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
236 views

Current on both sides of a capacitor

When a capacitor is connected to a DC circuit, what ensures that the current on both sides of the capacitor is the same? When charges arrive at one end of the capacitor they stop moving; presumably ...
0
votes
1answer
402 views

first law of thermodynamics and conservation of energy

The first law of thermodynamics is referred to as a reformulation of the law of conservation of energy. I am not sure to fully understand this relationship. My way of picturing it is the following. ...
1
vote
2answers
291 views

Why does mechanical energy have to equal zero to find escape velocity?

A object orbiting the earth has total mechanical energy equal to \begin{align*} E^{mech} = \frac{1}{2} m v^2 - \frac{GMm}{r} \end{align*} with $M$ the mass of the earth and $r$ the distance. My course ...
0
votes
0answers
41 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
141 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 ...
1
vote
1answer
140 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
1answer
69 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 ...
1
vote
1answer
124 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. 4–...
0
votes
1answer
86 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
2answers
141 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
0answers
84 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 ...
2
votes
2answers
363 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
380 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 ...
0
votes
2answers
64 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
51 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
129 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 ...
0
votes
5answers
162 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
1answer
113 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 ...
0
votes
2answers
207 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
vote
3answers
118 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 ...
0
votes
2answers
46 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 ...
6
votes
4answers
117 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
1answer
337 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
149 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 increase/...
-1
votes
2answers
125 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 ...
4
votes
1answer
279 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: ...
0
votes
2answers
45 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 ...
0
votes
1answer
27 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
0answers
25 views

How does impulse lose energy in rotation?

I have this kind of empirical signal in Electronics where the signal is 1D current measurement (AD current coming from dynamic processes) such that the wire rotates in a big volume of water/mud: ...
0
votes
1answer
59 views

Impact on car without energy dissipation

This must be a relative simple question, but the answer I am obtaining doesn't appear to be logical. Say a vehicle has a velocity of 15 m/s and an effective mass of 1000 kg including occupants. 1) ...
0
votes
3answers
280 views

What happens to half of the energy in a circuit with a capacitor?

For a simple circuit with a battery supplying a voltage V to a capacitor, let us assume that the charge on the capacitor is Q. Now, the work done by the battery or the energy supplied is given by the ...
1
vote
2answers
123 views

Energy of black hole

If a black hole attracts and engulfs light, then why doesn't the energy of the black hole increase to form a star once again? Black holes form when a star burns off its energy and collapses, so it ...
1
vote
2answers
653 views

Block slides down a movable ramp, 2 approaches yields different answer

My Question is exactly as this Phys.SE post: block slides on smooth triangular wedge kept on smooth floor.Find velocity of wedge when block reaches bottom The Problem is I used 2 methods Momentum/...
1
vote
2answers
68 views

Proof that oscillations in 1d potential well occur between certain points

In >>this<< situation, a particle with energy $E$ will oscillate between the positions $x_1$ and $x_2$ indicated on the diagram. This simple fact is taught in many introductory courses however I ...
0
votes
1answer
57 views

Using energy conservation for equation of motion

I am trying to get $\phi(t)$ out of this equation: $$\frac12 J \dot\phi^2 + mgl ( 1- \cos\phi) = E_0.$$ I know it's a common method to say $\sin(\phi) \approx \phi $ and $\cos(\phi) \approx 1 $, but ...
2
votes
4answers
378 views

Coupling fission and fusion and disappear all mass. Why doesn't this happen?

I'm confused about the fact that in a fission process the masses of the "products" are less than the mass of the "reactants". And in a fusion process the masses of the "products" are less than the ...
1
vote
2answers
500 views

Are photon energies conserved in general relativity?

As I understand it, both Maxwell's wave equation and the null geodesics of general relativity are scale invariant. Thus an electromagnetic wave can be shifted along a null geodesic without changing ...
0
votes
2answers
166 views

Conceptual question regarding work

Here is a problem I am having some trouble with: The solution is below, my question is, why is conservation of energy valid in this situation. From my understanding, just before the box hits the ...
0
votes
1answer
122 views

Greater momentum than initial?

The question is : Heavier object A, initially at rest, is struck by lighter object B. Is it possible for object A to have a larger final momentum than the initial momentum of object B? The ...
-1
votes
2answers
98 views

Finding the initial velocity of projectile [closed]

What is the formula for calculating initial velocity of projectile using the conservation of energy given the mass, height, angle ?
-2
votes
1answer
93 views

Why does this not make infinite energy?

Here's an idea I had for infinite energy: Get into a spaceship, and fly to a star or other large body and get into orbit. Then just harvest your kinetic energy somehow and keep going indefinitely. My ...
2
votes
3answers
202 views

Violation of conservation of energy?

Imagine a perfectly spherical mirror with 100% reflection. Imagine a point source of light in the center. The point source keeps radiating light. Will the light undergo destructive interference ...
1
vote
2answers
537 views

Why doesn't Beta Decay violate the laws of physics?

In Beta decay, a neutron decays into a proton, and "throws out" an electron at high speed. However, this, to me, suggests that the law of conservation of mass is not being kept here. Neutrons have a ...
1
vote
1answer
90 views

Why did the electric potential energy fall in this situation?

Suppose you have two capacitors configured as shown in the image below. One of them has charge $q$ and the other one is uncharged. Then, you close the switch, allowing charges to flow. After ...
7
votes
3answers
937 views

Where does the majority of the mass of the usual matter come from? [duplicate]

I apologize in advance to experts for the naivety of the question. It should be a duplicate but I didn't find any satifying question or answer about that. The proton is composed by two up quarks ...
4
votes
2answers
180 views

Does the dynamical Casimir effect generation of real photons create energy from nothing?

The experiments reported in recent years demonstrating the creation of real photon pairs from the vacuum via the dynamical Casimir effect have made me wonder if this process demonstrates the creation ...
12
votes
4answers
2k 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 ...
0
votes
2answers
155 views

Velocity and transfer of energy for a body in free fall

I just need to check if my understanding of the transfer of gravitational potential energy to kinetic energy is correct. Is anything wrong below? Say if a body were suspended at a height h above the ...
0
votes
2answers
87 views

a question about work needed to move a charge towards another one [duplicate]

Consider two charges $q_1>0$ & $q_2>0$ separated by a distance $d_1$, and we want to move charge $q_2$ to a distance $d_2$ from $d_1$ where $d_1-d_2>0$. I need to calculate the work ...
1
vote
3answers
79 views

A captious work problem: same paths but same forces?

A man jumps onto a chair. A man climbs onto a chair by putting a leg first and then the other. In both cases, the work has been the same. TRUE or FALSE...? Spoiler!: The path is the same, so the ...