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.

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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 ...
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1answer
29 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 ...
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40 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 ...
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109 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 ...
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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 ...
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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?
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1answer
19 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 ...
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6answers
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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 ...
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34 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. ...
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1answer
60 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 $6000$ ...
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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 ...
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522 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 - \dfrac{1}{\...
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1answer
18 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 here'...
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34 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 ($B$)...
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24 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 ...
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1answer
32 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 ...
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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 ...
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1answer
64 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 ...
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1answer
19 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 ...
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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 ...
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3answers
108 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 ...
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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%.
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1answer
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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 ...
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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 ...
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1answer
54 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 ...
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1answer
58 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 = \frac{1}{2}{m}{v^...
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1answer
44 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 ...
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1answer
53 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 $Q$ is ...
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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 ...
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1answer
38 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 ...
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1answer
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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 ...
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Do all forms of energy fall under kinetic and potential energy?

I know that energy is recognized through motion. Even in the mass-energy equivalence a velocity is present even though it is a rest-energy (Not really sure if this would count as a potential energy ...
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34 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 ...
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1answer
153 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 ...
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1answer
26 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
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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 ...
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2answers
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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 (...
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1answer
48 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 ...
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1answer
42 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 ...
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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 ...
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2answers
116 views

How can I find the height for a looping pendulum? [closed]

I have a ball attached to a string, hanging on a rod placed $L$ meters above the floor. I release the ball and string at a 90 degree angle and let it loop around another horizontal bar after it ...
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2answers
83 views

Why has a molecule less energy than the uncombined atoms?

My book says, A molecule as compared to the atoms from which it is formed is more stable because it possesses energy lower than the energy of the uncombined atoms. This difference in energy is due ...
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1answer
109 views

Is there conservation of momentum if there's conservation of energy? [closed]

The equation for conservation of momentum: $$m_1\vec{v}_1 + m_2\vec{v}_2 = m_1\vec{u}_1 + m_2\vec{u}_2$$ The equation for conservation of energy: $$\frac 12m_1v_1^2 + \frac 12m_2v^2_2 = \frac ...
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1answer
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Can we in the far future have an energy source made from vacuum energy, antimatter, dark energy, Higgs boson? [closed]

Can we in the far future have an energy source made from vacuum energy, antimatter, dark energy, Higgs boson and all kind of these like stuffs?
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1answer
66 views

Where did all the photon energy go?

From the wikipedia page on dark energy, in reference to the total mass-energy of the universe: The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, ...
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How do I show that the total energy of a particle moving in an electrostatic field is constant?

Basically the Lorentz force says that: $$F=qE+qv × B$$ I am asked to show that if a particle moves in an electrostatic field $$E= - ∇V(x,y,z)$$ and ANY magnetic field, then the energy: $$\frac{1}{...
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1answer
68 views

Why does antimatter and matter fuse to release energy even though they will cancel each other perfectly?

I have seen many physics people talk about antimatter as a evil twin of matter and when they come in contact with each other leave massive amount of energy instead of only void. If antimatter and ...
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1answer
55 views

Energy and Momentum: Impact

Boat A has just lost another race to Boat B. In a fit of rage, the driver of Boat A (mass = 500 kg) turns his boat towards Boat B (mass = 450 kg) which is at rest as the driver gloats over yet another ...
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Does a conservative form of the Chew-Goldberger-Low MHD equations exist?

I wonder if is it possible to bring Chew-Goldberger-Low MHD equations to a conservative form? Conservative form of the system of equations of ideal MHD with isotropic pressure is well known and can be ...
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2answers
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Condition for looping the loop

Consider a ball tied to a string and it is imparted a velocity we have studied that condition for looping the loop is that tension at the uppermost point must be zero, but why is this condition ...