Energy is a quantity which gives an overview of the amount of work doable by the system.
25
votes
9answers
14k views
If photons have no mass, how can they have momentum?
As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects ...
37
votes
14answers
7k views
Why does kinetic energy increase quadratically, not linearly, with speed?
As Wikipedia says:
[...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $mv^2/2$.
Why does this not increase linearly with speed? Why does it take so much ...
5
votes
3answers
520 views
Explain how (or if) a box full of photons would weigh more due to massless photons
I understand that mass-energy equivalence is often misinterpreted as saying that mass can be converted into energy and vice versa. The reality is that energy is always manifested as mass in some ...
21
votes
7answers
3k views
Why does holding something up cost energy while no work is being done?
I read the definition of work as
$$W ~=~ \vec{F} \cdot \vec{d}$$
$$\text{ Work = (Force) $\cdot$ (Distance)}.$$
If a book is there on the table, no work is done as no distance is covered. If I ...
8
votes
2answers
733 views
Does $E = mc^2$ apply to photons?
Photons are massless, but if $m = 0$ and $E = mc^2$ then $E = 0c^2 = 0$. This would say that photons have no energy, which is not true.
However, given the formula $E = ℎf$, a photon does have energy ...
-2
votes
3answers
586 views
Anti-laser: How sure we are that energy is transported?
Reading this PE question
can-we-transport-energy-over-infinite-distances-through-vacuum-using-light, a related question arises naturally:
Is energy transported (by light)? -- (I did believed in this ...
32
votes
7answers
7k views
What Is Energy? Where did it come from?
The simplistic undergrad explanation aside. I've never really understood what energy really is. I've been told that it's something when converted from one kind of something to another kind does some ...
14
votes
3answers
844 views
Amplitude of an electromagnetic wave containing a single photon
Given a light pulse in vacuum containing a single photon with an energy $E=h\nu$, what is the peak value of the electric / magnetic field?
24
votes
10answers
3k views
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 ...
26
votes
7answers
3k views
Is a hard drive heavier when it is full?
Browsing Quora, I saw the following question with contradicting answers.
For the highest voted answer:
The bits are represented by certain orientations of magnetic fields
which shouldn't have ...
7
votes
3answers
2k views
When is the Hamiltonian of a system not equal to its total energy?
I thought the Hamiltonian was always equal to the total energy of a system but have read that this isn't always true. Is there an example of this and does the Hamiltonian have a physical ...
19
votes
5answers
2k views
Do photons gain mass when they travel through glass?
Please correct me if I'm wrong, but I believe that photons slow down when travelling through glass. Does this mean they gain mass? Otherwise, what happens to extra kinetic energy?
I understand now ...
6
votes
3answers
948 views
Maxwell's Demon Constant (Information-Energy equivalence)
New Scientist article: Summon a 'demon' to turn information into energy
The speed of light c converts between space and time and also appears in e=mc^2.
Maxwell's Demon can turn information supplied ...
1
vote
2answers
592 views
Why is there a $\frac 1 2$ in $\frac 1 2 mv^2$?
For elastic collisions of n particles, we know that momentum in the three orthogonal directions are independently conserved:$$ \frac{d}{dt}\sum\limits_i^n m_iv_{ij} =0,\quad j=1,2,3$$
From this, it ...
2
votes
2answers
743 views
Is the E-cat for real?
Does this thing really do what they say?
http://www.youtube.com/watch?v=EhvD4KuAEmo
If it does, it looks like this will probably be the biggest breaktrough in science ever :)
4
votes
3answers
682 views
Black holes and positive/negative-energy particles
I was reading Brian Greene's "Hidden Reality" and came to the part about Hawking Radiation. Quantum jitters that occur near the event horizon of a black hole, which create both positive-energy ...
4
votes
3answers
618 views
How much of the energy from 1 megaton H Bomb explosion could we capture to do useful work?
The world is full of nuclear warheads being stockpiled. Controlled fusion power seems a long way away. Could we put these warheads to better use by exploding them in a controlled way and capturing ...
3
votes
6answers
4k views
Is fire matter or energy?
I wanted to know that if FIRE is Matter or Energy. I know that both are inter-convertible. but that doesn't mean there is interconversion taking place just like nuclear fusion or fission. If it is ...
2
votes
4answers
3k views
What is pure energy in matter antimatter annihilation?
I used to read the term "pure energy" in the context of matter antimatter annihilation,
are them photons? is it other form of heat, are particles with mass ?
What does it refer to?
thanks
12
votes
2answers
4k views
Why can't energy be created or destroyed?
My physics instructor told the class, when lecturing about energy, and that it can't be created or destroyed. Why is that? Is there a theory or scientific evidence that proves his statement true or ...
0
votes
5answers
887 views
Is the law of conservation of energy still valid?
Is the law of conservation of energy still valid or have there been experiments showing that energy could be created or lost?
11
votes
2answers
329 views
How does rest mass become energy?
I know that there's a difference between relativistic rest mass. Relativistic mass is "acquired" when an object is moving at speeds comparable to the speed of light.Rest mass is the inherent mass that ...
3
votes
1answer
330 views
Is there something like the Poynting vector for hydraulic circuits?
The Poynting vector is a representation of the energy flux in electromagnetics, showing the amount and direction of power flow at different points in space. In electric circuits, the energy is not ...
3
votes
2answers
158 views
Dark Energy saving the world
Ok, my understanding of physics is still limited, hoping to change that in the following years, but I have a question.
Dark energy is what provokes the expansion of the universe, so, in theory, it ...
1
vote
1answer
103 views
Is light affect by gravity? Why?
I would like to know if light is affected by gravity, also, I would like to know what is the more correct for the definition of gravity: A force that attracts bodies with mass or force that attracts ...
4
votes
2answers
380 views
Energy operator
Does the Hamiltonian always translate to the energy of a system? What about in QM? So by the Schrodinger equation, is it true then that $i\hbar{\partial\over\partial t}|\psi\rangle=H|\psi\rangle$ ...
14
votes
4answers
549 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 ...
4
votes
1answer
151 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. ...
9
votes
2answers
588 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 ...
4
votes
1answer
153 views
Is this formula for the energy of a configuration of 3 fluids physically reasonable?
I have studied for a couple of months now a mathematical model of the energy of a configuration of immiscible fluids situated in a fixed container such that the fluids fill the container. In other ...
2
votes
2answers
366 views
Is the potential energy in a compressed spring a Lorentz invariant?
The total energy of an object comes from the time part of the four-momentum, and so isn't a Lorentz invariant. On the other hand, is the potential energy of a compressed spring a Lorentz invariant?
2
votes
4answers
2k 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 ...
2
votes
2answers
344 views
Mass in terms of energy
Given the relationships between mass and energy in relativity, and given that particles with mass can be created given energy over the threshold energy, and vice-versa, can we say that mass is simply ...
1
vote
1answer
110 views
Specific electron energy gap values $E_{i+1}-E_i$ vs. photons with arbitrary energy $\hbar \omega$
The energy levels of electrons in an atom are quantized $E_i$. A photon of a specific momentum $\vec p$ and energy $$\omega=(E_{i+1}-E_i)/\hbar$$ hits an atom and gets absorbed. Okay now say the ...
1
vote
2answers
584 views
Conservation of Energy in a magnet
When a permanent magnet attracts some object, lets say a steel ball, energy is converted into for instance kinetic energy and heat when attraction happens, and they eventually collide. Does this imply ...
1
vote
2answers
117 views
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, ...
1
vote
0answers
145 views
Will earth rotation be slowed down? [duplicate]
Possible Duplicate:
Are tidal power plants slowing down Earth's rotation?
Since we have various energy acquiring facilities that harvest energy from earth rotation. (e.g. wind, tidal ...
0
votes
1answer
333 views
Elastic Collision And Momentum
The question I am working on is, "Two blocks are free to slide along the friction-less wooden track shown below. The block of mass $m_1 = 4.98~kg$ is released from the position shown, at height $h = ...
0
votes
1answer
162 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 ...
0
votes
3answers
10k views
Charging 12V 150Ah battery
I want to charge a 12V battery of 150Ah with a solar panel. The solar panel specs is
12V, 25 Watt.
Can anyone please provide me how to calculate that how much time it will take to charge the battery? ...
-4
votes
1answer
182 views
Tricky Conservation of Momentum problem: find the ratio of the carts by mass percentage lost [closed]
A wagon is coasting at a speed $v_A$ along a straight and level road. When 42.5% of the wagon's mass is thrown off the wagon, parallel to the ground and in the forward direction, the wagon is brought ...
39
votes
9answers
3k views
What makes running so much less energy-efficient than bicycling?
Most people can ride 10 km on their bike. However, running 10 km is a lot harder to do. Why?
According to the law of conservation of energy, bicycling should be more intensive because you have to ...
14
votes
1answer
333 views
a priori validity of $W=\int Fdx$ in relativity?
There are lots of different ways of arriving at the relativistic relations involving mass, energy, and momentum such as $E=mc^2$ and $m^2=E^2-p^2$ (the latter with $c=1$). One that I've seen in some ...
5
votes
1answer
225 views
Historical and philosophical reflexions about the concept of energy? [closed]
Perhaps you know the books:
Concepts of Mass in Contemporary Physics and Philosophy
Concepts of Force: A Study in the Foundations of Dynamics
by Max Jammer, which discusses mass and force from a ...
7
votes
9answers
877 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 ...
6
votes
2answers
267 views
Photon energy - momentum in matter
$E = h\nu$ and $P = h\nu/c$ in vacuum.
If a photon enters water, it's frequency $\nu$ doesn't change.
What are its energy and momentum : $h\nu$ ? and $h\nu/c$ ?
Since part of it's energy and momentum ...
5
votes
5answers
2k views
Why there is a 1/2 in kinetic energy formula? [duplicate]
Possible Duplicate:
Why is there a $\frac 1 2$ in $\frac 1 2 mv^2$?
Hèllo, I have a question about kinetic energy formula.
As you know, in kinetic energy formula, we have:
...
14
votes
6answers
3k views
Is two cars colliding at 50mph the same as one car colliding into a wall at 100 mph?
I was watching a youtube video the other day where an economist said that he challenged his physics professor on this question back when he was in school. His professor said each scenario is the same, ...
12
votes
6answers
2k views
Why is torque not measured in Joules?
Recently, I was doing my homework and I found out that Torque can be calculated using $\tau = rF$.
This means the units of torque are Newton meters. Energy is also measured in Newton meters which are ...
8
votes
3answers
604 views
Is the normal force a conservative force?
Most of the time the normal force doesn't do any work because it's perpendicular to the direction of motion but if it does do work, would it be conservative or non-conservative?
For example, consider ...
