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

0
votes
2answers
29 views

How does a Venturi Injector comply with energy conservation?

How does a Venturi Injector comply with energy conservation? When the water flow enters the narrower cross section its velocity increases and its pressure decreases. When the pressure gets lower ...
0
votes
0answers
26 views

What is the kinetic energy of a proton as it is falling towards the core of the star?

Given: Star's mass = $1.99e10^{30}kg$ Type II supernova occurs (in a simple form: the outer layers of the star fall into the now inactive core at speeds of 126000km/s) $10^{44}$ joules of energy is ...
0
votes
2answers
100 views

Proof of conservation of energy with the relativistic definition of force

Professor Susskind proved conservation of energy in one of his lectures by taking the classical definition of force ($F = ma$) and by showing that its time derivative is zero. How can we do that with ...
-1
votes
0answers
25 views

Can the Betz limit be overcome?

The Betz Limit states that the energy extracted from a turbine can be no more than $\frac{16}{27}^{th}$ of the energy present in the wind flowing through it. Extracting more energy would make make the ...
2
votes
0answers
216 views

How do I calculate the energy balance of a trompe?

I was quite fascinated by the concept of an ancient type of air compressor, called a trompe. It entrains air bubbles into a falling stream of water via the Venturi effect, and extracts the air at a ...
0
votes
1answer
25 views

Work done by friction on a body?

I know that when a body slides over a surface, the work done by friction is not stored as potential energy in the body. It is dissipated in the form of heat. But why is it not stored as potential ...
-2
votes
0answers
24 views

Would momentum be conserved in teleportation? [on hold]

Defining teleporation loosely as at least quantum tunneling all of your particles and preserving the arrangement. If that doesn't work, assume the mechanisms of the most basic definition of ...
6
votes
4answers
276 views

Do virtual particles actually physically exist?

I have heard virtual particles pop in and out of existence all the time, most notable being the pairs that pop out beside black holes and while one gets pulled away. But wouldn't this actually violate ...
2
votes
1answer
43 views

Spinning rubber ball with equatorial ridge

I have this rubber ball with something like a very slight equatorial ridge (sort of like Saturn's moon Iapetus) which I often spin around on my desk. I keep noticing that no matter the inclination of ...
0
votes
3answers
87 views

How is energy converted when massless spring is attached to a rigid support

Say a massless spring of length l is attached to a rigid support. It is extended to length l + x. Now at this position, the ...
0
votes
1answer
43 views

Classical Mechanics — Sign of work done

It seems that work has two possible ways to decide it's sign: Whether you take the perspective of the system or the surrounding (whether you consider work done on the system as positive, or work done ...
0
votes
1answer
69 views

A confusion regarding a paragraph in The Feynman Lectures

Under the chapter entitled Conservation of energy, under the section of Gravitational potential energy, there is this following paragraph : A very simple weight-lifting machine is shown in Fig. ...
0
votes
0answers
24 views

Problem on Conservation of Energy in Pulley System [closed]

See the masses in Figure (1) which start out at rest. (i) Find the velocity of the 14 kg mass just before it hits the ground. (ii) Find the maximum height reached by the 8 kg (and don’t ...
0
votes
1answer
118 views

In a CMCS 2-body system, why does the speed of the particles after collision stay the same?

A particle $m_1$ is traveling with velocity $v$ toward a stationary particle $m_2$. The velocity of the center of mass is given as $v_c=\frac{m_1}{m_1+m_2}v$. Changing to a moving coordinate system, ...
0
votes
1answer
92 views

Isolation of a Physical System and Energy

I'm trying to grasp basic concepts of energy and i have a little doubt regarding conservation of energy. According to Feynman book and Wikipedia, the Law of Conservation of Energy states that ...
3
votes
1answer
91 views

Perpetual Mobile and Gravitation

I have fundamental question about what is called the “law of conservation of energy”. We all hear about the tidal power stations which using the tidal power. The source of the tidal power came from ...
28
votes
6answers
5k views

Total energy of the Universe

In popular science books and articles, I keep running into the claim that the total energy of the Universe is zero, "because the positive energy of matter is cancelled out by the negative energy of ...
0
votes
2answers
78 views

Definition for potential energy

I came across this definition for potential energy: If we let $T$ be the Kinetic energy, we have that: $$T = \frac{1}{2}mv^2 \implies T = \frac{1}{2}m{x'}^2$$ $$T'= mx'x'' = F(x)x' \implies \\T = ...
2
votes
2answers
123 views

What is the mechanism of particle anti-particle annihilation

My question is loaded with assumptions so to minimize them, I would like to ask it with respect to an electron and anti-electron annihilating. When I think of annihilation, I think of electron and ...
1
vote
1answer
26 views

Loss in energy while charging a capacitor

Suppose a capacitor having capacitance $C$ is being charged using a cell of emf $E$. By the time the capacitor is fully charged, the cell has supplied $QV$ energy while the potential energy of the ...
4
votes
2answers
355 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 ...
2
votes
0answers
48 views

Non-local gravitational energy tensor

The well-known derivation of the Landau-Lifshitz gravitational energy pseudotensor, relies on several requirements: 1) that it be constructed entirely from the metric tensor 2) that it be index ...
6
votes
4answers
2k views

A problem of missing energy when charging a second capacitor

A capacitor is charged. It is then connected to an identical uncharged capacitor using superconducting wires. Each capacitor has 1/2 the charge as the original, so 1/4 the energy - so we only have 1/2 ...
1
vote
1answer
24 views

How may one calculate the escape velocity of a binary system at a given radius from the COM?

Apologies if this is a bit general, even some pointers in the right direction would be great (reading material etc.) Given an object of mass $m_3$ travelling outward at speed $v$ from the centre of ...
6
votes
3answers
374 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 ...
2
votes
4answers
713 views

The “binding energy” of bonded particles adds mass?

This is a follow-up my previous question. Several of the answers indicated that the mass of a particle (atom, proton, etc.) increase with the "binding energy" of it's component particles - the energy ...
0
votes
0answers
26 views

What forms does the energy of a photon get converted to after absorption

Assume an object which has a high absorption rate of light. Now my question is, if you shine light on the object, what all forms of energy does the object acquire? I know there will be an increase in ...
0
votes
2answers
38 views

How the mass is conserved during continuous expansion?

If universe is continuously expanding doesn't it contradict the fact that mass is conserved? Because if mass is conserved so how is universe expanding?
0
votes
0answers
40 views

Does mixing violate conservation of energy?

If two fluids in an isolated system mix, the entropy of the system increases. The volume and the number of particles in the system, however, are constant. Thus $$dU=TdS+PdV-\mu dN=TdS>0$$ which ...
0
votes
2answers
29 views

Kinetic energy dissipation in braking a vehicle

Let's say a vehicle that weighs 20t is hauling along at 50m/s and we want to brake it down to a full stop. The kinetic energy we need to dissipate into heating up the brakes is ...
6
votes
4answers
89 views

Energy conservation without action principle?

The normal tagline for energy conservation is that it's a conserved quantity associated to time-translation invariance. I understand how this works for theories coming from a Lagrangian, and that this ...
0
votes
5answers
109 views

How is energy conserved in this scenario?

There are two objects A and B of equal mass (m). They are laying in close proximity to each other. Object A is then moved further from object B, to a distance r. Since there is gravity between these ...
1
vote
3answers
363 views

Example of law of conservation of energy

Law of conservation of energy states that "Energy of the universe remains constant. Energy can neither be created nor be destroyed, but it can be transformed from one form to another". So I ...
1
vote
2answers
288 views

Picturing Feynman's argument about perpetual motion

So, there is a certain paragraph in Fenyman's book that I'm struggling with for quite some time. It says: We imagine that there are two classes of machines, those that are not reversible, which ...
1
vote
3answers
62 views

Can lift contradict conservation of energy?

In cruise flight of an aircraft lift does no work, it just holds the aircraft aloft. In order to climb aircraft pitch up and need excess energy because the thrust vector now has a vertical component ...
4
votes
1answer
383 views

Sufficient conditions for the energy to be not conserved?

I'm almost embarrased to ask this question because I thought I was by now very confident with classical mechanics. Someone has stated that given a mechanical system with a Lagrangian $L$ s.t. ...
0
votes
2answers
41 views

In what form is the energy stored in this situation?

Consider a hypothetical situation, A person is pushing a block of some arbitrary mass in vacuum (no external forces are involved) and due to this, if I am not mistaken, the person is doing work and ...
0
votes
1answer
24 views

Bead on a rotating wire - Conservation of angular momentum, fix points

Lets consider a wire in the x-y plane which rotates with constant angular velocity $\omega$. The coordinates of a bead, which is forced to stay on this wire, can then be expressed as $$x=r \cos(\phi) ...
0
votes
2answers
105 views

Why does this capillary action generator not work?

So I was doing a bit of reading. Apparently the the obstacle to generating energy from the forces driving capillary action is breaking the surface tension at the top of a capillary tube. It is just ...
1
vote
2answers
185 views

Why isn't jumping against a wall an elastic collision?

According to this calculator http://www.abecedarical.com/javascript/script_collision1d.html when low mass object hits high mass object it is reflected gaining opposite velocity almost the same as ...
0
votes
2answers
72 views

Do transformers lose energy?

EDIT: The title should rather be how/why transformers lose energy My idea of a transformer is that it is composed of two separate wire windings around some metal core. The purpose is to ...
3
votes
1answer
37 views

Does an Inductor maintain it's energy?

Consider a simple LCR ac circuit; generally I (might) understand that the charge exchange between capacitor and inductor would induce a harmonic current flow, but I remain confused for two reasons: ...
-1
votes
2answers
70 views

If energy can't be created and can't destroyed [duplicate]

Then wouldn't be logical that all the energy in the big-bang-created-universe will equal to the energy that was released by the Big Bang? The sum is zero? Edit The cited dup is asking "Did the law ...
0
votes
1answer
163 views

Was Joule's experiment able to show: thermal energy = $mgh$

Is my understanding of this experiment correct? There is the quantity $MC\delta \theta$ associated to the heat which I think is not an absolute value to be compared with $mgh$ since the $\delta ...
0
votes
1answer
124 views

Changing momentum of moving trolley

We have a trolley of mass $m$ and it is moving at a velocity $v$ along a smooth horizontal plane. It is full of water, and is leaking water at a constant rate out of the bottom of the trolley, i.e ...
1
vote
1answer
190 views

Conservation of energy when focusing black body radiation on another black body

This question about whether or not it is possible to focus black-body radiation to make something hotter than the radiation's source was answered mostly negative: the second law of thermodynamics ...
3
votes
2answers
782 views

How to understand Feynman's reasoning about perpetual motion?

I'm studying Feynman's Lectures on Physics, and I'm not really understanding his reasoning here: Consider weight-lifting machines $\overline{}$ machines which have the property that they lift one ...
1
vote
1answer
92 views

Pendulum problem where it hits a peg swings around and lands on top of it

Using energy equation from when it is released to when it reaches its highest point after hitting the peg I get change in kinetic energy = 0 and change in potential due to gravity = 0 Does the string ...
0
votes
1answer
23 views

Calculating time from energy [closed]

I am trying to calculate the time of a movement using the energy of the system. If $$E=\frac{m}{2}\dot{x}^2 + V(x)$$ where $V(x)$ is the potential energy (assuming the potential is time-independent) ...
0
votes
2answers
40 views

If I make a object enter a planet's gravity it hits the surface with huge velocity, where is it gaining k.e. from?

If I make a object enter a planet's gravity range,it hits the surface with huge velocity,where is it gaining k.e. from? If i keep sending more and more heavy things like above and some how harness the ...