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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2answers
121 views

Energy carried by photon not conserved?

In an imaginary frame of reference traveling with a photon, the length of the path traveled is 0. If the length of the path is 0, isn't it similar to say that the photon is either at the source or at ...
0
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1answer
43 views

Why is the change in height for mass be $2h$ in this question? [closed]

Could anyone tell me why the height is $2h$?, and is the total gpe in a system where the masses are balanced 0?
5
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4answers
4k views

How to tell if the collision is elastic or inelastic?

I'm a programmer and a game developer, not a mathematician or a physicist. So please go easy on the math :) I know two things: How to find the new velocities of two objects after an elastic ...
1
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3answers
146 views

Why don't we consider electrostatic energy of the pair in the case of pair production?

I have seen this Wikipedia article and many others, but in none of them I find any mention of the electro-static energy of the generated pair. Why? I mean, the energy conservation should be written ...
2
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2answers
355 views

Poynting's Theorem simplified?

How can I apply Poynting's theorem to any system that has a magnetic field & electric field, to state conservation of energy? How does Poynting's theorem state conservation of energy in EM? I ...
1
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2answers
64 views

Who of these cyclist travel takes less time? [closed]

There are two cyclist. They start off with equal velocity $v_0$. The first one bikes a straight path, while the other bikes through a valley, something like this: We assume friction doesn't affect ...
7
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4answers
655 views

Optimal speed for the water wheel

The hydroelectricity plants extract the potential energy of highly deployed massive object (water) as it falls down. Without turbine, all that energy would be converted into speed (kinetic energy) at ...
1
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1answer
192 views

Conservation of energy and the 'crazy ball' product

Well I'm not sure how many people remember the crazy ball - a small ball made of rubber which bounced like crazy. What I noticed is that the ball seemed to bounce higher than the point from which it ...
2
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3answers
591 views

Hamiltonian mechanics and conservation of energy?

Can anyone explain to me Hamiltonian mechanics relation to conservation of energy? I'm not very good at mathematics, and I know it's important into understanding Hamiltonian mechanics. However, can ...
1
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1answer
42 views

Explanation of binding energy in decays

Everyone knows that the mass of a system is less than the mass of its components, with the equation: $M = \sum_i m_i - BE(M) $ Now, if we consider a general decay, lets say $A \rightarrow \sum_i ...
0
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1answer
135 views

Aside from Noether's theorem, what other concepts would explain energy conservation?

Energy is defined more in the mathematical sense, and tends to show true with observations in the physical world. But why is energy conserved aside from "Noether's theorem"? In a closed system that ...
1
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3answers
125 views

Placing two similarly charged particles in space

Now, I will make a hypothetical situation. Assume that we place two similarly charged particles (lets take electrons) in space. Imagine that there is no other force acting on the particles except the ...
2
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0answers
73 views

Double pendulum find first integral [closed]

Consider the following situation of a double pendulum in 2D. We found the moving equations as $$ \ddot{\theta_1}=-L_1\sin\theta_1 + \frac{m_2}{m_1}\cos\theta_2\sin(\theta_2-\theta_1),\\ ...
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0answers
56 views

Energy of a system

I'm considering a ring of mass $m$ sliding along a cardioid with equation $r=a(1+\cos\theta)$. We let the angle between the downward vertical and the radius vector be $\theta$. The ring is attached to ...
2
votes
1answer
78 views

Time evolution of states - Is total energy constant or not?

Suppose the state of the particle is given as follows: $$ |\psi_{(t)}\rangle = \frac{1}{\sqrt2} \left( e^{-\frac{i\omega t}{2}} |0\rangle + e^{-\frac{3i\omega t}{2}} |1\rangle \right) $$ Where the ...
8
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2answers
338 views

Speed of gravity

Consider two objects presented in the figure below. Objects have equal masses and are separated by a distance of 60 light seconds. Assume that we move left object by 3 light seconds to the left in ...
3
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1answer
221 views

Visualization of electromagnetic field [duplicate]

In the Wikipedia article about electromagnetic radiation one can find the following picture: But shouldn't the E and B field be 90$^\circ$ out of phase? In the depicted way the energy isn't ...
4
votes
1answer
4k views

How much power would it take to stop a bullet with a magnet?

If a 7.5 gram copper-jacketed lead bullet (say, a 9x19mm Parabellum) was travelling at 360 m/s, how much power would it take to diamagnetically stop it in the space of one meter? This question comes ...
6
votes
4answers
326 views

Do magnets redshift light?

Do magnets redshift light? Suppose we have an extremely powerful magnet (say the size of the Sun) and we have a smaller paramagnetic material above it (say. Titanium Brick which is ...
2
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1answer
2k views

Does perpetual motion machine really exist?

I recently saw a video on Youtube about a perpetual motion machine. Here is the link: http://www.youtube.com/watch?v=OS1KXMsE2qk It's on Robert Boyle's flask. Where does the energy come from?
3
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2answers
335 views

Conservation of energy (or lack thereof) in Doppler cooling [duplicate]

I did not find my question answered elsewhere, so here it is. I'm doing a project in my first optics course, and we are reading a bit about Doppler cooling. I understand that a laser is tuned to a ...
0
votes
2answers
244 views

Conservative Forces & Conservation of Energy?

I'm trying to relate them, I'm trying to find the key relation that would show how the conservative forces serve conservation of energy. How would they relate? Also, how are non-conservative forces ...
2
votes
1answer
94 views

Minimum Power Required

Here is a question, I found two methods to solve it, differing in the answers and I'm not sure why a method is incorrect. Question: A fire hose of cross section area ...
1
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1answer
37 views

Understanding the energy transffered/used from a power source?

A source of power(from a generator,battery,etc...) is used to power a system of any kind. We can calculate the energy used via this formula $E$ = $IV$ x $t$ ? For example, a battery that is connected ...
47
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4answers
10k views

If I'm floating in space and I turn on a flashlight, will I accelerate?

Photons have no mass but they can push things, as evidenced by laser propulsion. Can photons push the source which is emitting them? If yes, will a more intense flashlight accelerate me more? Does ...
-1
votes
1answer
120 views

Energy Conservation [closed]

Supose that I throw a ball on the surface of the Earth at a given instant t. Chosing the referencial as the surface of the Earth, the PE is zero (and the KE is different from zero). Imagine that when ...
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2answers
385 views

if the universe is flat does it mean it exists only in our mind as math? [closed]

My dad, who is a retired mathematician, has this attitude, which I think we all have as kids, that ultimately reality is made of stuff. End of story. If you look around yourself in the world, ...
1
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1answer
57 views

Energy in nuclear decays

After a nuclear decay is it a necessity that the total energy of the products is more than the energy of the original particle before decaying? (NB: by 'energy' I don't intend to include mass-energy ...
1
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1answer
62 views

Energy conservation in diamagnetic levitation

How do we account for conservation of energy in diamagnetic levitation when no external energy is required?
2
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2answers
222 views

Extra energy in quantum tunneling

In quantum tunneling, the probability of finding an electron inside the potential barrier is non zero . So we can actually find an electron which had an energy $E$ in a place where classically it ...
-1
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2answers
250 views

Do Quantum Energy Generators really produce more energy than they consume?

I'm not a science expert, just someone whose interested. I was taught that you cannot produce more energy than the amount put in. I received a thing on facebook asserting that a Quantum Engery ...
2
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0answers
44 views

Would a large, small mass object in orbit experience induced rotation

Imagine a large (multiple earth radii), very small-mass ring orbiting The Sun. Half of the ring would be closer to The Sun than the outer half. Since orbital velocity decreases with distance, two free ...
3
votes
3answers
570 views

How is angular momentum conserved if a bullet hits a wheel?

Suppose my system involves: 1) A mounted wheel with some outward flap 2) A bullet already in motion Initially the net angular momentum is 0 and the net kinetic energy is just that of the speeding ...
1
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1answer
943 views

Does time travel violate conservation of mass/energy? [closed]

Imagine I exist at time $t_1$ and my mass is $m$. At time $t_2$ I time travel back to $t_1$. At time $t_1$ there is now a net increase of mass/energy in the universe by $m$. At time $t_3 = t_2 - x$ ...
0
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1answer
91 views

Doing a spring experiment, and need to find the stretch required

I'm doing an experiment where I have to calculate the stretch of a spring needed for it to launch itself into a trashcan located some distance away. The spring is located on the edge of a ramp that is ...
1
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0answers
88 views

How Long does it take for a roller coaster cart to reach point $x_1$? [closed]

Assume that we have a particle with weight m and initial speed v0, that follows a roller coaster path y(x). Assuming no energy loss to friction or air resistance, the total energy in the system is ...
0
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3answers
1k views

Why isn't angle of launch used in this kinetic energy equation?

Question: A stick is thrown from a cliff 27 m high with an initial velocity of 18 m/s at an angle of 37 degrees above the horizontal. a) use the law of conservation of energy to determine the speed ...
6
votes
2answers
206 views

What happens when two high-speed objects going opposite directions crash into a black hole?

Imagine the following: A-> [] <-B where [] is a black hole, and A and B are two objects of same mass, traveling with the same speed right at the black ...
0
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1answer
392 views

What happens to the acceleration of a body undergoing constant force?

Here's the problem I have, specifically relating to a rocket in deep space: A rocket burns fuel at a constant rate. Assuming its mass remains constant, what happens to its acceleration? I seem to get ...
4
votes
2answers
674 views

Energy conservation when capacitor charges in a zero resistance circuit

We know that when a capacitor charges from a source $E$, it stores energy of $E=\frac{1}{2}QV $. This is derived without taking into consideration any resistances present in the circuit. We also ...
-1
votes
2answers
65 views

Difference between two equations

So again I have two equations: $$K= \frac{1}{2} m v^2 + \frac{1}{2} I \omega^2$$ And $$K_w = \frac{1}{2} I \omega^2 $$ What's the difference between these two? Thanks.
6
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3answers
320 views

Is energy conserved in general relativity? Does $\nabla_aT^{ab}_{\rm matter}=0$ represent the conservation of energy and momentum?

For example, the radiation dominated cosmology, the energy density of radiation is proportional to $a^{-4}$ and the volume is proportional to $a^3$, where $a$ is the scale factor. So the total energy ...
1
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1answer
114 views

Could an orbiting mass generate power?

If an heavy object (e.g. 10 tons) orbiting around Earth at 370 miles high, is connected with a cable back to Earth, we assumed either Earth is going to pull the mass or vice versa (or it will fall ...
3
votes
1answer
87 views

The Energy involved in the work done here?

When a wire that has current $I$ flowing within it and its in a magnetic field, the wire experience the Lorentz force, and that force moved the wire over a certain distance $x$(no matter how small), ...
0
votes
2answers
124 views

How can I apply Conservation of Energy in electromagnetism?

How can I apply C.O.E to a system that applies magnetic & electric fields at the same time to do work, and convert energy from one form to another? Let assume we have a conductor that moves ...
1
vote
2answers
15k views

Loop-the-loop question [closed]

Problem: The marble rolls down the track and around a loop-the-loop of radius R. The marble has mass $m$ and radius $r$. What minimum height $h$ must the track have for the marble to make it around ...
0
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3answers
439 views

Why is energy conserved? [duplicate]

From basic principals, how does one prove that energy is conserved? Or a little more specifically - Why does this hold: $$\Delta \mbox{ PotentialEnergy} + \Delta \mbox{ KineticEnergy} = 0 $$ Or, for ...
0
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2answers
46 views

Initial velocity to travel a certain distance

When launched from a planet with mass M and radius R, what initial velocity is necessary for an object to travel a distance r before it gravitates back towards the planet? Sort of like trying to find ...
5
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1answer
121 views

Do particles keep on emitting energy?

Accelerated electrons emit electromagnetic waves, but those at rest do not. So accelerated electrons emit energy and those at rest do not. (At rest hypothetically, I know it is not permissible ...
0
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1answer
90 views

negative energy cancelling energy

In what circumstances can negative energy cancel out energy? If a positron and electron are created they cancel out as one of them has negative energy. Are there any other ways that negative energy ...