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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Does stopping the same bike and rider at the same velocity with the front brake require less energy than the back brake?

It's the same body made by the rider and the bike moving at the same speed. So, even though braking on the front/back alters the normal forces on the opposite wheels thus creating more friction with ...
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2answers
62 views

Relativistic deceleration and energy

It's common knowledge that due to relativistic effects, accelerating from 0.8c to 0.9c takes a lot more energy than accelerating from 0.1c to 0.2c. However, what's the case with deceleration? Does it ...
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2answers
504 views

Why does a wind turbine have only three blades? [duplicate]

Why not four or five or even more? Intuitively, the more leaves the more power. So, what is the reason?
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1answer
125 views

Why is wave energy zero at maximum deviation?

The equation for the energy density of a longitudinal wave in a thin rod is $$w = \rho\cdot \left(\frac{\partial \xi}{\partial t}\right)^2$$ So the energy density seems to be 0 when the deviation ...
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4answers
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What is all energy made of? [duplicate]

I know it sounds like a weird question to ask but I find it unusual nobody has actually told me. I know what it does but not what it is... I have some kind of idea that it's actually matter and matter ...
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0answers
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What is the kinetic energy of a proton as it is falling towards the core of the star?

Given: Star's mass = $1.99e10^{30}kg$ Type II supernova occurs (in a simple form: the outer layers of the star fall into the now inactive core at speeds of 126000km/s) $10^{44}$ joules of energy is ...
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9answers
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Why do solar panels not have focusing mirrors?

Most of the solar panels that I have seen do not have any mirrors, etc., but usually solar cookers have mirrors. What is the reason for solar panels not having focusing mirrors?
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0answers
63 views

Kinetic Energy and reference frame

Two space ships A and B moving through space in the same direction. Space ship A moves 2m/s faster than B, its weight is 1kg and it has a 2 Watt engine. It is accelerating with full power for one ...
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0answers
50 views

What fills the space between the galaxies, stars and planets? [duplicate]

What fills the space between the galaxies, stars and planets with the exception of gas clouds, dust, atoms, elementary particles and electromagnetic radiation? What the authoritative opinions exist on ...
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1answer
148 views

Will my induction hob be powerful enough? [closed]

I'm looking to buy an induction hob to heat some water and I would like to know if its going to be powerful enough. If so, can it be done in, say, 1 hour? Additionally, what power rating could I go ...
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1answer
68 views

Energy and momentum of a relativistic electron

The question is to find the magnitude, $p$ of the electron's momentum in the unit of MeV/$c$, given that the kinetic energy of the electron is 2.53 MeV. The answer provided by the book says, ...
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1answer
441 views

Feynman Lectures: Why a non-reversible weight lifting machine cannot lift higher that a reversible one?

Consider weight-lifting machines—machines which have the property that they lift one weight by lowering another. Let us also make a hypothesis: that there is no such thing as perpetual motion ...
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1answer
129 views

Preventing Heat Escape

Is is possible to completely prevent heat from escaping from a closed container? Here is a diagram of vacuum flask, which tries to implement the design - Vacuum Flask prevents heat from escaping ...
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4answers
137 views

Can a photon have little to no energy and/or speed?

Can a photon move more slowly than the speed of light and behave 'non-relativistically,' so to speak. Perhaps another way to express my thought is: could we stop a photon from moving?
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2answers
228 views

Relationship between Fermi Energy and Average Energy

Electrons are in a one dimensional box of length 2L. What is the relationship between Ef, the fermi energy and E, the average energy? The answer is Ef/3 I looked up the formula for the relationship: ...
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2answers
84 views

Is possible to synthesize diamond at relatively lower pressures (6 to 7 GPa vs 12 GPa)?

Pure binderless nanopolycrystalline diamond can be produced from graphite when the graphite is treated with more than 12 GPa pressure and at least 2100°C temperature . If we treated graphite or ...
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1answer
166 views

Coin rolling, but not sliding. Kinetic energy

A homogeneous coin of mass M rolls, without sliding, along the x-axis with the axis of rotation being parallel to the z-axis. Let $\Theta$ be moment of inertia regarding that axis and $\vec{V}$ be ...
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1answer
223 views

Gough-Joule effect in rubber band

When a stretched rubber band is heated, surprisingly, its tension will increase. What is the physics behind this?! Another thing. The Entropic spring model says, for a good specimen, that the ...
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1answer
293 views

Cylinder inside a cylinder - moment of inertia

A homogeneous cylinder with radius a and mass m rolls in a hollow cylinder with radius R. Determine the kinetic energy of the cylinder as function of $\dot{\theta}$. I'm sorry for ...
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0answers
18 views

Cylinder rotating inside another cylinder. [duplicate]

A homogeneous cylinder with radius a and mass m rolls in a hollow cylinder with radius R. Determine the kinetic energy of the cylinder as function of $\dot{\theta}$. I'm sorry for ...
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1answer
65 views

Relationship between gravitational time dilation and energy?

The rate that a stationary clock slows down near a massive object, relative to one far away, can be read off from the Schwartzschild metric: ...
2
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1answer
100 views

Optimal size of a windmill for a given windspeed

Here is the problem: Assume that you have some constant wind speed. I want to run a windmill but I need to decide how big a windmill I want. The size is characterized by the length of the blades, $r$. ...
3
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1answer
110 views

Perpetual Mobile and Gravitation

I have fundamental question about what is called the “law of conservation of energy”. We all hear about the tidal power stations which using the tidal power. The source of the tidal power came from ...
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3answers
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How does liquid convert to gas on getting thermal heat energy?

Say for example, when we heat, water converts to steam gas. How does it happen? What happens underneath giving rise to breaking of bond between molecules in liquid state and spreading them in gas ...
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1answer
152 views

Total energy of neutrons and protons

In a stable nucleus, are the total energies of neutrons and protons same?
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2answers
243 views

Question about kinetic energy

I have a question about kinetic energy: Imagine two vehicles with velocities $v_1$ going at $30 \frac{\text{m}}{\text{s}}$ and $v_2$ at $50\frac{\text{m}}{\text{s}}$. If they hit each other, will ...
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1answer
99 views

Mathematical expression of energy storage

I don't know if anyone can help out there, but I guess it won't hurt to ask!! I'm trying to develop an idea which is as follows. Put simply, imagine a flat sheet of material which, when distorted ...
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1answer
32 views

Plot of fusion energy output growth? [closed]

What would a chart of energy output of elements of mass X fused with hydrogen look like? What about X fused with X?
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1answer
213 views

Interaction Energy vs Force

I'm having a hard time determining the relationship/differences between interaction energy and forces. Say we have a system of two charged particles. Each particle will exert a force on each other ...
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0answers
36 views

Reactor for electrical energy design base assumption inefficient?

Im no physicist or engineer but as I observe our methods to obtain electrical energy I noticed a pattern where we cause a exothermic reaction, the resulting energy heats water, that turns into vapor, ...
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3answers
181 views

Is it possible for man to break Earth into 2 parts? [closed]

Many countries have extreme devastating nuclear weapons. Also they have weapons in very large numbers. I want to ask that Is it possible for man to break earth into 2 parts with the help ...
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0answers
65 views

question regarding work energy theorem [closed]

The question says A smooth track in the for of a quarter circle of radius 6 lies in the vertical plane. A ring of weight 4N moves from $P_1$ to $P_2$ under forces $F_1$,$F_2$ and $F_3$. $F_1$ is ...
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0answers
148 views

What is the equation to calculate the strength and radius of an electromagnetic pulse?

With this interesting answer on the blast force of a uniformly charged electron sphere, came another interesting question. What would be the strength and blast radius of an EMP launched from such a ...
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1answer
78 views

How much energy is saved when using luggage with wheels? [closed]

How much energy is saved when using luggage with wheels as opposed to carrying the luggage? Thanks.
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2answers
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How do I express the interaction energy between two charged spheres?

Consider two identical insulating spheres each with radius $R$ and uniform charge $Q$ through their volume. They are separated from their centers by a distance of $d>2R$. Here is my general ...
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2answers
192 views

When do we see particles to be in a superposition of energy states?

I have two doubts: Exactly when does this happen? and If we are in a superposition of states (lets say E1 and E2) and the particle absorbs a photon, what will happen? If E3-E1 = hf, will it go to E3? ...
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1answer
97 views

Definition of internal energy

I have read some thermodynamical and statistical physics books but it feels like they always gloss over the definition of internal energy. In wikipedia they included terms like Kinetic energy. But ...
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2answers
99 views

Kinetic energy dissipation in braking a vehicle

Let's say a vehicle that weighs 20t is hauling along at 50m/s and we want to brake it down to a full stop. The kinetic energy we need to dissipate into heating up the brakes is ...
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1answer
197 views

How much energy is required to compress one engine cylinder full of air?

I was recently wondering how much energy would be required to start a car engine by manually cranking it. I was talking to someone who said the largest load you would have to deal with in the ideal ...
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1answer
93 views

Solving the quantum well gives you eigen energies gives $E_n$, are these energies in conduction band or valence band?

I wonder if the energies $E_n$ that is derived from solving the SWE for the quantum well can be considered as energies in the conduction band or the valence band. In other words is $E_1$ is lowest ...
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1answer
81 views

Question about energy moving through one object to another [closed]

I think this question is part physics, and maybe part chemistry. I'm making electronic drums. On the top of the drum there's a flat surface (the drum head) you hit with a stick anywhere on the head. ...
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2answers
166 views

Energy loss without work or heat?

Suppose we have a mass-spring system in equilibrium as in the above part of the picture(it's a frictionless surface). Then I attached a weightless rope to the mass from one end and a car from the ...
2
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1answer
523 views

Coefficient of friction on a loop-the-loop!

Here is a question that I thought of while doing a physics lab in class. We dropped a marble from a certain height down a ramp and around loop-the-loop and let it fly off of a table and then we were ...
8
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2answers
386 views

Do virtual photons have a frequency?

Real photons do have frequencies, which is directly related to its energy. So, can virtual photons that take part in EM interactions have frequencies too? When my hand is pressed up against a glass ...
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2answers
279 views

What is internal energy?

My physics book says: The total potential energy and kinetic energy possessed by the particles that make up an object, but excludes the potential and kinetic energies of the system as a whole. I ...
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1answer
50 views

Interpretation for negative energy of curves

Let $(M,g)$ be a Lorentz manifold and $\gamma :[a,b] \to M$ a differentiable curve. I understand we define the energy of $\gamma $ as: $$E[\gamma] = \frac{1}{2} \int_a^b ...
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2answers
66 views

According to calculations why the current of a battery of a mobile is high?

If you pay attention to the features of your mobile's battery the voltage would be around 3-4V and power is 4-5 Wh (14400-18000 Ws) and with calculating the current you can find a very high ...
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2answers
157 views

Why the electric potential energy the charges carry in a electric circuit does not vary with its distance to the opposite terminal?

I understand that with wires assumed to be ideal, there is no resistance and subsequently no force acting on electrons, hence no work done and p.d. across any segment of wire. However, as the ...
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2answers
180 views

Is the Einstein Energy-Momentum equation $E^2 = p^2c^2 + m_0^2c^4$ valid only for Free Particles?

Is the energy -momentum relation $$E^2 = p^2c^2 + m_0^2c^4$$ satisfied only by free particles or even bound particles? Does the Energy refer to total Energy(including potential) or only (kinetic ...
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1answer
328 views

Can the total amount of energy in the entire universe be quantified?

Classical thermodynamics leads one to believe that if energy is transferred, and the universe is heading for maximum entropy, then back extrapolating to moments before the big bang, one could find a ...