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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2
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2answers
170 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 ...
0
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0answers
17 views

frequency shift in background radiation, where is the missing energy? [duplicate]

It is known that the dilatation of the universe cause a shift in frequency in the background radiation toward lower frequencies. It is also known that photons with lower frequency have lower energy ...
15
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7answers
3k views

Why can't we store light in the form of light?

We can store cold (ice), heat (i.e. hot water bag) and electrical charge (batteries). We can even "store" a magnetic field in a magnet. We can convert light into energy and then, if we want, back to ...
2
votes
1answer
43 views

Why is the Carnot engine the most efficient?

It seems that the only condition used in proving that the Carnot engine is the most efficient is that it is reversible. More specifically, the Carnot engine can be run in reverse as a refrigerator. ...
1
vote
1answer
85 views

making an equation dimensionless

I have a balance of energy equation as following (for a spherical particle that colliding with a spherical fluid droplet) Left hand side is for before collision and RHS for after that: ...
2
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1answer
115 views

Proof of the conservation of the energy functional for the Gross-Pitaevskii equation?

From the Gross-Pitaevskii equation \begin{equation}i\hbar\frac{\partial\psi}{\partial t}=\left(-\frac{\hbar^2}{2m}\nabla^2+V+g|\psi|^2\right)\psi\end{equation} using the variational relation ...
1
vote
2answers
20 views

Understanding a graph of energy conservation with bounded and unbounded motions?

This graph is from the physics undergraduate text "Classical Mechanics by Douglas Gregory". Above this graph was the statement: What I didn't understand is- as stated in the under [*paragraph], ...
3
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4answers
120 views

Virtual particles/quantum tunneling - conservation of energy?

I'm confused as to how the above phenomena can take place since arent they breaking the law of conservation of energy (even, if temporarily)?
5
votes
5answers
366 views

Kinetic energy vs. momentum?

As simple as this question might seem, I failed to intuitively answer it. Let's assume there is a $10,000$ $kg$ truck moving at $1$ $m/s$, so its momentum and KE are: $p=10,000$ $kg.m/s$ and ...
3
votes
2answers
99 views

How to model energy loss in a rotating body?

I recently asked a question about modeling instability in a rotating rigid body. I now realize that I was mentally confounding two different effects: The "Dzhanibekov effect" in which a rigid ...
0
votes
1answer
53 views

Spring on a cart problem [closed]

The problem is two dimensional. At t = 0, a stationary cart at initial position (0,0) starts to accelerate with acceleration a in the negative "x-axis" direction. On the cart, there is an isosceles ...
1
vote
1answer
36 views

Calculating speed of spheres in an asymmetric Newton's cradle?

If we assume there is an asymmetric Newton's cradle with 3 spheres of different masses and the first one is initialized with some certain speed can we calculate the outcome speed of the all the three ...
0
votes
2answers
108 views

Perspective and changes in kinetic energy

Say you have two planes flying next to each other at the same speed and one decides to pick up speed by burning a tank of rocket fuel. If someone on the ground wanted to know that plane's new speed ...
6
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2answers
116 views

Was the Big Bang actually cold?

As I understand, from watching the Discovery Channel, the total amount of energy in the universe is zero. As such, people like Hawking explain that the universe can be created out of nothing ...
0
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0answers
82 views

Difference of rotationnal velocities start/end change the sum of work? [closed]

An arm of length $d$ is turning clockwise at $w1$. A red disk of radius $r$ is turning at $w2$. $w1$ and $w2$ velocities are in labo frame reference. I define $w2'$ the rotationnal velocity in the arm ...
2
votes
3answers
323 views

Two dimensional elastic collisions with varying angle of incident

If in an elastic collision I know all initial values and that mass for each object remains constant throughout the collision (but different from one another) how can I determine their final velocity ...
20
votes
6answers
10k views

Newton's cradle

Why, when one releases 2 balls in Newton's cradle, two balls on the opposite side bounce out at approximately the same speed as the 1st pair, rather than one ball at higher speed, or 3 balls at lower ...
0
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0answers
13 views

Muscular energy transformations with regards to gravity

Maybe this has more to do with biology than I know, but I have been wondering just what kind of energy muscles use and convert when lifting a weight off the ground or lowering a weight back to the ...
0
votes
1answer
26 views

What happens to K.E. in matter antimatter annihilation?

If I have two matter and antimatter particles, say an electron and a positron, each moving towards each other with a certain speed, they annihilate after the collision. Does the energy of the photons ...
0
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0answers
59 views

Adiabatic Expansion with expanding control volume

I have a tube with a length $L$ and diameter $d$ that is open on 1 side. At a certain point (say $x$) from the closed end of the tube, I have a gas with a high pressure. At this point, $x$, there is ...
6
votes
2answers
1k views

Mass converted to energy in a common fire?

In a common wood fire such as a campfire, is matter converted to energy or is it simply an exothermic chemical reaction and all the mass can be accounted for in the ash and soot?
1
vote
0answers
93 views

Friction and work from torque

I would like to understand where is the error in the third case, for that I gave 2 easier cases where I'm able to find the energy from heating is equal to the energy lost by torque. Case 1/ Purple ...
42
votes
10answers
11k 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 ...
1
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1answer
47 views

Is change in kinetic energy invariant?

Consider the inelastic collision between two bodies. This question follows on from this: Is the coefficient of restitution frame independent and energy conservation?. One of the answers to this ...
0
votes
1answer
41 views

What happens to the energy of photons when two light waves with plane wavefront interferes destructively? [duplicate]

When I began learning about optical interference, I came to know about destructive interference in which light waves cancel each other. How the energy is still conserved ? I found that the ...
14
votes
5answers
999 views

How was the formula for kinetic energy found, and who found it?

My questions mostly concern the history of physics. Who found the formula for kinetic energy $E_k =\frac{1}{2}mv^{2}$ and how was this formula actually discovered? I've recently watched Leonard ...
0
votes
3answers
432 views

Conservation of Angular Momentum, as related to a flywheel

Trying to work out some pesky flywheel dynamics for a project I'm working on, would love some for your assistance to better understand the underlying concepts. For a given flywheel (thin-walled ...
1
vote
1answer
112 views

What prevents this rotational perpetuum mobile from working? [closed]

As a newbie in physics I imagine this device and I would like to know why it can't increase its energy. Note I study the sum of energy during one second (or less) after set friction ON. The device: ...
0
votes
1answer
41 views

Why do we have to use an integral in this scenario to figure out $v_{max}$? [duplicate]

$$\int_0^R mg\frac{r}{2}dr=\frac{m}{2}v_{max}$$ and can't write $$\frac{m}{2}v_{max}=mgR$$ don't we usually equal potential energy to kinetic energy? A stone is falling through a tunnel towards the ...
2
votes
1answer
36 views

How many properties like charge, mass etc any quanta or particle must have?

How many properties are required to measure full energy of a fundamental particle? I know $E=mc^2$, but what about charge, spin, etc? Which full equation would give me all parameters of any particle?
1
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6answers
482 views

Can energy be created and destroyed?

The indroduction of the principle of conservation of mechanical energy has been tremendously useful from the practical point of view. But .. Consider the case in which we shoot an electron up in the ...
28
votes
6answers
2k views

Why are rockets so big?

I'm curious why rockets are so big in their size. Since both the gravitational potential one need to overcome in order to put thing into orbit, and the chemical energy burned from the fuel, are ...
2
votes
4answers
107 views

Conservation of potential energy for a wormhole

So you managed to build a stable traversable wormhole. Somehow you managed to acquire the exotic negative-tension materials with sufficient densities to make it all work. Now you place opening A of ...
60
votes
10answers
6k 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 ...
1
vote
2answers
91 views

Do electrons “check in” at the quantized energy radius before they leap?

Quantum jumps inside atoms always have the same energy, at least in a hydrogen atom when jumping from $n=1$ to $n=2$, like from a 1s1 to a 2s1 state. My question is, if an electron can be anywhere in ...
0
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2answers
42 views

Inelastic collision theoretical problem - kinetic energy

I was wondering if there is a possibility where in inelastic collision the final kinetic energy is not less than the initial kinetic energy.
5
votes
3answers
181 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 ...
0
votes
1answer
55 views

Can Gauss' and Ampere's Laws be written in terms of the divergence of an energy four-vector?

In the first 20 minutes of this video, Susskind derives the continuity equation for charge conservation: $$\dot{\rho}+\nabla\cdot\vec{J}=0$$ (Where $\vec{J}=\frac{\partial\dot{q}^m}{\partial A^m} ...
1
vote
1answer
60 views

Heat Equation with In-Depth Radiation Exact Solution

I am looking to solve the heat conduction equation in a semi-infinite solid with in-depth radiation on the domain $-\infty < x < 0$. The governing equation of this problem is: $$\rho c ...
2
votes
1answer
47 views

momentum and energy of the rods

I have two electromagnets and I attach them to two rods(same mass) such that, first one will be attached to centre of first rod and the second one will be to the end of second rod. Now I bring them ...
3
votes
1answer
3k views

How to understand the work-energy theorem?

How to understand the work-energy theorem? I took a short lecture on physics for engineering last week. The lecturer emphasized that the work done on an object will cause the kinetic energy change as ...
0
votes
6answers
403 views

Why is kinetic energy stored as potential energy in a body during its motion against gravity?

Everyone knows kinetic energy is converted to potential energy in the body when it moves up against the earth's opposing gravitational force. But I am facing some problem with this. What I learnt ...
3
votes
5answers
110 views

Mass and Energy

Would the mass of burnt firewood be equal to the mass of firewood before burning? Then where does that heat come from? According to Einstein's equation, $E=mc^2$ Shouldn't there be some mass going ...
8
votes
2answers
2k views

Does a cooling mass lose mass as it radiates?

Assume we have a red hot cannonball in space. It starts off with mass M at 1000K. Later it has cooled by radiation to 100K. Has the mass decreased?
5
votes
2answers
128 views

I need help figuring out what is wrong in this aspiring perpetuum mobile

Credits: My question is motivated from a question from another user (One disk/ring in double rotation and sum of energy), I just reformulated what I think he tried to ask into, what seem to me, ...
0
votes
1answer
35 views

Finding the total energy of satellite in orbit with the Earth

I'm given a problem where a satellite with mass $m$ is orbiting the Earth with mass $M$ and radius $R$ ($m<<M$) in a elliptical orbit. The ellipse has semimajor axis $a$ and semiminor axis $b$. ...
0
votes
0answers
29 views

Where did the energy of the universe come from? [duplicate]

Last week my Physics teacher asked the class this question: If the internal energy of the universe is constant, then where did all the energy come from originally?
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2answers
2k views

Finding max height of a mass launched by a spring?

I have the following problem on my homework. The actual problem has numbers, but I figure using symbolic notation will help me understand the concept in the long run. A block of mass $M$ is launched ...
33
votes
13answers
3k views

Where does the extra force generated by a lever come from?

Given an object being pulled down to earth by a force of 1000 lb: If a machine were to lift it off the ground for me, and I grabbed on to the rope before it was released, there is no way in the world ...
0
votes
1answer
58 views

Energy Imparted by a Magnetic Field

As I understand it, when you switch on an electromagnet it generates a magnetic field that pervades all space, imparting potential/kinetic energy to every object with which it interacts. Where does ...