# 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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### Is the normal force a conservative force?

Most of the time the normal force doesn't do any work because it's perpendicular to the direction of motion but if it does do work, would it be conservative or non-conservative? For example, consider ...
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### Does the amount of gravitational potential energy in the universe increase as it expands?

It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
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### Relationship between height and velocity in conservation of mechnical energy

I'm a high school physics student, and we recently did a lab on the conservation of energy where we measured the speed of a marble at varying heights on a rollercoaster track. We were supposed to ...
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### How can gas from compressed air can “take” heat from surrounding environment?

I have recently been reading about why a can of compressed air gets cold when the air it contains is discharged. From what I understand the change from a liquid to a gas requires energy and therefore ...
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### Why is a washing machine unable to spin-dry a duvet?

Background story: I put my duvet (synthetic filling) into a washing machine, and the machine was unable to spin-dry it. My mom told me: "It's because it is full of air". I was not satisfied with the ...
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### Total Energy of the Universe? [duplicate]

I've heard the total energy is zero, but I've also heard it cannot be said to be zero since there's so much unknown stuff in the universe. Is that true?
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### Elastic potential

I have a doubt: elastic potential energy is given by: $U=\frac{k}{2}x^2+K$ but does elastic potential exist? (for example: potential gravitational energy is given by $U=mgz+K$ and gravitational ...
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### Shouldn't the change in kinetic energy be more in a moving elevator from a stationary frame of reference?

Consider an elevator moving down with uniform velocity. A person standing inside watches an object fall from the ceiling of the elevator to the floor. Say the height of the elevator is $h$. Then the ...
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### Conservation of Energy in the Universe [duplicate]

Possible Duplicate: Is energy really conserved? Why can’t energy be created or destroyed? One of the laws of the universe that dazzles me the most is the law of conservation of energy. I ...
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### How does rest mass become energy?

I know that there's a difference between relativistic and rest mass. Relativistic mass is "acquired" when an object is moving at speeds comparable to the speed of light. Rest mass is the inherent mass ...
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### Escape velocity from Earth

We know the escape velocity from the Earth is 11.2km/s. Isn't it the velocity required to escape from earth if we go normal to the surface of earth? i.e while we derive the formula for the escape ...
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### How efficient is the human body?

This question sort of comes to mind when hearing how efficient an internal combustion engine is turning chemical energy in mechanical energy (something like 20-40%) with lots of excess heat. As an ...
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### Quantum Conservation versus classical conservation

If energy is conserved in all quantum mechanical interactions, how are there classical interactions in which energy is not conserved, given that classical interactions are a macroscopic approximation ...
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### Why can't a superconductor make a DC motor self sustaining?

Superconducting wire can host a low current magnetic field. I do not know if it supports a corresponding electrical field. Can a superconducting wire that sustains a current accelerate a DC motor? ...
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### Energy Conservation Law validity

I will probably be laughed out of town for saying this, but why can't the Energy Conservation Law be broken? Everybody thought electricity to motion was impossible until Faraday made his motor, ...
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### What happens to orbits at small radii in general relativity?

I know that (most) elliptic orbits precess due to the math of general relativity, like this: source: http://en.wikipedia.org/wiki/Two-body_problem_in_general_relativity I also know that something is ...
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### Conservation of Energy in a Capacitor

Consider a parallel-plate capacitor in free space. A negatively charged point particle with initial velocity $v$ passes through the space between the pair of parallel plates (with an initial path ...
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### Elastic collision in two dimensions

Suppose a particle with mass $m_1$ and speed $v_{1i}$ undergoes an elastic collision with stationary particle of mass $m_2$. After the collision, particle of mass $m_1$ moves with speed $v_{1f}$ in a ...
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A car is driving down a mountain ($v=90 km/h=25 m/s$, when the driver realizes that brakes aren't working. He try to lose velocity going up an inclined ($20°$) plane, with a friction coefficient of $k=... 2answers 139 views ### Energy problem: What's wrong here? A car ($m=540\,\text{kg}$) engine, has a power of$60\,\text{kW}$. The static friction coefficient between wheels and road is$k=0.6$. How long does it take to reach the speed of$27.7\,\text{m/s}$, ... 2answers 3k views ### How do I approach this conservation of energy problem, symbolically In the absence of nonconservative forces such as friction and air resistance, the total mechanical energy in a closed system is conserved. This is why that when I toss an object directly upwards, the ... 4answers 195 views ### Is the mathematical truth 1+1=2 analogous to the conservation of energy? [closed] They seem to express the same concept in different fields. 5answers 2k views ### Is there a mathematical derivation of potential energy that is *not* rooted in the conservation of energy? For simplicity I'll consider only gravity, but in general this question only applies to conservative forces. As per my understanding, the way one gets to the equation for gravitational potential ... 5answers 12k views ### Does matter with negative mass exist? Or does it exist mathematically? Is it really inconsistent with a common-sense, mathematics or known physical laws? As far as I understand, if it exists, it must be far away from the "positive" ... 2answers 2k views ### CPT Violation and Symmetry / Conservation Laws Ok, so I remember reading that every conservation law has a corresponding symmetry (i.e. conservation of momentum is translational symmetry, conservation of angular momentum is rotational symmetry). ... 5answers 20k views ### Can I take heat from the air and convert it to electricity? Its a summer day and the air in my house has been heated up. I could switch on my air conditioning, but then I'd be using energy from the grid in order to reduce the amount of energy in my house. ... 3answers 1k views ### Vacuum energy and perpetual motion The part of the Einstein equations of general relativity referred to vacuum energy, introduce a repulsive term in gravity. This means that as the space become bigger and bigger, vacuum part become ... 1answer 272 views ### Understanding Work and the conservation of energy We have a car with a mass of$780 kg$with travels with a speed of$50 km/h$. The car brakes and after$4,2m$is stops completely. Warmth is created. Calculate the friction. I solved this easily, by ... 2answers 2k views ### Finding an equation relating the mass of a blade of a wind turbine to its velocity I'm writing up my physics coursework and I thought I'd try and find an equation described in the title. This is my attempt: Is it correct? 3answers 6k views ### Conservation of energy in objects at terminal velocities In vacuum, object free falling under gravity, the sum of Gravitational Potential Energy(GPE) and Kinetic Energy (KE) is a constant. The GPE is a decreasing side of a quadratic and KE is a increasing ... 3answers 1k views ### Energy-time uncertainty and pair creation Usually, the energy-time analogue of the position-momentum uncertainty relation is quoted as$\Delta E \Delta t \geq \frac{h}{4 \pi}$. This has interpretational issues and such. But, with a suitable ... 2answers 1k views ### Many-worlds: Where does the energy come from? With regard to the theory that each time a wave function collapses the universe splits so that each possible outcome really exists - where does all the energy required to create all the new universes ... 0answers 78 views ### Energy can't be created or destoryed? [duplicate] Possible Duplicate: Conservation law of energy and Big Bang? If energy can't be created or destroyed then how did the big bang create energy? and if energy can't be created then does that mean ... 5answers 18k views ### Is the total energy of the universe constant? If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy? 10answers 2k views ### Is it possible all matter in the universe emerges from nothing? If the Universe is flat and the total energy of the universe can be zero (we don't know if it is, but many theorists support the idea, i.e. at BB initial conditions: t = 0, V = 0, E = 0) then is it ... 3answers 1k views ### Where does the energy for repulsion in charges come from? If I bring together 2 objects with the same charge, they repel. But even after the repulsion has taken place, there is no loss of charge in the objects. Where does the energy come from then? 3answers 1k views ### Conservation of Energy in Different Frames of Reference [duplicate] Say I have a bucket of fuel that can produce 150J of energy by combustion. No matter what frame of reference an observer or the bucket of fuel is in, since the configuration of molecules stay the same,... 2answers 2k views ### what happens when I roll a gyroscope along its axis of spin Say: I have a gyroscope that is spinning in the xy plane along the z axis. I then roll its spinning axis by some angle theta Now I know the gyroscope will resist my attempting to change its axis of ... 2answers 544 views ### Is an electric lamp a transducer? [closed] Silly thought. A transducer, by definition, is a device that converts variations in one form of energy to another. An electric lamp converts electricity into visible light - the brightness may vary ... 2answers 2k views ### What is the most efficient machine for translating gravitational potential energy of one mass into kinetic energy of a different mass? As the question states, what is our current best machine for translating falling gravitational potential energy, such as a large weight, into launching a smaller projectile vertically? A lever? A ... 3answers 5k 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 ... 3answers 5k views ### Is there a deep reason why springs combine like capacitors? I was solving a practice Physics GRE and there was a question about springs connected in series and parallel. I was too lazy to derive the way the spring constants add in each case. But I knew how ... 2answers 602 views ### Conservation Laws in Photon Parametric Down-Conversion As Wikipedia explains, one photon passing through a crystal sometimes down-converts to two photons. Wikipedia says total energy and momentum are conserved by just considering the three photon states; ... 3answers 13k views ### Is the law of conservation of energy still valid? Is the law of conservation of energy still valid or have there been experiments showing that energy could be created or lost? 1answer 5k views ### What happens when you shake a can of soda? Of course the CO2 comes out of the liquid, and it will even build up a lot of pressure if needed. The question is what happens at a molecular level to cause it to leave the liquid? 4answers 15k views ### Where does the electricity, generated by a solar panel, go if you don't use the electricity? I'm sorry if this question is too trivial for this Q&A forum. I am a layman when it comes to physics (though I did cover the high-school physics courses). I was wondering what happens to the ... 1answer 981 views ### Bernoulli equation from energy conservation I have derived the energy conservation equation: \frac{\partial}{\partial t} \left [ \frac{1}{2} \rho v^2 + \varepsilon + \rho \phi \right ] + \frac{\partial}{\partial x_j} \left [ \... 2answers 4k views ### First integral of an equation of motion:$\mu\ddot r=-\frac{k}{r^2}\$

I've got an equation of motion (EOM), which is $$\mu\ddot r=-\frac{k}{r^2}$$ How do I find the first integral of this EOM? I'd appreciate it if someone could show me the steps involved. I should ...