The law of conservation of energy, which states that the amount of energy in a system is constant. For questions about Earth's environment, see the climate-science tag instead.

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What is the mass of individual components in a gravitationally bound system?

When material of rest mass M falls from infinity onto a black hole accretion disk, it gets heated and then emits so much light that the energy radiated away can measure up to about 30% or so of M c^2. ...
5
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5answers
895 views

Does the stress-energy tensor contain the equations of motion?

Derivatives $\nabla_i T^{ik}=0$ of a stress-energy tensor of physical system express conservation laws. Whether contains a stress-energy tensor also the information on the equations of motion of ...
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2answers
12k views

Work done by spring over distance

I'm working through a problem involving energy conservation. Unfortunately, I cannot calculate the work done by a spring. ...
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1answer
158 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 ...
2
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1answer
161 views

Kinetic energy transfer in matter annihilation?

What happens to the kinetic energy of matter when it is annihilated? Is it released in the resultant explosion? In that case shouldn't it be $E=(mc^2 + \frac{1}{2} mv^2)$ ?
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3answers
3k views

Conservation of Momentum/Energy collision Problem

I'm working on a physics problem in preparation for the MCAT and there's this particular problem that's troubling me. I don't know if it's a bad question or if I'm not understanding some sort of ...
2
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3answers
102 views

Certain material heating water in a recipient

I don't know how to resolve a problem, but I don't want the answer since I'm almost going to have it resolved. What the problem says is we have 85 liters of water at 7ºC in an iron pot of 29kg. We ...
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4answers
12k views

Why can't energy be created or destroyed?

My physics instructor told the class, when lecturing about energy, that it can't be created or destroyed. Why is that? Is there a theory or scientific evidence that proves his statement true or ...
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1answer
186 views

I think I disprove this with kinematics, but energy says it is right

! Here is my kinematics argument. For now I am only going to look at ball 2 and ball 3. Make note of the following data. $|v_0| = 10m/s$, $y_0 = 10m$, $\theta_2^0 = 30^0$, $\theta_3^0 = -45^0, g = ...
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2answers
229 views

Can conservation of energy be applied if trajectory is not smooth?

In this video http://www.khanacademy.org/video/conservation-of-energy?playlist=Physics Khan academy explains conservation of energy for a falling object. He looks at an object falling ...
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1answer
492 views

What happened to the work done by friction here?

Problem/Solution ! Question What happened to the work done by friction BEFORE it touched the spring? Why was that neglected? Also they say there is no physical meaning behind the negative root, so ...
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1answer
533 views

Thermal energy generated due to loss in kinetic energy when observed from two different frames of reference

A body is moving with a velocity $v$ with respect to a frame of reference $S_1$. It bumps into a very heavy object and comes to rest instantaneously, its kinetic energy $$\frac{1}{2}mv^2$$ as ...
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4answers
11k views

Where do electrons get their ever-lasting circulating energy?

We all know (or maybe know) that to move, we need to spend energy. If you want to drive a car, you gotta spend gasoline. We also know that energy can't be created (first law of thermodynamics, and ...
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3answers
374 views

Is energy conserved in decay of hydrogen atom in superposed state?

This looks like a paradox. Let's say we have an hydrogen atom. Superposition of states could be possible for electrons. But if an electron is in a superposition, I guess it could decay into a lower ...
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1answer
267 views

What “I” should use in Rotational Energy formula $(I \omega^2)/2$

$\text{Rotational Energy} = \frac{1}{2} I \omega^2$. What $I$ should be used? $I$ as a inertia tensor matrix = stepRotation * inverse moment of inertia * inverse stepRotation; Or I as moment of ...
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2answers
230 views

curvature tensor component capable of doing work on $T_{\mu \nu}$

I'm wondering what part of the curvature tensor is able to do work (and hence transfer energy) in matter. I'm wondering if this tensor: http://en.wikipedia.org/wiki/Stress-energy-momentum_pseudotensor ...
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2answers
761 views

Does the global financial system violate laws of thermodynamics and energy conservation? [closed]

For the past years I am thinking about relating concepts from physics to concepts from economics. Especially since the financial crisis made obvious the instability of the financial system I ask ...
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3answers
453 views

Is there a measure of internal energy flow?

A system might have internal energy and/or kinetic energy. Kinetic energy in classical mechanics is a form of energy the object has, only because of its relative movement to other objects. If you ...
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0answers
680 views

Bungee jump physics

Question: A bungee jumper jumps from a bridge. The length of the loose rope is 30 m. When the jumper reach the lowest point possible, the rope stretches 10 m. What is the final stretch of the rope, ...
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3answers
888 views

Is mass-energy conversion in chemical reactions experimentally observable

This is a common point of argument on internet forums. I think it is fairly well established theoretically that there is a very small amount of mass converted to energy in an exothermic chemical ...
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2answers
1k views

Mass converted to energy in a common fire?

In a common wood fire such as a campfire, is matter converted to energy or is it simply an exothermic chemical reaction and all the mass can be accounted for in the ash and soot?
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3answers
6k views

Force as gradient of scalar potential energy

My text book reads If a particle is acted upon by the forces which are conservative; that is, if the forces are derivable from a scalar potential energy function in manner $ F=-\nabla V $. I ...
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4answers
2k views

Work done by the Magnetic Force

The magnetic part of the Lorentz force acts perpendicular to the charge's velocity, and consequently does zero work on it. Can we extrapolate this statement to say that such a nature of the force ...
2
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1answer
686 views

Why does salt on snow create Ice?

I think I kind of understand this process but I would like someone to explain it more completely. For those who aren't aware here is the scenario I'm talking about: As terrain maintenance at my ...
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2answers
475 views

Kinetic energy puzzle

System S1 moves at constant speed V with respect to S0 in one dimension: ...
2
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1answer
439 views

Finding Speed with the Energy Principle - Nuclear Fission

The question goes along the lines: Uranium-$236$ fissions when it absorbs a slow-moving neutron. The two fission fragments can be almost any two nuclei whose charges $Q_1$ and $Q_2$ add up to ...
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2answers
393 views

What is changing in latest light bulb technologies?

I'm confused with the latest home lightning bulbs. Understanding filament bulbs was easy. For example take 220V, 100W filament bulb: Power = $V^2/R$ Filament gets heated and emits energy in the form ...
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1answer
3k views

How to calculate calorie expidenture from lifting a weight?

Am I correct to say that when a human lifts a dumbbell from, say, 2 feet off the ground to 6 feet off the ground, he will have increased the potential energy of the weight and thus will have burned ...
6
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2answers
389 views

Conservation of energy in a non-linear oscillator

I have a homework question about a "non-linear oscillator". I actually have an answer to this question, but the answer I get is stronger than what is needed according to the question. The question ...
2
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1answer
71 views

stress work of uniformly deforming continuum

I have a volume which is deforming (using explicit time-integration scheme) uniformly with velocity gradient $L$ and stress tensor $\sigma$. I would like to determine work done by the volume ...
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0answers
675 views

Would Portal-style portals transmit gravity? [closed]

In the video game Portal, there are often puzzles which must be solved by gaining a large amount of momentum. Typically, this is accomplished by putting one portal on the ground and another directly ...
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3answers
2k views

photons in expanding space: how is energy conserved?

If a photon (wave package) redshifts (streches) traveling in our expanding universe, is it's energy reduced? If so, where does it go?
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1answer
296 views

Energy storage and battery banks [closed]

Why is it not possible to store windfarm energy in battery banks? A lot of energy isn't being used when wind farms are at peak and we have no way of storing it. This is wasteful. So why don't we use ...
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3answers
666 views

The “binding energy” of bonded particles adds mass?

This is a follow-up my previous question. Several of the answers indicated that the mass of a particle (atom, proton, etc.) increase with the "binding energy" of it's component particles - the energy ...
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0answers
110 views

What is the energy conversion efficiency of a computation device like a modern CPU? [duplicate]

Possible Duplicate: How efficient is a desktop computer? I believe the energy conversion efficiency of a modern CPU is extremely low, because it dissipates pretty much all of it as heat, ...
3
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1answer
280 views

If 100% of the energy from the sun is reflected back into space

100% of the energy from the sun is reflected back into space, it's just shifted from a low-entropy state to a high-entropy state, and from a high frequency (ultraviolet) to a low frequency (infrared). ...
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4answers
35k views

What is the relationship between kinetic energy and momentum?

I can't seem to figure out the relationship between $E_k$ and $p$ or $F$. I understand that the units are pretty different. But for example: A bullet with a mass of 10.0g is moving at the speed of ...
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6answers
691 views

Why is global conservation of energy not considered a tautology?

This question is in reference to my downvoted answer to this active physics.SE question. More than one user has indicated that it is simply wrong and I am having trouble understanding why. My point ...
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5answers
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Conservation law of energy and Big Bang?

Did the law of conservation of energy apply to the earliest moments of the Big Bang? If so, what theoretical physics supports this? I hear that Einstein's theory of relativity disputes the law of ...
2
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1answer
138 views

potential energy of an object due to other two objects

consider object A with mass $m_{A}$ and positional vector $\overrightarrow{r_{A}}$ object B with mass $m_{B}$ and positional vector $\overrightarrow{r_{B}}$ object C with mass $m_{C}$ and positional ...
2
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2answers
401 views

Mechanical energy problem - please help [closed]

Quite new to all this - simple question. "A ball starting from rest rolls down a hill and reaches 10m/s at the bottom. If the same ball starts rolling down the hill from an initial velocity of 5m/s, ...
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1answer
698 views

Lawrence Krauss' Theory of Origin

I don't understand how a vacuum, the absence of matter, can hold energy. How can it hold energy when Einstein proved that matter is energy? And a second related question; how does the energy in a ...
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1answer
533 views

Kinetic energy loss when a rigid body falls into water

Assume you have a rigid body falling into the ocean at terminal velocity. Also assume that the rigid body does not break on impact. How could you figure out how much kinetic energy would be lost in ...
6
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1answer
442 views

Where does the energy go, when light is blocked by polarisation

I've been looking around about LCD monitors, and how they polarise light. When a pixel needs to be black, the light is "twisted" so that it can't go through the polarising sheet in front. What happens ...
16
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2answers
871 views

If energy is only defined up to a constant, can we really claim that ground state energy has an absolute value?

Sorry if this is really naive, but we learned in Newtonian physics that the total energy of a system is only defined up to an additive constant, since you can always add a constant to the potential ...
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9answers
2k views

How to explain independence of momentum and energy conservation in elementary terms?

I'm trying to explain to someone learning elementary physics (16 year old) that linear momentum and energy are conserved independently. I'm not a professional physicist and haven't tried to explain ...
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4answers
3k views

Energy conservation and interference

I have a problem with energy conservation in case of interfering waves. Imagine two harmonic waves with amplitudes $A$. They both carry energy that is proportional to $A^2$, so the total energy is ...
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2answers
773 views

When matter and anti-matter collide

Do they create energy? Or do they just disappear with zero energy? If they create energy when disappearing, that means it takes energy to create them, right? If they disappear into zero energy, ...
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5answers
1k views

If the universe is expanding, what is it expanding into?

If the universe is expanding, what is it expanding into? When the big bang happened where did it occur? When the big bang happened how did it occur? Where did the energy come from? Energy can not ...
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4answers
826 views

Redshifting of Light and the expansion of the universe

So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light ...