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.

learn more… | top users | synonyms

1
vote
1answer
34 views

Free fall in non-uniform field

Imagine I'm a space-diver, with mass $m_1 $, 500 miles above the Earth's surface at $x_i$. I want to calculate my position, velocity, and acceleration as a function of time, accounting for the Earth's ...
2
votes
1answer
28 views

Energy conservation around a black hole

In the Schwarzschild black hole, the Killing vector "time translation" $k^a$, so that the following quantity is conserved along a geodesic: $$E = -g_{ab}k^au^b = (1 - ...
2
votes
1answer
53 views

Is Phone-Charging Plant Product Even Possible?

Bioo, a new company making the rounds in the press, claims to produce "electricity from plants' photosynthesis." Specifically, their website claims that a single potted plant with their integrated ...
0
votes
0answers
12 views

Derivation of Poynting theorem in matter

In most textbooks I have read they derive the Poynting theorem using the Maxwell's Equation in vacuum and the fact that the force density $f=\pmb{E} \cdot \pmb{J}$. Then they just generalize it ...
1
vote
1answer
38 views

What are the assumptions behind the Lagrangian derivation of energy?

What are the assumptions behind the Lagrangian derivation of energy? I understand that we're searching for a function $L$ that describes a set of physics so that solving the energy minimization ...
2
votes
0answers
34 views

Is expectation value of the Hamiltonian always the energy? [duplicate]

There are time dependent & space dependent systems (magnetic fields) and time independent (particle in a box or harmonic oscillator). In the latter the expectation value is the 'average' energy ...
0
votes
1answer
39 views

Conservation of energy and realm of possibility

The law of conservation of energy states that energy cannot be created or destroyed. Based on this principle, you can safely conclude that any effect resulting from a cause must somehow keep all ...
0
votes
1answer
35 views

Expansion of the Universe and Conservation of Energy [duplicate]

How the expansion of the universe doesn't violate the principle of the conservation of energy? If we would put a spring between two objects, the distance would get bigger between them, so would the ...
0
votes
0answers
18 views

Collision time of two particles [duplicate]

The question I am trying to answer is the following. Consider two particles of mass $m_1,m_2$, initially at rest at a distance $2a$ apart. They are attracted to each other via Newton's law with ...
4
votes
3answers
70 views

Where does the energy of a photon trying to escape a black hole go?

I've heard "light cannot escape a black hole" explained several ways. One is that if a photon inside the event horizon tries to escape a black hole it loses energy to gravity. As it loses energy its ...
1
vote
1answer
74 views

How does the uncertainty principle relate to quantum fluctuations?

I found a webpage that just kind of mentions the uncertainty principle lightly but doesn't really go into detail as to why we need it in the first place when considering quantum fluctuations and ...
0
votes
0answers
17 views

Conservation of energy in a sound wave

I have two ultrasonic transducers, an emitter and a receiver, and I'd like to know how the energy of the spherical wave is conserved. I guess the energy is proportional to its amplitude and it ...
0
votes
1answer
31 views

Why can't an ocean liner be powered by an engine that takes heat from the ocean water and eject ice cubes?

The following excerpt comes from the physics textbook Understanding Physics, by David Cassidy, Gerald Holton, and James Rutherford: A ball lying on the floor will not somehow gather energy from ...
3
votes
1answer
94 views

Can $E=mc^2$ be derived from the Minkowski spacetime metric? $s^2=x^2+y^2+z^2-(ct)^2$?

Can $E=mc^2$ be derived from the Minkowski spacetime metric? $s^2=x^2+y^2+z^2-(ct)^2$? If so, how? Can the equivalence of mass and energy be derived from the Minkowski spacetime metric? Has someone ...
-5
votes
2answers
74 views

If I spinned a fan in a vacuum it will keep spinning forever. Why can we not make energy out of it? [duplicate]

Suppose we created a vacuum and spinned a turbine inside it with some amount of force. According to newton's second law it will keep spinning as there is no air resistance, so why can we not make ...
1
vote
2answers
35 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
1answer
18 views

Spheres collide with merry-go-round [closed]

Four spheres, with uniform densities $\rho_1, \rho_2, \rho_3, \rho_4$ and radii $r_1, r_2, r_3, r_4$, respectively, roll without slipping with constant velocities $v_1, v_2, v_3, v_4$ along tracks ...
1
vote
6answers
80 views

Why does positive work done by internal conservative forces $\implies$ decrease of potential energy?

Potential energy can be thought as the amount of work that the force can potentially do on the point because of its position. $$W=-\Delta U=U_{initial}-U_{final}$$ A positive work done by a force ...
0
votes
0answers
32 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
1answer
44 views

Interference of light waves question

We were recently asked to solve a question in class which goes as follows: In a modified Young's double slit experiment, a monochromatic uniform and parallel light beam of wavelength $6000$ ...
1
vote
2answers
55 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 ...
4
votes
3answers
480 views

Poynting's theorem - energy conservation

Poynting's theorem: $$\int_V\left(\vec{E}\cdot\vec{J}\right)\,\mathrm dV = -\dfrac{\partial}{\partial t}\int_V\dfrac{1}{2}\left(\epsilon_0 E^2 + \dfrac{1}{\mu_0}B^2\right)\,\mathrm dV - ...
0
votes
1answer
17 views

Max theoretical efficiency of a weight powering a car [closed]

Say we had a 700 gram car. There is a 2 kg weight on a launcher that is supposed to power it. What is the maximum theoretical speed it takes to travel 8 meters? Its a little hard to describe so ...
0
votes
0answers
30 views

Find the Energy release in this Nuclear Reaction (Fission)

Plutonium undergoes nuclear fission according to the equation below: The value of $x=3$. The question is to estimate the energy released in this reaction. I know the Binding Energy per nucleon ...
0
votes
0answers
21 views

Intensity of interfering light waves

I had a conceptual question above light wave interference. Suppose that two light beams, each of an irradiance $I$ interfering on an area $A$ of a screen, such that all of the light from each beam ...
-1
votes
1answer
27 views

why laser beams don't get reflected (or knocked away) when they intersect with each other?

laser beams are photons with the same frequency and the same direction, but according to the wave-particle duality, photons have mass. but if we shoot two masses and they intersect at some point ...
0
votes
0answers
22 views

How is energy conserved in a Crookes tube?

In a Crookes tube electrons are accelerated by an electric field and gain kinetic energy. If the electric field is provided by a capacitor the capacitor would be discharged by the anode current when ...
1
vote
1answer
54 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
17 views

About Electric potential

when we bring a unit positive charge from infinity to a point in the electric field EF does work on the charge and external work is also done on the charge in same amount but in opposite sign. then ...
64
votes
3answers
3k views

Why do travelling waves continue after amplitude sum = 0?

My professor asked an interesting question at the end of the last class, but I can't figure out the answer. The question is this (recalled from memory): There are two travelling wave pulses moving in ...
3
votes
3answers
84 views

Car Crash Question… (Conservation of Momentum?) [duplicate]

Dumb car crash question. I think this is a conservation of momentum problem. In one scenario, two 4000 lb cars (car A and car B) each traveling at exactly 35 MPH in opposite directions have a ...
0
votes
0answers
15 views

Could we challenge the imposibility of the second law of thermodynamic by the Adiabatic Process of the first law of thermodynamic?

by this question I mean in adiabatic process there is no heat exchange, so in efficiency formula if we substitute zero then the efficiency would be 100%.
1
vote
1answer
37 views

Mechanical energy $E<0 \implies $ the system is in a bound state?

In a gravitational field if a body has negative mechanical energy, then the system body-planet (or star) is in a gravitationally bound state. The explanation that I have in mind for that is: since ...
1
vote
0answers
25 views

Why don't big bang photons conserve mass and energy? [duplicate]

A photon from the big bang has lost most of its momentum and energy. What does it push against? Does it break the 'laws' of conservation of energy and momentum? Is there any possibility that momentum ...
-1
votes
1answer
51 views

Law of conservation of energy and time [closed]

Consider the following in a space (with out any external influences like gravitation) Let say i have two identical bodies b1,b2 and i applied a force F on them to make them accelerate a. After t time ...
0
votes
1answer
52 views

Finding speed of an object on an inclined spring [closed]

question posed here: http://puu.sh/nV7rw/504a7be88a.jpg method: Conservation of energy $E_{{mech}_i} = E_{{mech}_f}$ $U_i + KE_i = U_f + KE_f$ $\frac{1}{2}{k}{x^2} + {mgh_i} + 0 = ...
-1
votes
0answers
42 views

Energy conservation on Maxwell's wheel

Why is it that in Maxwell's wheel most of the wheel's potential energy is transformed into kinetic rotational energy? You may find some background information in the here. I was reading that document ...
0
votes
1answer
42 views

Conditions for which conservation of energy works

I was solving the following problem, and the first idea I had was conservation of energy. 27.19. Fusion Reactor. If two deuterium nuclei (charge $+e$, mass $3.34 \times 10^{-27}$ kg) get close ...
3
votes
1answer
45 views

Conservation of Energy of two charged blocks connected by light spring

Two identical blocks resting on a frictionless, horizontal surface are connected by a light spring having a spring constant $k = 100 N/m$ and an unstretched length $L_i=0.400 m$. A charge $Q$ ...
-1
votes
1answer
219 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 ...
3
votes
1answer
36 views

Swing: why does the body position modify the amplitude?

When a person swings, why does the amplitude of oscillations increase if the person changes the body position ? That is, when descending and approaching the vertical position, if the person extend his ...
0
votes
1answer
21 views

Interial Momentium Question

I had a question for my Physics 101 class and was wondering if anyone could share some insights. The question was stated as follows. A pottery wheel of radius 0.5 m and mass 15 kg rotates ...
0
votes
0answers
25 views

A magnetic spring generator with an induction coil conservation of energy?

I was thinking about a spring oscillator experiment where a mass is pulled down and it oscillates back and forth. I imagined that in a perfect vacuum the system would oscillate forever. Then I thought ...
-1
votes
1answer
71 views

Energy Conservation in Kinetic Power Plants [duplicate]

Quite recently the company Rosch has developed a new kind of power plant that supposedly utilizes the buoyancy effect to generate electricity. The apparatus consists of a vertical conveyor belt with ...
1
vote
1answer
24 views

Why does the kinetic energy work theorem apply to inclined surfaces?

Work is supposed to be force x distance, how did we end up with work equaling the difference of hights x gravitational acceleration ? Sorry for my poor understanding :D
0
votes
2answers
57 views

Confusion about Conservation of Mechanical Energy

I'm having a bit of difficulty understanding how to apply the idea of conservation of mechanical energy to a problem. One of the questions on my homework reads: A physics student shoves a 0.50-kg ...
2
votes
2answers
25 views

Motion after a collision of objects not involved in it [closed]

I'm confused about collisions where there are parts of the objects not involved in it. How do these parts move after the collision? I'll make an example. Consider a cart with an incline attached ...
1
vote
1answer
45 views

Change in energy during orbital decay

Suppose we have a planet orbiting a star and the planet radiates away some amount of energy $\Delta E$. I want to find by how much the orbit decays (i.e the change in orbital radius). One way to do ...
1
vote
1answer
41 views

Kinetic Energy: Difference in energy for observers [duplicate]

Consider a stationary 10kg bike; if I apply a force of 10J then $KE=\frac{1}{2}mv^2$ so $v=\sqrt\frac{2KE}{m}=1m/s$. If we consider the same example again, but from the point of view of a stationary ...
3
votes
2answers
119 views

1st Law of thermodynamics (conservation of energy) and quantum tunneling

Doesn't quantum tunneling contradict the 1st Law of thermodynamics? As far as I remember my school physics teacher told us that quantum tunneling is the reason why we can observe the alpha decay, as ...