# Questions tagged [energy-conservation]

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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### How long must escape velocity be maintained?

Escape velocity from Earth's surface is 11.2 Kilometres/second. How long would one need to maintain this escape velocity to actually escape Earth's gravitational pull? Must this 11.2 km/s velocity ...
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### A two-level system absorbs a detuned photon. Where does the extra energy go?

Let's consider simple two-level system with frequency gap of $\omega_0$ between ground and excited state. Now, when we turn on external electromagnetic field with frequency $\omega < \omega_0$, ...
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### Calculating kinetic energy?

Would this be a valid equation to calculate kinetic energy created from a drop from a height: $$E_{kinetic} ~=~ v_{vertical}tmg$$ Velocity multiplied by time gives distance. Distance multiplied by ...
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### Is energy “destroyed” when walking?

Conservation of energy states energy can't be destroyed, but isn't energy used up when walking in a straight line? If your not walking up a slope, kinetic energy isn't converted to gravitational ...
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### When work is performed solely by magnetism, is there an equivalent loss of energy from the magnetic field?

When two magnets are placed within appropriate proximity and released, the attractive force will perform work and bring them together. Work is performed overcoming friction. Can we measure a ...
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### Conservation of Energy in Special Relativity

In classical Newtonian mechanics, from what I understand, conservation of energy stems from the fact that all known forces are conservative forces, and vector calculus tells us that they can be ...
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### Existence Of Electric Field Lines [closed]

Can an Electric Field with field lines Like So Exist: http://puu.sh/tWkJ One Of my friends said it couldn't as the field lines here are not conservative ; so it cannot exist ; Is he right? Or can ...
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### Finding Distance an Object Travels Up an Incline After Launch

I've been doing a review for an introductory physics course final. I have a question on one problem though. Here is the problem: A mass (M=2kg) is placed in front of a spring with k=900N/m, ...
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### Can a force in an explicitly time dependent classical system be conservative?

If I consider equations of motion derived from the principle of least action for an explicitly time dependent Lagrangian $$\delta S[L[q(\text{t}),q'(\text{t}),{\bf t}]]=0,$$ under what ...
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### Matter - Antimatter Reactory Practicality

With current technology, would the energy released by a matter-antimatter annihilation be more than the energy needed to created the antimatter in the first place? Would it be worth it? Just curious, ...
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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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### What happens to the energy when waves perfectly cancel each other?

What happens to the energy when waves perfectly cancel each other (destructive interference)? It appears that the energy "disappear" but the law of conservation of energy states that it can't be ...
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### Is the energy conserved in a moving frame of reference?

Consider this situation: When the box is at the bottom of the frictionless incline, it will have a velocity of $v_f$. The person is an inertial frame of reference that moves at a constant velocity ...
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### How did the inflaton field “add” energy to the universe?

How did inflation add energy to the universe? What mechanism did this occur by? In other words, where did that energy come from? Was it due to the quantum fluctuation (or that scalar field rolling ...
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### Conservation of Energy and Momentum Regarding Forces - clarification needed

The other day, my teacher stated something along the lines of, "Conservation of momentum is not violated by the actions of internal forces, but the conservation of energy is violated. Energy is ...
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### How can a Higgs decay to heavier products than its mass?

How is it possible that a higgs at ~125 GeV can decay into 2 W bosons @ ~ 80 GeV a piece (for example)? Shouldn't a particle only be allowed to decay to lighter particles + energy? Diagram copied ...
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### wind versus compressed air

What would the calculation look like when computing the wind pressure of a moving vehilce versus compressed air pressure pushing against the the moving vehicle, how much compressed air pressure would ...
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### How is information related to energy in physics?

I recently attended a talk by Dr. Ravi Gomatam on 'quantum reality', where the speaker suggested, that conservation of energy is not a fundamental law, and is conditional, but the conservation of ...
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### Are information conservation and energy conservation related?

as evident from the title, are both, conservation of energy and conservation of information two sides of the same coin?? Is there something more to the hypothesis of hawking's radiation other than ...
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### On what basis do we trust Conservation of Energy?

I'm happy to accept and use conservation of energy when I'm solving problems at Uni, but I'm curious about it to. For all of my adult life, and most of my childhood I've been told this law must hold ...
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### Have red shifted photons lost energy and where did it go? [duplicate]

I think the title says it. Did expansion of the universe steal the energy somehow?
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### Quantum perpetual motion

Perpetual motion describes hypothetical machines that operate or produce useful work indefinitely and, more generally, hypothetical machines that produce more work or energy than they consume, whether ...
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### Long-Life High Altitude Balloon

Normally high-altitude balloon experiments end with the balloon popping and the payload falling back down to be reclaimed. But if a second balloon was attached to the payload, one which was only ...
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### Conservation of Energy and the Poynting Theorem

Conservation of energy in an electrical circuit can be expressed by Ampere's law $$\nabla \times \textbf{B} = \mu_o \textbf{J} + \epsilon_o \mu_o \frac {\partial \textbf{E}} {\partial t}$$ when ...
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### Is the mass to energy physical process an exact mirror of the energy to mass process?

For my level of understanding the only explanation of mass to energy, ie nuclear weaponry, is limited to a simple summing game where a mass deficit is expressed as energy. For the 'reverse' process, ...
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### Why is it wrong when I try to use Newton's Law?

A 25 kg block is connected to a 30 kg block by a light string that passes over a frictionless pulley. The 30 kg block is connected to a light spring of force constant 195 N/m, as in Figure P13.59. The ...
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### Canonical transformations and conservation of energy

I have an important doubt about the nature of canonical transformations in hamiltonian mechanics. Suppose I have a one-degree-of-freedom lagrangian system, whose hamiltonian depends explicitly on ...
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### What causes destruction in car crash?

Suppose a car crashes at a speed $v$ against a wall and comes to a stop. Now if the car crashes at $2v$, does that mean it suffers twice as much destruction, if that can be objectively measured? If ...
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Since the Banach-Tarski paradox makes a statement about domains defined in terms of real numbers, it would appear to invalidate statements about nature that we derived by applying real analysis. My ...
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### Equation $H(q,p)=E$ is the equation of motion or energy-conservation law?

I do not completely understand, why do we consider Hamilton–Jacobi equation $H(q,p)=E$ as equation of motion, whereas it is looks like energy-conservation law?
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### Would a thinner atmosphere (such as high altitude) yield better solar panel energy generation?

I am curious. If you were to put a solar panel up in high altitude or on the surface of the moon, both of which have much less atmosphere to reflect/refract light, would that solar panel produce more ...
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### Is there a valid Lagrangian formulation for all classical systems?

Can one use the Lagrangian formalism for all classical systems, i.e. systems with a set of trajectories $\vec{x}_i(t)$ describing paths? On the wikipedia page of Lagrangian mechanics, there is an ...
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### Where does the energy come from when a current heats a wire (resistor)?

I'm trying to figure out an example from a textbook (Demtröder -- Experimentalphysik 2, pg. 198) where the energy transport caused by a current is depicted: Assume you have a wire (with some ...
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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. ...
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### 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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### 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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### 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 ...
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### 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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### 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 ...
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### 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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### 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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### I think I disprove this with kinematics, but energy says it is right

http://img710.imageshack.us/img710/914/engkin.jpg! 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. $|v_0| = 10m/s$, $y_0 = 10m$,...
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### 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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### What happened to the work done by friction here?

Problem/Solution http://img832.imageshack.us/img832/1313/88791065.jpg! Question What happened to the work done by friction BEFORE it touched the spring? Why was that neglected? Also they say there ...
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### 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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### 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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### 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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### 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 ...