Questions tagged [energy-conservation]

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 answers
232 views

3D perfectly elastic collision between two points

There is a high probability, I think, that this question is a duplicate of some other question ... but to may knowledge, it hasn't been posed in this exact manner: Assume we have 2 points, $P_1$ and $...
1 vote
0 answers
53 views

Does laser trap atom cooling change the electron shell levels?

Do the electron orbits change during the cooling of the atom in a laser trap? When cooling down, do the electrons come closer to the nucleus of the atom or do they move away? Or do the electrons leave ...
0 votes
3 answers
46 views

Closest distance of approach [closed]

Suppose a charged body $q_0$ is at rest. Another charged body $q_1$ at infinite distance starts moving towards $q_0$ with velocity $v_1$. Gradually the velocity of $q_1$ will decrease and that of $q_0$...
-1 votes
0 answers
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Will the mango dislodge? [closed]

A boy throws a rock having a mass of 0.2 kg with a speed of 16 m/s to dislodge a mango of mass 0.25 kg which is 10 m above the ground. Will the mango dislodge?
1 vote
1 answer
219 views

Help understanding a spring-mass system where mass is not attached

This is the scenario: We have a spring resting on a surface with no mass on it. Let the $y$ position of the top of the spring be $l_0$. When we put an object with some mass on top of the spring, the ...
1 vote
3 answers
5k views

When is mechanical energy ever conserved for inelastic collisions?

So as I was taking a test today I encountered an intriguing question, the question came in two parts. The first parts, which I answered quite easily, was about whether or not mechanical energy is ...
0 votes
1 answer
48 views

Total rotational energy of oval-shaped object [closed]

I am analysing the rotational energy of an oval-shaped ball (after taking a series of videos of a rugby ball bouncing), through the following 3 axes of rotation...: ...I calculated the angular ...
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2 answers
51 views

How did Noether use the total time derivation to get her conservation of energy? [duplicate]

I was informed by @hft that by combining the total time derivation: $$\frac{dL}{dt} = \frac{\partial L}{\partial x}\dot{x} + \frac{\partial L}{\partial \dot{x}}\ddot{x} + \frac{\partial L}{\partial t}$...
-1 votes
2 answers
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Convert ambient heat into electricity without violating laws?

This is an idea I've been playing around with for a while, even though many people tell me it's impossible. However, as far as I can tell, it does not actually violate the laws of physics. I am ...
1 vote
1 answer
527 views

What is the relationship between flow work and velocity of gas?

As we know that for open systems, we have energy conservation equation as $$h+\frac{v^2}{2}+zg=const.$$ $$h=u+pV$$ $$u+pV+\frac{v^2}{2}+zg=const.$$ Here $pV$ is flow energy. We know that all these ...
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0 answers
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What's the "energy formula" for a car with chemical potential energy, friction and drag? [closed]

I'm trying to figure out the "energy formula" corresponding to a car that is circulating on a highway and that is affected by friction and drag at the same time. The problem is that, since ...
1 vote
4 answers
487 views

Understanding conservation of energy in a pulley problem

I am trying to check if my understanding of conservation of energy is correct. Imagine a pulley problem like so with $m_2$ heavier than $m_1$ and the pulley is ideal (in the original problem I ...
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0 answers
23 views

Electromagnetic stress-energy-momentum tensor and stress tensor

The purely spatial components of the energy-momentum-stress tensor for a perfect fluid are clearly related the components of the ordinary stress tensor inside the fluid. My question if this case is ...
-1 votes
1 answer
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Hypothetical gravitational force depletion [closed]

Hypothetically, let's assume humanity/ someone would manage to use solely gravity for power generation... Would the gravitational force get smaller, or could it even get completely depleted?
4 votes
3 answers
750 views

What would happen if I took empty space and stretched it?

There was a talk at my school by Rocky Kolb and he claimed that they derived (or it might have been ''experimentally found'', I don't remember) that the mass of empty space must be/is on the order of $...
-2 votes
1 answer
70 views

Yet another perpetual motion machine: does this imply large selective membranes are not possible? [closed]

So I was just thinking about the buoyant force and came up with what seemed like a simple perpetual motion based on it. Obviously such things are not physically possible so I'm trying to figure out ...
4 votes
1 answer
48 views

Does a water heater consume more energy if constantly kept on than if it is made to heat the water on demand?

Suppose I have a water heater, an insulated tank of water that when turned on heats water to a predefined higher than ambient temperature and keeps it at that temperature. Suppose I don't need the ...
1 vote
1 answer
59 views

Where does the excess energy go in this problem?

Two identical plastelin balls were suspended on a non-extensible weightless string with the same length L, which are fixed at the same point. One of the balls was deflected 90 degrees from the ...
2 votes
1 answer
237 views

Potential Energy in a System and its Relation to the Total Energy

In this answer it was stated that potential energy is a property of a system and not an individual particle. If we have two particles (1 and 2) interacting via a conservative force, we can write an ...
2 votes
3 answers
78 views

How conservation of energy looks like in the moving frame?

It is obvious that a car accelerates by converting its chemical energy in the fuel to produce kinetic energy to accelerate itself. If the energy is lost into friction and heat, the car will slow down. ...
1 vote
2 answers
121 views

Why did Noether use the Lagrangian for her conservation of energy theorem?

So I know that for Noether's conservation of energy theorem, the Lagrangian is used. However, I know that the Lagrangian doesn't always equal energy. So why did she use the Lagrangian and not other ...
26 votes
13 answers
5k views

Microscopically, are all collisions really elastic collisions?

I teach grade 12 physics and am about to introduce collisions. I am explaining that in elastic collisions, kinetic energy is conserved and in inelastic collisions, kinetic energy is not conserved. The ...
0 votes
2 answers
458 views

What happens to the energy of an absorbed photon?

Photons are electromagnetic wave fields that travel at the speed of light. When they are absorbed by an object, their momentum and energy is transferred to the absorbing object. What is that ...
0 votes
1 answer
51 views

How is a motorcycle whip possible?

For clarification on what I'm talking about, please see this video link at 0:40 https://youtu.be/YAUOFy1QyiE?t=40 The riders appear to be breaking the laws of physics by rotating one way, and then ...
0 votes
4 answers
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Is the total gravitational energy in the sun greater than the energy that is produced by the sum total of the nuclear fusion contained therein?

I think this is off topic, maybe an astronomy question. Unlike a chemical chain reaction or a fission chain reaction I believe fusion cannot sustain inself. Is more energy going out than is going in ...
3 votes
2 answers
341 views

Why do we give so much importance to energy, i.e., the conserved quantity under time symmetry?

In almost all equations—from GR to QFT—energy conservation is a tool for solving those equations, but we know that energy on large scales is not conserved. Why do we still use this (not) conserved ...
2 votes
0 answers
37 views

On energy conservation and time-ordered interaction diagrams

I've found the following lectures given by professor Mark Thomson (here's his particle physics web page); in the third one, Interaction by particle exchange and QED, it's stated that time-ordered QM ...
1 vote
1 answer
51 views

Statistical Analysis of motion under central force

My question is very simple. If we were to plot the motion of a particle inside circular walls governed by a central attractive force with perfectly elastic collisions, would the statistics of the ...
0 votes
1 answer
365 views

Equivalence between LHV and energy balance in combustion

I have seen in many textbooks the following equation for a combustion chamber $$ \eta \, m_{fuel} LHV = m_{out}h_{out}(T_{out}) - m_{in}h_{in}(T_{in}). \quad (1) $$ Consider now the stoichiometric ...
1 vote
1 answer
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Are one-dimensional bifurcations dissipative or conservative?

I have elementary questions about one dimensional bifurcations. It should be remarked that I have also searched and read previous posts of Physics stack exchange. However, these did not help me with ...
1 vote
4 answers
770 views

Resistive heating - calculating how much a wire will heat up before it melts

I wanted to offer a simple model for students to work with about resistive heating. Since $P=\frac{V^2}{R}$ we can calculate how much energy will be put into a wire over time, if we know the voltage ...
1 vote
3 answers
136 views

Energy conservation in a system of two bodies

I have no idea why my previous post was marked as a homework question where the solution was clearly visible. I was asking to clear a concept of mine regarding system of bodies. However,i will keep ...
0 votes
2 answers
95 views

Piano on the moon

Imagine a piano on the moon. You strike a chord. Since there is no atmosphere, there is no medium for the sound to travel. so where does the sound energy go? Does it just dissipate as another form of ...
0 votes
3 answers
56 views

Conserving energy in newtonian mechanics

If an object is stationary on ground,then it's potential energy is $0$ and kinetic energy is $0$ as well. So it's mechanical energy is $0$. But we know energy is always conserved. Then how does that ...
0 votes
1 answer
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Where is the excess energy going to when a solar inverter is operating a photovoltaic (PV) panel away from the maximum power point (MPP)?

The MPP is the maximum power point of a solar panel and a PV inverter will typically try to find this MPP to yield the maximum energy ouput. But sometimes it might be neccessary to operate the panel ...
4 votes
3 answers
1k views

Is David Tong incorrect in this remark about classical mechanics in his QM lectures?

In page 11 of his Quantum Mechanics lectures, we have the following quote: It turns out that not all classical theories can be written using a Hamiltonian. Roughly speaking, only those theories that ...
1 vote
1 answer
272 views

Kinetic Energy change in particle to wall collision

The Question A particle of mass m is moving in the +x direction with a speed v. It is approaching a massive wall which is also moving in the +x direction, but with a speed u < v (see figure) The ...
0 votes
1 answer
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Transport of energy in DC current

In alternating current (AC), electrons barely move a few centimeters along the conductor, with the usual frequencies and intensities in electric lines. That fact, together with Maxwell's equations, ...
4 votes
1 answer
56 views

Non-Conservative Potential for Charged Residues in Proteins

I am struggling with this question. I want to determine whether the potential of a protein is conservative or not if its dielectric constant depends on the local density of the protein. My model only ...
37 votes
5 answers
27k views

Kinetic energy with respect to different reference frames

I'm having problems understanding the following situation. Suppose two 1-tonne cars are going with the same orientations but opposite senses, each 50 km/h with respect to the road. Then the total ...
0 votes
1 answer
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Internal work in system made up of a ball and the earth

Assume I am considering my system to be the earth, a ball, and the gravitational interaction between them. No air resistance. Assume the ball has a nonzero initial velocity upward. I understand there ...
11 votes
4 answers
11k views

Intuitive meaning of factor 2 in formula of vertical throw max height $h=v^2/2g$

This is a question about a simple thing. The simplified expression for maximum height in vertical throw is $h=\frac{v^2}{2g}$ , could anyone explain intuitively (analogies are welcome) why there is a ...
0 votes
1 answer
176 views

Conservation of Kinetic Energy?

A rod is hinged in a vertical plane. If we throw a ball at the rod's lowest point with some velocity, and they undergo elastic collision, can the total mechanical energy be conserved? What about the ...
0 votes
2 answers
84 views

Which law of thermodynamics is broken here? [closed]

This question came in the Dhaka university admission exam 18-19. A heat engine in each cycle does positive work and loses energy as heat with no heat energy input. Which law of thermodynamics does the ...
0 votes
2 answers
60 views

Can kinetic energy be transferred between two objects even if they are not in contact?

This question is better explained with a thought experiment. It is inspired by this answer, stating that the amount of work done depends on the inertial frame. Consider a one-dimensional space with ...
0 votes
1 answer
41 views

Conservation on energy in newtonian gravity

For the electromagnetic field, we can define the EM energy: $$\mathcal{E}_{EM}= \frac{1}{2} \int_V \left(\varepsilon_0\boldsymbol{E}^2+\frac{\boldsymbol{B}^2}{\mu_0}\right)dV$$ And because charged ...
3 votes
2 answers
314 views

Does energy conservation imply time invariance?

This is similar to this question: Is the converse of Noether's first theorem true: Every conservation law has a symmetry?. However, the answer given there is very technical and general. I am only ...
0 votes
0 answers
38 views

Why does the Navier-Stokes energy equation neglect the pressure-volume work?

A strictly conservative form of the Navier-Stokes energy equation reads (Schlichting, Gersten: Boundary Layer Theory 2017, eq. 3.57) $$\frac{\partial \rho e_t}{\partial t} + \mathrm{div}\left(\rho e_t ...
1 vote
0 answers
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Some questions related to the energy of a viscoelastic bar

Let's consider a problem of free longitudinal vibrations of an one dimensional infinitely long elastic relaxing bar (Maxwell material) with constant cross section. The bar's displacement $u$ and ...
0 votes
1 answer
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What does it mean when "work done cannot be regained"?

This question came in the Rajshahi University admission exam in 2014-15 Q) Which one is true in the case of conservative force? (A) Energy is wasted (B) The work done cannot be regained (C) The law of ...

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