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)

-1
votes
1answer
34 views

How can I get 100% fuel efficiency from an IC Engine in a car?

I understand that the current technology of internal combustion (IC) engines gives about 20% for a normal car. I want to understand what I can do to achieve 100% or near 100% efficiency in the same ...
0
votes
2answers
30 views

Trouble understanding non-conservative force and work

I am having some trouble defining whether a force is conservative or not. In the example that we are working on a mass on a pulley system is used to cause a slender bar to rotate from rest to a ...
1
vote
0answers
23 views

Light bulbs Flashing on when disconnected

For the 2nd time I've had a lightbulb flash on in my hand after removing from the power socket. The first time the overhead light in my hallway was out for a couple of days before I got around to ...
0
votes
1answer
54 views

Is fire more harmful than a phone?

I have read somewhere that the higher the frequency of electromagnetic radiation, the higher the damage it causes to your body, and visible light has a very high frequency in comparison to microwave ...
0
votes
0answers
26 views

What is the physical meaning of potential energy in a band diagram of a semiconductor?

According to this slide (from this website), the potential energy of an electron in a semiconductor is defined by the difference between the Energy level of the conduction band and the fermi level. ...
6
votes
1answer
143 views

Optimal size for a bike tire

While I was riding my bike funnily enough I wondered if there was an optimal size for a bike wheel. So I suppose that is my question, What is the optimal size for a bike wheel? If the bike accelarates ...
8
votes
6answers
16k views

Why does higher acceleration minimize a car's fuel consumption?

I generally try to optimize my car's fuel consumption when driving, using my car's real-time MPG gauge and average-trip MPG indicator. Until recently, I believed the slower the acceleration, the ...
0
votes
1answer
21 views

Since a dipole has some potential energy,it should be able to do some work !? What is it?

We say a dipole has potential energy.What work can a dipole do ?.For eg : A ball on a shelf has a PE because it has the potential to fall down and do some work on the ground if shelf wasnt there.What ...
0
votes
0answers
21 views

How are negative energy orbits around a Black Hole defined?

I have read several times that within the ergosphere or a Kerr Black Hole, it is possible to have particles that have an orbital energy value of less than 0. However, I do not understand the concept ...
0
votes
0answers
21 views

What is Phase Transition in Cosmology and where does the energy of it comes from?

There are different theories on how our universe will end. The Big Crunch, The Big Rip, The Big Freeze but I also heard about Phase Transition on a space science program on a science channel, what ...
1
vote
1answer
43 views

Energy diagram of a planet with fixed mechanical energy

Consider the following energy diagram for the motion of a planet about a star. The centrifugal potential curve can be represented once I fixed the angular momentum $\vec{L}$ of the planet. To fix the ...
1
vote
1answer
36 views

Why does the kinetic energy of a particle moving in circular motion increase when the turn radius decreases and no torque is acting?

Why does the kinetic energy of a particle moving in circular motion increase when the turn radius decreases and there is no torque acting? E.g. if a planet is rotating about its axis and it shrinks to ...
1
vote
2answers
41 views

Why is Kinetic Energy = (-) Total Energy and Potential Energy = 2 $\times$ Total Energy?

I came across this relation while reading on the Bohr atomic model. Are there any other forces for which these relations hold good?
0
votes
3answers
7k views

Work, Energy & Power - Body slides down a hemisphere

A small body of mass $m$ slides down from the top of a hemisphere of radius $r$. There is no friction between the surface of the block and the hemisphere. The height at which the body loses contact ...
0
votes
3answers
80 views

Zero Potential Energy Change in Raising a Mass

Suppose I am able to lift a body of mass $m$ up using a constant force $F=mg$ so that net acceleration is zero on it. I want to find the potential energy and the total energy of the body when I have ...
0
votes
1answer
32 views
0
votes
1answer
35 views

Inconsistency in solving energy in free space given electric potential

So we have the equation $$ W= \frac{1}{2} \int_{volume} \rho_v V\,dv$$ and then after applying a vector identity which is $$\nabla \cdot (V\vec{D})=V(\nabla\cdot \vec{D})+\vec{D}\cdot (\nabla V)$$...
0
votes
3answers
25 views

ionizing radiation and energy

At the website "How Stuff Works" an article (radiation sickness) states that when radiation knocks an electron from an atom, energy (specifically 33 electron volts) is released which heats up the ...
0
votes
1answer
43 views

Wavefunction of a system of particles

A three-dimensional volume $V$ contains a certain number $N$ of electrons and they can't escape the volume $V$. Assume for simplicity that the potential $\mathcal{V}(\mathbf{r})$ is zero in all the ...
0
votes
0answers
36 views

When should gravitational potential energy be included in the law of conservation of energy

I have a problem that says: A block of mass 0.249 kg is placed on top of a light, vertical spring of force constant 4 975 N/m and pushed downward so that the spring is compressed by 0.090 m. ...
0
votes
0answers
22 views

Compare a normal spring and a torsion spring

Consider a torsion spring with a known spring constant $K_c$. I would like to calculate the required $K_l$ for an equivalent linear spring, which would store the same amount of energy when compressed. ...
1
vote
2answers
90 views

Which coordinate is to be considered for the energy of simple pendulum?

For an simple harmonic oscillator energy can be represented as in picture. Consider in particular picture (b) with the energy as a function of the coordinate $x$. Consider now a simple pendulum. The ...
0
votes
1answer
672 views

How do I derive the formula of coefficient of restitution?

I have learnt in collision mechanics about the term Coefficient of restitution, $\mathrm{e=\frac{v_{2f}-v_{1f}}{v_{1i}-v_{2i}}}$. But the sad part is that, in my book only the formula is there. My ...
1
vote
2answers
59 views

How does conservation of energy work?

I understand that the law of conservation of energy states that energy cannot be created or destroyed, it can only change from one form to another. This means that the total energy before a certain ...
3
votes
2answers
204 views

Climbing Stairs and Calories Burnt

I climb stairs to work (20 floors) every day . The least amount of work my body is doing by gaining potential energy (P.E) = mgh where m is mass , g is acc. due to gravity and h is height . Assuming ...
0
votes
1answer
27 views

The highest photon energy ever measured and transplanckian energies

What is the highest energy of we have measured of a photon by any physical experiment? Has a transplanckian energy photon been measured? Can we study them -if they exist- with special relativity?
1
vote
1answer
78 views

Statistical Physics: How do we derive this equation?

I'm reading through Statistical Physics by F. Mandl and there is a step in arriving at an equation that I don't follow. He uses $$P = \sum_r p_r \left(-\frac{\mathrm dE_r}{\mathrm dV} \right) \tag{4....
0
votes
4answers
339 views

Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?

In classical physics we often cast an analogy between translational and rotational systems Force < > Torque Energy < > Rotational Energy Momentum < > Angular Momentum and considering SI ...
-1
votes
2answers
86 views

How can work be done during collisions? [closed]

Consider a collision between two material points, with no external forces acting on the system. Linear and angular momentum of the system are always conserved, while the kinetic energy of the system ...
1
vote
3answers
62 views

Work energy theorem: variation of kinetic energy as sum of terms given by the work of single forces

Work-energy theorem states that $$W_{\sum F_i}=\Delta K,$$ where I expressed with $W_{\sum F_i}$ the work done by the resultant of forces ${\sum F_i}$ and with $K$ the kinetic energy. In general it ...
5
votes
4answers
3k views

Are neutrinos affected by gravity?

Layman here, but EE and BS physics. I know that light is affected by gravity. But are neutrinos? During the collapse of a star into a neutron star, as the electrons join protons to form neutrons (e....
3
votes
0answers
54 views

What is the abstract idea behind energy? [duplicate]

Many theories use something called energy to explain reason behind certain phenomenon, but I have not been able to grasp the idea of energy. Maybe it is because I have grown in a society where the ...
1
vote
1answer
170 views

How to calculate the electric energy caused by eletrostatic field stored in a region, given $V$? [closed]

I have problem in apply the rules to find the energy stored in free space here is the problem Find the energy stored in free space for the region $$0.002<r<0.003m,\quad 0<\phi<\frac{\...
0
votes
0answers
25 views

Accelerating voltage relationship to energy

if I have the accelerating voltage, how can I obtain the energy in eV? Basically, I asked someone for the energy, but instead they gave me the accelerating voltage. What more data would I need to ...
0
votes
3answers
93 views

Lifting and putting down a weight

A man in a gym lifts a weight and then puts it down where it was before. 1) What can be say about the work done by the man to the weight? 2) Can it be equal in absolute value to the work made by ...
1
vote
0answers
36 views

Does energy produce a gravitational force [duplicate]

$E=mc^2$. From this, I would assume that any form of energy (not just rest-mass energy, but kinetic energy as well) would produce a gravitational force. Am I being too naive in my application of ...
1
vote
2answers
40 views

Turning Work into Thermal Energy in Joule’s Experiment

In Joule’s famous experiment where he determined the specific heat capacity of water, he used a paddle to increase the temperature of the water. That is, the kinetic energy of the paddle went into the ...
2
votes
2answers
894 views

Work done by a non-conservative force and change in potential energy

I know that the work done by a non-conservative force is equal to the change in total mechanical energy (from Work-Energy Theorem). But I read in a place that "Non-conservative forces don't affect PE"....
-4
votes
1answer
43 views

Put energy into void [closed]

what happens when you drop matter or energy of any type into the void , by void I mean a space of absolute emptiness with nothing more than darkness and total absence of every kind of energy , ...
1
vote
2answers
93 views

Physical meanings of kinetic energy

While studying energy on Sears & Zemansky's University Physics, I came up with a doubt on the meaning of kinetic energy. The book gives two possible physical interpretations of this quantity. ...
0
votes
0answers
30 views

Could Something Infinitely Heat Up?

If there was a perfect environment, and something was able to continue to heat up indefinitely, what would happen?.. What would be the outcome of something continuously heating up?
1
vote
1answer
65 views

Work-energy theorem for a system

While studying energy conservation on Morin I found this explanation about the work-energy theorem for a system. The work–energy theorem stated before is relevant to one particle. What if we are ...
0
votes
1answer
41 views

Would High Explosive helped free Shackleton's ship, trapped in pack ice?

This question is based on physical concepts regarding presssure waves and crystallography, but I thought it might be helpful to include a brief historical context first. From Wikipedia: Loss of Polar ...
0
votes
1answer
36 views

How would you get the velocity in this problem? (and what did I do wrong) [closed]

This problem is supposed to be solved by using the conservation of energy but I do not really know how to solve it that way.
1
vote
0answers
28 views

Reason behind the heat capacity of water

Water has a high specific capacity. Why can water release a large amount of energy without a great change in temperature?
0
votes
0answers
26 views

Microscopic Definition of Heat and Work

If I am given a statistical System, then I can define state-variables like Energy, Entropy or other Observables, and then I can (at least for equilibrium states) give the Change of Energy as: \begin{...
1
vote
0answers
32 views

How to derive kinetic energy from the Lagrange equations? [duplicate]

I'm having trouble deriving the kinetic energy from the Lagrange equations. For reference, I'm following Landau and Lifshitz book, "Mechanics," which can be found for free at Archive. In any case, I'...
-1
votes
1answer
227 views

What prevents this magnetic perpetual motion device from working? [closed]

Two days ago, I thought of this device, which may seem to rotate indefinitely(in all 3 directions). I have two questions. Is this perpetual motion machine already known? If it is, could you please ...
1
vote
3answers
185 views

Is work-kinetic energy theorem useless?

This is how my book explains it: $W_{net} = \delta K$ Since the net work is tied to changes in kinetic energy and changes in speed, a mass must accelerate in order for net work to be nonzero. ...
0
votes
0answers
30 views

When space containing an extended mass contracts, what delivers or absorbs energy?

As my understanding goes, general relativity and the many theories that have sprung from it allow for the contraction and expansion of space. When such expanding (or contracting) space contains a ...