# 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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### 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. ...
24 views

### 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 ...
37 views

### 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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### 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 ...
128 views

### 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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### 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 ...
141 views

### 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 ...
74 views

### 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 ...
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) ...