hbar \omega$

The energy levels of electrons in an atom are quantized $E_i$. A photon of a specific momentum $\vec p$ and energy $$\omega=(E_{i+1}-E_i)/\hbar$$ hits an atom and gets absorbed. Okay now say the ...
1
vote
1answer
2k views

What arrangement of sound waves would be needed to heat air in a typical sized room?

From what I understand, sound is simply the jostling of the molecules that make up the air in a specific pattern, widely known as waves. I also know that these are longitudinal waves. If we were to ...
1
vote
1answer
665 views

Calculating laser wavelength/power to cause emission of light in a gas?

I've become interested in volumetric displays. In particular I'm looking at a pair of lasers scanning a volume of gas such that the intersection of the lasers would supply enough energy to cause the ...
1
vote
1answer
377 views

Why does an ice rink use a primary and secondary coolant?

A normal refrigeration cycle uses one coolant, so why does vapor compression cycle of an ice rink you a primary coolant and a secondary coolant?
1
vote
2answers
505 views

Maxwell-Boltzmann distribution and total energy per unit volume

We know that $$n(E) ~=~ \frac {2 \pi (N/V)}{(\pi k_B T)^{3/2}} E^{1/2} e^{-E/(k_B T)} dE,$$ where $V$ is total volume. If then, how do we derive total energy per unit volume from this equation?
1
vote
1answer
617 views

How did the energy/entropy/volume/pressure/temperature relationship exist at the Big Bang and how did it evolve thereafter?

According to the current Big Bang with inflation cosmological model? I was under the mistaken impression that there was very low volume, very high temperature/pressure, very low entropy and the Big ...
1
vote
1answer
373 views

2d or 1d conduction in this scenario?

There is a rectangular fin attached to a heat exchanger with a base temperature of 350K. The fin has uniform properties and experiencesa uniform heat generation. It also experiences heat transfer with ...
1
vote
1answer
3k views

In what situations do I use the characteristic length of a fin to find the surface area?

So I'm learning about fins in heat transfer and it seems that there are two separate formulas for the surface area of a rectangular fin of length L, width w and thickness t. The fin is attached to a ...
1
vote
1answer
993 views

Work and Area under a Curve relating to Hooke's Law

If it takes work W to stretch a Hooke’s-law spring (F = kx) a distance d from its unstressed length, determine the extra work required to stretch it an additional distance d (Hint: draw a graph and ...
1
vote
1answer
101 views

Terminology question about energy

I'm looking for the appropriate term to use for what gets "used up" as potential energy is converted to heat and work. For example, some of the the energy in solar radiation is converted by ...
1
vote
1answer
166 views

Will a photon emitted from something moving quickly have a shorter wavelength?

If a photon is emitted from a light source moving at any speed, the photon will nonetheless always move at c (assuming it is emitted in a vacuum.) If the speed of a photon's emitter cannot influence ...
1
vote
2answers
536 views

Spin angular momentum of a system of particles : Is there any energy associated with it?

Consider a system of point particles , where the mass of particle $i$ is $μ_i$ and its position vector is $\vec{r}_i$. Let $\vec{r}_\text{cm}$ is the position vector of the center of mass of the ...
1
vote
2answers
51 views

Energy transferring question

If I have a spring being compressed by two bodies, A and B, with different masses, how much energy would be transferred to each one when they are released and the spring expanded?
1
vote
1answer
249 views

Depalma Free Energy fields

Few years ago I read some papers about Free Energy, written by Bruce de Palma, a physicist who is said to be the inventor of the N-Machine, which is an device that works with free energy latent in the ...
1
vote
0answers
20 views

Are all kinds of energy quantized? [duplicate]

I was reading this answer of David Z about meanig of "Quantum" that this question came in mind: If light is quantized (i.e. is a integer multiples of some unit $\hbar c/k$), are other kinds of energy ...