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.
7
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2answers
775 views
Where does the electricity, generated by a solar panel, go if you don't use the electricity?
I'm sorry if this question is too trivial for this Q&A forum. I am a layman when it comes to physics (though I did cover the high-school physics courses).
I was wondering what happens to the ...
2
votes
1answer
246 views
Bernoulli equation from energy conservation
I have derived the energy conservation equation:
\begin{equation}
\frac{\partial}{\partial t} \left [ \frac{1}{2} \rho v^2 + \varepsilon + \rho \phi \right ] + \frac{\partial}{\partial x_j} \left [ ...
3
votes
2answers
636 views
First integral of an equation of motion: $\mu\ddot r=-\frac{k}{r^2}$
I've got an equation of motion (EOM), which is
$
\mu\ddot r=-\frac{k}{r^2}
$
How do I find the first integral of this EOM? I'd appreciate it if someone could show me the steps involved. I should ...
2
votes
1answer
160 views
Would the Moon drift away from the Earth due to extraction of tidal energy? [duplicate]
Possible Duplicate:
Why does the moon drift away from earth?
It seems to me that, due to conservation of energy, the moon would drift away from the Earth if humans began extracting large ...
4
votes
1answer
299 views
How is dark energy consistent with conservation of mass and energy?
I initially thought that dark energy must in some way violate conservation of mass and energy since the component of the energy density of space that comes from dark energy is constant, and space is ...
6
votes
4answers
156 views
Is energy extensivity necessary in thermodynamics?
Given a partition of a system into two smaller systems, the energy $U$ is devided into $U_1$ and $U_2$, with
$$U=\mathcal{P}(U_1,U_2):=U_1+U_2,$$
so that $U_2$ is given by $U-U_1$. Here the ...
1
vote
2answers
262 views
Standing wave and energy flux
Here is a problem I have been asked that I do not know the answer. Consider two ideal wave generators (it can be sound generator or whatever) separated by a distance L and facing each other. At t=0 ...
1
vote
1answer
164 views
What controls whether a ball will skid or roll?
A billard ball is struck with a cue. The line of action of the applied impulse is horizontal and passes through the center of the ball. The initial velocity $v_0$ of the ball, its radius $R$, its mass ...
3
votes
2answers
1k views
Are there devices which convert thermal energy to electric energy?
Converting optical energy to electric energy is a huge business based on the photovoltaic effect. Is there an analogous effect for phonons? Are ther devices which convert phonon energy to electric ...
0
votes
2answers
94 views
Really basic mechanics and Galilean Relativity question
Consider two solid objects: A and B.
System 1:
A <----- B
10m/s
System 2:
A -----> B
10m/s
...
1
vote
3answers
167 views
Neutron decay and electron anti neutrino $n\to p + e + \bar{\nu}_e$
Why do we need neutrino to explain neutron decay?
Is there any evidence regarding existence neutrinos in the context of
$n\to p + e + \bar{\nu}_e$?
2
votes
1answer
134 views
Why don't cancelling wavefunctions for two different particles give zero total wavefunction?
Let $\left|a\right>=e^{i(kx-\omega t)}$， $\left|b\right>=-e^{i(kx-\omega t)}$ be two neutral particles in the 1D free space without any interaction. Then ...
2
votes
1answer
147 views
Man on treadmill and energy/mass conservation
Suppose we put a very obese man inside a large air tight isolated room with a treadmill and he runs on the treadmill for 24 hours and at the end of the day he sees his weight has come down (mostly ...
2
votes
1answer
96 views
extracting energy from cosmological expansion
This question is a more concrete reincarnation of an old question about energy conservation in GR.
Are there mechanisms to extract energy from the cosmic rate of expansion? putting some extremely ...
1
vote
2answers
128 views
Unexpected potential energy increase during Tic-Tac drop
I dropped a Tic-Tac:
(no worries, it happened before), and I noticed as it bounced on the floor that it would first jump 20 cm high, and at the next bounce for instance 50 cm high. Shouldn't it ...
3
votes
1answer
379 views
Rabi oscillation
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 $\omega < \omega_0$, ...
0
votes
2answers
99 views
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 ...
1
vote
1answer
119 views
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 ...
3
votes
2answers
178 views
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 ...
2
votes
1answer
349 views
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 ...
2
votes
1answer
194 views
Existence Of Electric Field Lines
Can an Electric Field with field lines Like So Exist:
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 it be made to ...
0
votes
2answers
340 views
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,
...
6
votes
2answers
586 views
Can a force in an explicitly time dependent classical system be conservative?
If I consider equations of motion derived from the pinciple of least action for an explicilty time dependend Lagrangian
$$\delta S[L[q(\text{t}),q'(\text{t}),{\bf t}]]=0,$$
under what ...
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, ...
0
votes
2answers
1k views
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 ...
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 ...
9
votes
5answers
1k views
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 ...
0
votes
1answer
283 views
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 ...
4
votes
3answers
338 views
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 ...
1
vote
1answer
463 views
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 ...
-1
votes
1answer
70 views
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 ...
-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 ...
1
vote
1answer
328 views
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 ...
1
vote
2answers
468 views
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 ...
6
votes
3answers
752 views
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 ...
6
votes
2answers
310 views
Have red shifted photons lost energy and where did it go?
I think the title says it. Did expansion of the universe steal the energy somehow?
3
votes
3answers
344 views
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, ...
1
vote
3answers
426 views
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 ...
1
vote
2answers
636 views
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 ...
1
vote
2answers
97 views
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, ...
0
votes
1answer
361 views
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. ...
1
vote
2answers
390 views
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 ...
3
votes
2answers
800 views
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 ...
8
votes
5answers
611 views
Does the Banach-Tarski paradox contradict our understanding of nature?
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 ...
0
votes
3answers
144 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
2answers
316 views
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 ...
3
votes
3answers
544 views
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 ...
2
votes
1answer
540 views
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 ...
1
vote
1answer
128 views
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
votes
4answers
517 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 ...
