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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negative energy cancelling energy

In what circumstances can negative energy cancel out energy? If a positron and electron are created they cancel out as one of them has negative energy. Are there any other ways that negative energy ...
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Energy, time and force

Consider force $F$ acting on a body of mass 1000kg and the displacement be $s$. So energy required to do so is $F$x $s$. Now consider the same force causing same displacement on body of mass 1kg. ...
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How does someone move space?

I've heard that we've known components of the universe appear to be flying apart faster than $c$, and that this can happen because the space containing these materials are also moving apart. How does ...
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Perspective and changes in kinetic energy

Say you have two planes flying next to each other at the same speed and one decides to pick up speed by burning a tank of rocket fuel. If someone on the ground wanted to know that plane's new speed ...
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75 views

Relation between different quantum excitation energy, mass energy and kinetic energy

When a particle enters an excited state, the energy appears in its quantum wavefunction according to $E = h \nu$. Does the $E$ in this equation also include kinetic energy, and rest mass energy? ...
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163 views

How does electric current generate thermal energy?

How does free electrons moving through a wire cause random vibrational motion of the positive ions?
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592 views

What is the maximum theoretical efficiency of heat to electricity conversion?

I know that heat engines (heat to kinetic) are limited by Carnot cycle and that kinetic energy to electric energy conversion via standard generator reaches over 90%. However I would like to know ...
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127 views

How do you make a collision more elastic?

Currently, as described in this question, collisions cannot be elastic. However, is there a way to make a collision more elastic? Background: We're working on a project right now involving high ...
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1answer
108 views

Physical interpretation of $Q^i = \partial _\nu T^{i \nu}$

I'm trouble with exercise 1.8 of Carroll's Space-Time and Geometry: If $\partial_\nu T^{\mu \nu} = Q^\mu$, what physically does the spatial vector $Q^i$ represent? Use the dust energy momentum ...
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242 views

Question about heat engine efficiency

I would like to know how much thermal energy is converted to kinetic energy in a steam engine, or a more efficient means if available. I have done some research and found out that Carnot made some ...
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529 views

Calories burned doing 1 squat [closed]

To calculate the calories burned doing 1 squat, wouldn't it simply involve the distance that your center of mass is moving and your weight? I read some other article that said your legs act as levers, ...
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151 views

Where does energy in electricity come from?

In a simple battery-wire-lightbulb circuit the moving electrons in a wire generate electromagnetic field, and that field transports energy. And the bulb goes light. But where does the energy come ...
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How do you keep a fridge full of food cooler?

I just graduated and a mate moved into a flat (none of us are physicists by the way). So, were graduates, we got a new flat, and were broke. So, were now having a debate on how to keep a fridge ...
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Energy spent by motor in a period

Consider a single coil that is turning in a constant and uniform magnetic field thanks to a motor. The normal to the coil is given by: $${\bf u}(t)=\sin (\omega t){\bf u_x}+\cos(\omega t){\bf u_z}$$ ...
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Propulsion Energy in Special Relativity

I have a question regarding how to calculate the energy required to move objects within the frame of special relativity. To that extent, I understand the mass of an object as it approaches the speed ...
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2answers
7k views

Relation between work, kinetic energy and potential energy

We derived two equations in class. The work done between two points $A$, $B$ is equal to the difference between the kinetic energy at the last point and the one at the first point. The work done ...
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1answer
106 views

How do we know that time and distance are not discrete?

I know that it is believed that energy is discrete, in that it travels in quanta. I was wondering if there is any evidence which either proves or disproves something similar with both time and ...
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581 views

Composition of solar spectrum

I read some where that there are three types of UV and infrared rays namely UV-A, UV-B, UV-C and near infrared, mid infrared and far infrared. Which is the most abundant among the the three in ...
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Energy stored in fields

How to intuitively think of energy stored in a magnetic/electric field? Kindly answer in a bit simple terms without referring to mathematics.
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How hot would a disc have to be to create strong drafts that lift it upward?

I read this article on xkcd.xom, from its spin-off blog "What If?" and that answer made me wonder. It was mentioned in that article that a very hot but indestructible box would eventually be able to ...
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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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161 views

If human energy use were to increase by 2.3% per year, would we use all solar energy by 1400 years?

Assuming 2.3% per year exponential growth of human energy consumption (so, roughly speaking, that corresponds to multiplying consumption tenfold every century), it's argued that human annual energy ...
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2answers
214 views

Elastic potential

I have a doubt: elastic potential energy is given by: $U=\frac{k}{2}x^2+K$ but does elastic potential exist? (for example: potential gravitational energy is given by $U=mgz+K$ and gravitational ...
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Finding Rotational Kinetic Energy Of A Clock

The problem I am working on is: Big Ben, the Parliament tower clock in London, has an hour hand 2.70 m long with a mass of 300 kg, and a minute hand 4.20 m long with a mass of 100 kg (see figure ...
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Energy Question

Carl throws 100 g of Bohrium-262 (Half life: 0.10 seconds, undergoes alpha decay) to Sukhdeep with a initial horizontal velocity of 20.0 m/s at an initial height of 2.00 m above the ground. What is ...
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What is the formula for max kinetic and max potential energy of a spring?

What is the formula for max kinetic and max potential energy of a spring?
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Elastic Collision And Momentum

The question I am working on is, "Two blocks are free to slide along the friction-less wooden track shown below. The block of mass $m_1 = 4.98~kg$ is released from the position shown, at height $h = ...
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613 views

A Lawnmower Problem

For my physics class we were just given an exam that asked the following: A 12Kg lawnmower is pushed with a constant force along the 30* handle with 80N. Break the Forces into its vector ...
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4answers
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Mass-Energy relation

Einstein mass- energy relation states $E=mc^2$. It means if energy of a paricle increases then mass also increases or vice-versa. My question is that what is the actual meaning of the statement ...
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4answers
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Find total energy and momentum of an moving electron in a rest frame

I have an electron moving with speed $u'$ in a frame $S'$ moving with speed $v'$ relative to a rest frame $S$. How do I find the total energy and momentum of the electron in the rest frame $S$? I ...
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1answer
821 views

Energy used to stop / slow an object

I'm trying to workout how much energy (if any), I use (imagining me as an efficient machine rather than a complex bio-mechanical human) when I lower or catch a weight. I understand that when I push it ...
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1answer
67 views

Noise is a form of dissipation?

(Mechanical) noise is a form of dissipation? For example, when the computer fan turns it produces noise. This noise is a form of dissipation in addition to heat produced by the machine (computer)? If ...
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1answer
250 views

Decay of particle (for example $\pi^+$); Find energy [closed]

I don't understand how to solve this: A $\pi^+$ decays into a muon and neutrino. Find the pion's energy if max $E_\nu$ / min $E_\nu$ = 100/1; $m_\nu = 0$ $m_\pi*c^2 = 140\text{ peta-eV}$
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305 views

Does friction act in the same way in different directions?

A physics textbook is launched up a rough incline with a kinetic energy of $X$ joules. When the book comes momentarily to rest near the top of the incline, it has only gained ($X-20$) joules of ...
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Rotational kinetic energy during vertical circular motion of a particle

Why is it not necessary to take into account rotational kinetic energy when using the Law of Conservation of Mechanical Energy to solve vertical circular motion problems? After all, the particle is ...
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System moves away from equilibrium $\rightarrow$ it has energy added?

Suppose there is an isolated system $A$ at time $(-\infty, t_1)$, whose entropy is $S=S_{max}$, i.e. it is at thermodynamical equilibrium. Between moments $[t_1, t_2)$ the isolation is violated and ...
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392 views

Energy versus free-energy diagram

Energy versus free energy diagram. I haven't been able to find an adequate definition of these two terms in relation to each other. Could someone point me in the right direction, please? From Borrell ...
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1answer
777 views

Temperature change effected by electric heater [closed]

A 40-gallon electric water heater has a 10kW heating element. What will the water temperature be after 15 min of heating if the start temp is 50F degrees. There must be an equation. I can't find it ...
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118 views

People eating and gaining energy

Let's say we have 100 grams of chocolate which has 571 kilocalories (so it's about 2.4 megajoules) and we have barbell which weights 100 kilograms and need to lift it to 0.5 meters height. So how ...
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409 views

what does 'same energy as a ton of TNT' mean?

Very often the energy released in large-scale events (e.g., earthquakes, meteor impacts, etc) is described in terms of some number of 'tons of TNT'. This may be all well and good for those with an ...
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218 views

Internal Energy Terminology

I am interested in finding out whether the term 'Internal Energy' can be used in both of the following cases: In the case of a macroscopic body upon which light is incident, the internal energy ...
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789 views

Energy transfer to a driven oscillator

Why is the amount of energy transferred to a driven mechanical oscillator largest when the drive frequency is the same as the natural frequency of the oscillator? Why are they exactly out of phase by ...
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In general terms, what key elements are necessary for resonance to exist in a physical system?

I found a related question An Analogy for Resonance, and John Rennie gives a good explanation using description of the harmonic oscillator. But I'm really looking for an accurate and complete list of ...
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1answer
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Angular momentum & Energy using Yukawa's potential [on hold]

I was doing my Gravitation problems and I found this one that I'm unable to solve. Yukawa's theory for nuclear forces states that the potential energy corresponding to the attraction force ...
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1answer
37 views

If an electron tunnels and loses amplitude, but maintains energy; where does the rest of the amplitude go?

I'm assuming the 'amplitude' is kind of like the MeV it has on it, so could be seen as a product of the voltage applied to that electron in a field. But how can it 'lose' volts when passing the ...
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Is it possible to make use of the gluon field's energy to use is applications?

As we know most of the mass of the proton comes from the energy caused due to the interactions of the quarks with the gluon fields (reference link of this statement ...
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1answer
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Polarized Filtering Frequency Shift?

A polarized filter is exposed to a unpolarized light source. The output of the filter should be of lower intensity, hence lower energy. Should not the filtered light be of a lower frequency to ...
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2answers
28 views

How does matter get the energy to repel the gravitational effects of itself to to other matter

This has always puzzled me. As i stand on the earth i require energy to exert a force over gravity to enable me to keep standing. The net force between me and the the earth is zero and thus i remain ...
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141 views

What factors affect the velocity of an object moving down an inclined plane?

If you have a small toy car rolling down an wooden plank, what factors would increase or decrease the velocity of said car. In the experiment my team originally stated that adding mass to the car ...
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“Definition” of internal energy

Conversation of energy implies that if we have a thermally insulated system which goes from state 1 to state 2: $$\Delta E_{12}=E(2)-E(1)=\Delta W_{12}$$ and the 1st law of thermodynamics ...