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
vote
2answers
165 views

What will be the unit of mass in $E=mc^2$ when energy units change?

I know that conventionally the unit of $E$ in $E=mc^2$ is taken as joules when mass is in $\mathrm{kg}$. But I am doing some calculations of my own with different units of energy ...
0
votes
1answer
91 views

Lower state of energy for 'connected' molecules

Quote from Wikipedia: Another way to view surface tension is in terms of energy. A molecule in contact with a neighbor is in a lower state of energy than if it were alone (not in contact with a ...
2
votes
0answers
500 views

kinetic energy in collisions [closed]

We were hoping you could help us understand collision energy. Vehicle $ A $ is driving West at $35\space mph$ and weighs $ 1437 \space kg$. Vehicle $ B $ is driving North at $35\space mph$ and weighs ...
2
votes
1answer
289 views

What is meant by the rest energy of non-composite particle?

When talking about the rest energy of a composite particle such as a proton, part of the rest energy is accounted for by the internal kinetic energy of its constituent quarks. But what is physically ...
1
vote
2answers
403 views

Connection between momentum and energy

What is the connection between momentum and energy? Which of the answers is the correct? A particle can have zero momentum but energy. A particle can have zero energy but momentum. ...
1
vote
0answers
476 views

Is the $mL_c$ value for triangular and rectangular fins the same value?

I am looking at the solutions that my professor put up and I feel that he did something wrong. Here is the question and I will give my stab at the solution so you can see why I think that it is wrong. ...
1
vote
1answer
803 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 ...
0
votes
6answers
4k views

17 Joules of Energy From a Mouse Trap

Do you think it would be possible to get 17 joules out of a standard size mouse trap. By my math, it is a torsion coefficient of 3.45 or so out of the spring.
2
votes
1answer
301 views

Negative energies and a partition function

I'm writing down the partition function for a system, for which I know the dispersion relation $$E \left( \mathbf{k} \right) = \sqrt{ \left| \mathbf{k} \right|^2 + m^2 + \cdots }$$ The exact form is ...
2
votes
1answer
795 views

How do I find average temperature given a temperature distribution?

I was told to find the temperature distribution of a wire with a current going through it. So I found $$T(x)=T_{\infty}-\frac{\dot{q}}{km^{2}}[\frac{cosh(mx)}{cosh(mL)}-1]$$ I need to find the ...
3
votes
1answer
1k views

What does activation energy actually do?

Spontaneous (exothermic) chemical reactions often require a push from the addition of externally supplied energy. This energy is often called activation energy. What does activation energy actually ...
3
votes
3answers
470 views

storing energy (as mass)

When chemical energy is released mass is reduced, if only by a negligible amount. Presumably that's true for all energy. And presumably that works in reverse as well: storing energy involves an ...
2
votes
2answers
377 views

Does EM radiation (any, i.e. RF), or sound, radiate everywhere at once?

I am having trouble understanding electromagnetic radiation (or waves in general, be it EM or sound). If I have a 1 Watt speaker, is it infinitely divided and spread out so that everyone in every ...
-1
votes
2answers
208 views

Show that the energy levels of a particle in a specific potential are $E_n=(n+\frac{1}{2})h\omega-\frac{1}{2}\frac{F^2}{m\omega^2}$ [closed]

A particle of mass m moves on the x-axis under the influence of the potential $$V(x)=\frac{1}{2}m\omega^2x^2+Fx$$ Can anyone help me, using Schrödinger's equation in one dimension that the energy ...
3
votes
1answer
168 views

Abstract, generic derivations of energy

How generic can be derivation of energy? In a system with gravity and masses – it is potential energy and kinetic energy. What if a constraint would be specified that no mass and velocity should be ...
2
votes
4answers
1k views

Easy to understand equivalents for energy (kilowatt-hour)

Is there a database/webpage/other source for obtaining simple, easy to understand and meaningful equivalents for converted energy by a photovoltaic-site (Wh/kWh/MWh/GWh)? Something like "xxxkWh equals ...
1
vote
0answers
493 views

How equivalent are heat energy and work energy in connection with a spinning flywheel?

Let's say we have two identical spinning flywheels, that have arbitrary geometry, and are made of copper. Now we apply some heat energy at the center point of flywheel A, causing it to slow down a ...
3
votes
3answers
3k views

How much energy was consumed when we turn on/off light?

My parents told me to turn off the light when I am not using it. But I remember my physics teacher told me that the action of turning on/off a light can cause huge energy. I am wondering how much is ...
1
vote
1answer
96 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 ...
0
votes
2answers
101 views

Energy behind door knocking?

How do we estimate the energy released from knocking on a wooden table or a door?
0
votes
3answers
197 views

Lightning Power?

What is the power source of a lightning? It doesn't consume fuel, nor an atomic fission/fusion, but it discharges a large amount of energy. Is it feasible to create an artificial lightning using the ...
3
votes
2answers
2k views

Energy in an EM wave should depend on frequency

I just finished reading Feynman's Lectures on Physics vol.I, §34-9: "The momentum of light". The author explains that there is a relation between the wave 4-vector $k^{\mu}$ and the energy-momentum ...
-1
votes
2answers
146 views

Energy required to reach 1 wavelength [closed]

I was curious if there was a forumla to find the energy required to reach 1 wavelength in a given substance. (or a vacumn if that's too hard). I am also wondering if this number can tell us anything ...
0
votes
1answer
310 views

When to use Heat Diffusivity eqn and when to use Fourier's law to find temperature distribution?

Let's say that there is a circular conical section that has diameter $D=.25x$ without any heat generation and I need to find the temperature distribution. Originially I thought I could use the heat ...
0
votes
3answers
231 views

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?
1
vote
1answer
123 views

A terminological question about work and energy

Work is force applied over distance. Is it also reasonable to say that work is (the same thing as) the transfer of energy? When work is done, the equivalent energy is transferred. But if energy is ...
3
votes
1answer
247 views

Lifetime of battery

If I directly connect two terminals of 3V battery (negative to positive) using copper wire, would it lose all its charge faster compared to another 3V battery that is used to lighten a 1.5V bulb?
2
votes
2answers
1k views

Is the curvature of space around mass independent of gravity?

Is the curvature of space caused by the local density of the energy in that area?Could gravity be a separate phenomenon only arising from the curvature of space? For instance if the density of energy ...
0
votes
1answer
813 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 ...
3
votes
3answers
361 views

Simple question about a gas in a box with a moving wall

David Albert is a philosopher of Science at Columbia. His book "Time and Chance" includes this example (p 36). A gas is confined on one side of a box with a removable wall. "Draw the wall out, ...
2
votes
2answers
212 views

How much energy does lowering an object into a black hole generate?

An object of mass m is slowly lowered into a black hole of mass 1000 m. Is the amount of braking energy larger than $0.6 mc^2$? Now what if, after lowering the mass close to the event horizon, we ...
0
votes
1answer
32 views

How much energy should I give into each particle in this equation:

I am trying to recreate the results of an article (membrane simulation) and I have the following line: Both particles have the same soft radius, $U_{rep} (r)/\epsilon = \text{exp}\left\{ -20 ...
0
votes
2answers
1k views

Kinetic energy absorbing in order to avoid damages?

is this possible? To absorb kinetic energy and disable inertia force? http://www.youtube.com/watch?v=KnKPbAbJI0w http://www.youtube.com/watch?v=4_5oseSVUc4
1
vote
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 ...
1
vote
3answers
251 views

If humans were able to catch all sun energy reaching the earth for their use, will the climate change?

I guess that energy will be used up and, at the end, will contribute to heat the earth, so I see no big differences... please explain your point of view.
3
votes
2answers
2k views

Proof that Energy Momentum Tensor of Scalar Field Theory satisfies Weak Energy Condition

It's a question on Sean Carroll's Spacetime and Geometry, where we are supposed to prove that the energy momentum tensor of scalar field theory satisfies Weak Energy Condition (WEC). The energy ...
3
votes
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 ...
0
votes
1answer
218 views

Violation of conservation of energy and potential energy between objects

I would like to clarify my question. I have numbered them to be independent questions For any conservative fields, $\vec{F} = -\nabla U$. Which means the restoring force is opposite to the ...
2
votes
3answers
106 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 ...
31
votes
4answers
13k 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 ...
1
vote
0answers
98 views

Practical personal aircraft today? [closed]

Is it possible to build, today, a personal aircraft that not use an impracticable amount of fuel for everyday use? What are the physical concepts that could be used to build it?
4
votes
1answer
38k views

How much energy is in a lightning strike?

According to Wikipedia an average lightning has 1TW, the whole world used 16TW in 2006. (I suppose this means the same as 16TWh in one year?) Sometimes the lightning reaches 100kA. This peak last for ...
1
vote
2answers
273 views

Could someone remind me what this means again? $\nabla U = \pm F$

You know that for a potential function (conservative force/fields) that $\nabla U = \pm \vec{F}$ In math, we don't have that minus sign, we have only the plus one. What does it mean if you get rid ...
1
vote
2answers
450 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 ...
2
votes
1answer
549 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 ...
4
votes
4answers
12k 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 ...
4
votes
2answers
580 views

What is a single word that describes the idea of the second time derivative of energy?

I think about position, its time derivative speed, and its second time derivative, acceleration. I would like to identify a single word that can be used as a handle for the second time derivative of ...
4
votes
1answer
235 views

Is there a theory which treats particles as classical point singularities?

Is there a published theory that looks at all matter as occupying no space and only being felt because of its gravitational pull? We've been taught in school that matter has mass and occupies space. ...
2
votes
7answers
376 views

My math says this is wrong, but what part of energy conservation am I violating?

Basically energy of conservation says it doesn't matter what path I take, energy will be the same from before as it is after. So I took the block at the top's energy equal to the energy when it ...
-1
votes
2answers
169 views

Why is the work positive here?

Problem http://apcentral.collegeboard.com/apc/public/repository/ap11_frq_physics_b_formb.pdf Please refer to question 1f Solutions ...