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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51 views

Negative work done by dark energy

If I lift a book up, I applied a force opposite its weight so there's positive work done. Dark energy is said to be pushing galaxies apart and the energy is coming from this negative work done. ...
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Pinch point for heat and mass exchanger devices (like cooling towers)

As you know, there is a pinch point for heat exchangers... But can we introduce a pinch point for heat and mass exchanger devices (like cooling towers)? At cooling towers, the outlet water ...
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Piezoelectricity and conservation of energy

This is how I understand piezoelectricity: The molecules of a material have electric dipole moments. When a sample of the material is under mechanical stress, the molecules "slide"/"turn" and become ...
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1answer
34 views

The horizontal velocity $v$ that must be given to the mass $m$ to rotate the pendulum $90$ degrees

A compound pendulum consists of a uniform rod of length $L$ of negligible mass. A body of mass $m$ is fixed at the lower end and a body of mass $2m$ is fixed exactly in the middle of the rod. The ...
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2answers
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Perpetual motion using mass defect

First off, I know this wouldn't work due to it breaking thermodynamics and that there would also be heaps of energy dissipated to the surroundings, and fusion doesn't work yet, but I'm more interested ...
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5answers
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Conservation of Mechanical Energy in moving frame

A ball of mass $m$ is attached to a spring that is attached to a heavy moving cart (mass of cart $M >>$ mass of ball $m$) moving at velocity $V0$. The compressed spring with known spring ...
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How is energy conserved in a theoretical circuit where you put a lightbulb and two rechargeable batteries like… (diagram included)

In the beginning the first battery is fully charged, and the second is fully discharged. As the circuit is connected the discharging battery recharges a dead second battery and light is emitted from ...
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Why does conservation of energy appear to be violated? [closed]

I am currently working on a rigid body simulator. Before getting proper collision and rotations working, I decided to simulate the collision of spheres as axis-aligned objects. The simulation works ...
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1answer
106 views

Conservation of kinetic energy in two dimensional elastic collisions

I have a question where two particles collide with each other in a non central way. Mass of particle 1 ($m_1$) is $1\text{kg}$ and its velocity $v_1$ is $5\text{m/s}$ (moving along the x-axis) ...
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4answers
196 views

Can you derive the acceleration of a cylinder rolling down an inclined plane by ONLY using conservation laws?

I want to solve the standard problem of a cylinder rolling down an inclined plane by only using conservation laws. In particular I want to use the equation: $$E_{pot}=\frac{1}{2}mv^2+\frac{1}{2}I\...
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2answers
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Elastic potential energy and springs

The formula for elastic potential energy(for a spring) has been derived by assuming the following things: 1.Work done by a deforming force on a spring from a relaxed state (state where spring is not ...
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Gravity in classical energy

It's well understood that matter has mass, and more importantly gravity. I believe I am correct to say that gravity is more like an affect of matter. And not actually part of it. Ie mass bends space ...
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Is mass-energy conversion contradicting energy conservation? [duplicate]

If mass can be transformed into energy, and energy transformed back into mass, isn't this at odds with the law of conservation of energy? Because this would mean, that in theory, it would be possible ...
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How can the red shift due to Hubble flow be consistent with the law of conservation of energy? [duplicate]

Given that there is no change in relative kinetic energy attributable to the expansion of the universe, what accounts for the difference in energy emitted vs energy absorbed (i.e. the red shift ...
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Does a rock use up energy to maintain its shape?

A rock sitting on land, the ocean floor, or floating in space maintains its shape somehow. Gravity isn't keeping it together because it is too small, so I'm assuming it is chemical or nuclear bonds ...
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Conservation of Momentum vs. Energy [closed]

If a mass $m$ travelling at velocity $v$ collides ideally with a massless shaft mounted on a massless bearing with an identical mass on the other end of the shaft: Conservation of momentum requires ...
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1answer
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How does elastic potential energy translate into kinetic energy and momentum?

Let's assume there's a spring with 10,000J of elastic potential energy placed between two objects with a mass of 10kg each. Like this: 10kg< 10,000J >10kg I assume that the energy would be ...
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2answers
114 views

Proof of total energy is constant in a central force field

I saw many proofs of total energy is constant in a central force field. But all the proofs end up showing this formula $$m[{\dot r}^2 + r ^2{\dot \theta}^2 ]+ \int f(r)dr = E$$ is constant. But ...
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In the case of the Dirac Equation,what forbids the free electron to absorb a photon? (electron magnetic moment)

It is straightforward to show from relativistic kinematics that a free electron cannot absorb a photon, as shown in this previous thread. However, It is also known that using the Dirac equation, you ...
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221 views

Conservation of mechanical energy proof

Using Newtonian mechanics, how does one prove that the change in mechanical energy of a classical system of particles is equal to the work done by non-conservative forces? What assumptions are made? ...
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1answer
47 views

System of two masses interacting and the notion of work [closed]

Assume an isolated system consisting only of the earth and a basketball interacting through the gravitational force: No air, no other planets/stars etc. Are the following statements accurate? The ...
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1answer
52 views

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 ...
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1answer
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volume of liquid in a boiler based heating system

We're trying to improve energy efficiency for my dad's hothouse. The setup includes a gas boiler and a pump running the heated water thru special pipes to heat the grow space. My question is, leaving ...
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1answer
43 views

Difference between the angular momentum for a mass following a trajectory and a mass rotating around a rotational center

I struggle to find a satisfying reason and formulation, why for a point mass, that is fixed in a rotational system and the radius is changed (like pulling on a string with a small ball rotating ...
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3answers
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Does a heat engine inside of a refrigerator increase efficiency?

Consider a normal refrigeration cycle, except a heat engine runs off of the temperature difference between the evaporator and the condenser, as shown below. This heat engine powers the compressor, so ...
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Why is there discrepancy in conservation of energy and momentum in my modified version of Veritasium's bullet board experiment?

https://youtu.be/BLYoyLcdGPc This is the Veritasium's bullet board experiment: A bullet is shot from beneath a horizontal board. The first time it is shot right underneath its centre of mass, and the ...
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4answers
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I recently solved a problem where a tenis ball is kept on a basketball and basket ball is at some height from the ground [closed]

. When the system is released the tennis bounces back to a larger height than it was before. what is the reason for this? I mean the problem assumes elastic collisions. How can the tennis ball rise ...
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153 views

Block Sliding on a Wedge: Conservation of Energy

I have a question regarding a physics problem where a block is sliding down a wedge (which can be seen here). All of the solutions I've seen to this kind of problem involve the conservation of the ...
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3answers
64 views

Converting potential energy to kinetic energy

I know how potential and kinetic energy interrelate when you drop anything. But is it a general understanding in the community that a change in potential energy is always kinetic energy? I am ...
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1answer
96 views

Are particles an abstraction of energy, or their matters do indeed exist?

I'm trying to understand from where the first particles in quantum mechanics came from, and while doing research I got this information from an article about the origin of matter: In the beginning,...
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Conservation of energy on a spring. When are forces positive and negative?

I'm doing physics problems on my own and I cannot figure out when Potential Energies (U) are supposed to be positive and when negative in a Conservation of Energies problem. Take for example this ...
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3answers
69 views

A doubt related to Sound [closed]

According to laws of conservation of energy energy can neither be created nor be destroyed. We all know that sound is energy. But according to law of conservation of energy- sound can neither be ...
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4answers
103 views

Formula for potential energy? Conservation of energy?

How would we know what formula to use for potential energy? In my class, $mgh$ was used, but when dealing with a spring, it's ${1\over2}kx^2$. Is that because that's the elastic potential energy ...
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Time rate of change (the time derivative) of the internal energy.

I am reading in an article that according to the first law of thermodynamics, the time rate of change of the internal energy obeys the following relation, $\dot{E}+\int_A \mathbf{q} \cdot \mathbf{n} \...
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Is the violation of time-reversal always associated with the violation of time-translation and vice-versa?

Is the violation of time-reversal symmetry always associated with the violation of time-translation symmetry? What about the converse? Is it possible for one to be violated while the other remaining ...
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Why Pascal's Law is true and what is the mechanism for force amplification at molecular level?

"A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid" "A pressure change occurring anywhere in a confined incompressible fluid ...
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Why don't we use positrons from beta plus radiation as a source of anti-electrons for energy?

Why can't we use the positrons (let off by beta plus radiation) as a source of anti-matter, such that we can collide it with an electron for 100% efficient mass to energy conversion?
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Simple Harmonic Motion of a Pendulum Derivation Questions [closed]

A simple pendulum with mass m on a string of length l is released from rest at an angle of $\theta_0$ to the vertical. (i) Assuming that the pendulum undergoes simple harmonic motion, find an ...
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Use of conservation of energy [closed]

A ball starts rolling down a slope as shown in the figure below. All units shown are in meters. The floor has very little friction. Which of the following is true after a long time? options A. ...
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Why energy plays such an important role among other conservative quantities

I was reading books where I realized an interesting thing: In quantum, the evoluation of states/wave funciton is governed by hamiltonina operator $H$, which is bascially the energy operator. In ...
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Why isn't energy conserved in this collision problem? [closed]

I am stuck in a question where energy conservation is failing and momentum conservation is correct. I think I might be doing something wrong, that's why I'm asking this question. The problem is as ...
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2answers
228 views

Curve for fastest time down a ramp [duplicate]

I came across a physics experiment video showing three balls released from a point A, going down three different kinds of ramps leading to a point B (https://www.youtube.com/watch?v=61S0KW7e-rc) ...
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1answer
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In vertical circular motion of a mass attached to a rope, where does the elastic potential energy stored in the tension goes?

When a bob is attached to a rope where the rope has negligible mass, the bob undergoing circular motion will have its KE1+PE1 at any point of the circular motion equal to KE2+PE2 at another point of ...
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Release of energy during electron jumps [duplicate]

We took up the topic of particle nature of light recently.As the light is quantised when an electron jumps from higher to lower orbit it releases energy but as the energy is quantised it will release ...
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1answer
164 views

Explain Lenz Law with Conservation of Energy [duplicate]

My question is: If the bar magnet is moved towards a coil connected to a galvanometer and instead of a opposing magnetic pole a attractive pole i.e. south of coil and north of magnet is created due to ...
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2answers
248 views

Are these collisions equivalent?

Similar to the question if two cars with a velocity of 50 mph each colliding is the same as one car colliding with wall at 100 mph, I was wondering if the same amount of energy is produced when ...
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Conversion of Kinetic energy to electric potential energy [closed]

Two deuterium nuclei, separated by a large distance with same initial speed are travelling towards each other; the radius of the nucleus is given. How can I find the minimum speed of the nucleus? I ...
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1answer
116 views

Conservation of energy in rotational motion [duplicate]

Suppose a boy is standing on a platform which is free to rotate about an axis passing through its center. The Kinetic energy of the boy and the platform is K. If the boy stretches his hands so that ...
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112 views

Get relation from definition of stress-energy tensor and the conservation of energy

Starting from the following definition of stress-energy tensor for a perfect fluid in special relativity : $${\displaystyle T^{\mu \nu }=\left(\rho+{\frac {p}{c^{2}}}\right)\,v^{\mu }v^{\nu }-p\,\eta ...
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Is the mechanical power output of a car constant?

According to my textbook. The mechanical power output of a car is constant. My reasoning tells me otherwise though. According to a lot of pages on this forum related to the driving force of a car. ...