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# Questions tagged [energy]

Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding [tag:energy-conservation] if it is specifically about its conservation.

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### Why does kinetic energy increase quadratically, not linearly, with speed?

As Wikipedia says: [...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $\frac{1}{2}mv^2$. Why does this not increase linearly with speed? Why does it take so ...
55k views

### Why does holding something up cost energy while no work is being done?

I read the definition of work as $$W ~=~ \vec{F} \cdot \vec{d}$$ $$\text{ Work = (Force) \cdot (Distance)}.$$ If a book is there on the table, no work is done as no distance is covered. If I ...
46k views

### Why don't we use weights to store energy?

One of the main reasons why we haven't switched to clean energy is the lack of efficient storage methods - But, why aren't we using dead weights to store energy and draw it back later when needed? As ...
5k views

### Is it necessary to consume energy to perform computation?

As far as I know, today most of the computers are made from semiconductor devices, so the energy consumed all turns into the heat emitted into space. But I wonder, is it necessary to consume energy ...
6k views

### Where is the flaw in this machine that decreases the entropy of a closed system?

I was thinking about a completely unrelated problem (Quantum Field Theory Peskin & Schroeder kind of unrelated!) when the diagram below sprang into my mind for no apparent reason. After some ...
66k views

### What happens to the energy when waves perfectly cancel each other?

What happens to the energy when waves perfectly cancel each other (destructive interference)? It appears that the energy "disappear" but the law of conservation of energy states that it can't be ...
41k views

### What Is Energy? Where did it come from?

The simplistic undergrad explanation aside, I've never really understood what energy really is. I've been told that it's something when converted from one kind of something to another kind, or does ...
18k views

### What makes running so much less energy-efficient than bicycling?

Most people can ride 10 km on their bike. However, running 10 km is a lot harder to do. Why? According to the law of conservation of energy, bicycling should be more intensive because you have to ...
15k views

### Does a thrown ball have kinetic energy at the top of the curve?

I'm going through physics with my 5th grade child. There is a question and answer that indicates that a airborne ball at the top of the trajectory does not have kinetic energy. The diagram below ...
23k views

### If visible light has more energy than microwaves, why isn't visible light dangerous?

Light waves are a type of electromagnetic wave and they fall between 400-700 nm long. Microwaves are less energetic but seem to be more dangerous than visible light. Is visible light dangerous at all ...
17k views

### Could a candle theoretically melt iron?

The title question is rather illustrative. I suppose the real question would be: Is heat cumulative? Put back into an example: If I have a lit candle right beneath an iron bar, assuming the candle ...
13k views

### How is 6W equivalent to 40W, as claimed by adverts for LED light bulbs?

Every advert I come across for LED bulbs advertise them as the equivalent of a higher W incandescent bulbs. This makes no sense to me, if the room requires 40W to lighten it up then it'll always ...
12k views

### When a balloon pops and lets a brick fall, where does the energy come from?

Let's say a scientist attaches a 1 kg brick to a large helium inflated balloon, lets the balloon go, and then it reaches an altitude of 10 000 meters before it pops, dropping the brick. The brick ...
6k views

### What keeps mass from turning into energy?

I understand the energy and mass can change back and forth according to Einstein. It is fluid; it can go from one to the other. So, what keeps mass from just turning into energy? Is there some force ...
31k views

### Why can't energy be created or destroyed?

My physics instructor told the class, when lecturing about energy, that it can't be created or destroyed. Why is that? Is there a theory or scientific evidence that proves his statement true or ...
9k views

### The Sun is giving us a low entropy, not energy

While I was watching a popular science lecture on YouTube, I came across this sentence "Sun is giving us a low entropy, not energy" which was said by Prof. Krzysztof Meissner. I am not a ...
45k views

### Is a hard drive heavier when it is full?

Browsing Quora, I saw the following question with contradicting answers. For the highest voted answer: The bits are represented by certain orientations of magnetic fields which shouldn't have ...
8k views

### Was the 2013 meteor over Russia stronger than an atomic bomb?

Reports of the Russian meteor event (2013) say that it released more energy than 20 atomic bombs of the size dropped on Hiroshima, Japan: Scientists estimated the meteor unleashed a force 20 times ...
172k views

### How can momentum but not energy be conserved in an inelastic collision?

In inelastic collisions, kinetic energy changes, so the velocities of the objects also change. So how is momentum conserved in inelastic collisions?
45k views

### Why is torque not measured in Joules?

Recently, I was doing my homework and I found out that Torque can be calculated using $\tau = rF$. This means the units of torque are Newton meters. Work & Energy are also measured in Newton ...
9k views

### What's the real fundamental definition of energy?

Some physical quantities like position, velocity, momentum and force, have precise definition even on basic textbooks, however energy is a little confusing for me. My point here is: using our ...
11k views

### Why can electric cars recoup energy from braking, but a spaceship cannot?

It is said that in a spaceship, you need to spend as much energy to brake as you spent for accelerating. An electric car, however, charges its batteries while braking, thus it actually recovers energy ...
4k views

### What is behind the definitions of work and energy?

I am aware that there appear to be many similar questions on this site, but that is just because of the misleading title. I could not think of a better title that illustrates how different is this ...
15k views

### Newton's 3rd law… hitting drywall (which I break) vs hitting a brick (which breaks me)?

According to the Third Newton's law of motion: For every action there is an equal and opposite reaction. So, I understand that if I hit a brick wall with $50\, \mathrm{lbs}$ of force, the brick ...
5k views

### Are there any theoretical limits on the energy of a photon?

Is there any lower or upper limit on the energy of a photon? i.e. does the mathematical framework we currently use to study photons blow up when a photon surpasses a certain upper limit of energy? (or ...
6k views

### Do photons gain mass when they travel through glass?

Please correct me if I'm wrong, but I believe that photons slow down when travelling through glass. Does this mean they gain mass? Otherwise, what happens to extra kinetic energy? I understand now ...
5k views

### Why are line spectra only seen in gases?

This might be a stupid question but I could not find the answer in my textbook or on the internet with a few searches. So I believe when an atomic electron moves down to a lower energy level it ...
9k views

### Why do we need to know the shape of the slide to find the time to slide down it?

In my physics book after this solved example: A child of mass $m$ is initially at rest on top of a water slide at height h = 8.5m above the bottom of the slide. Assuming that the slide is ...
6k views

### Why is 7 TeV considered as a big amount of energy?

Considering that $7$ TeV is more or less the same kinetic energy as a mosquito flying, why is it considered to be a great amount of energy at the LHC? I mean, a giant particle accelerator that can ...
8k views

### Why doesn't light kill me?

Why does each individual photon have such a low amount of energy? I am hit by photons all day and I find it amazing that I am not vaporized. Am I simply too physically big for the photons to harm me ...
10k views

### Black holes and positive/negative-energy particles

I was reading Brian Greene's "Hidden Reality" and came to the part about Hawking Radiation. Quantum jitters that occur near the event horizon of a black hole, which create both positive-energy ...
26k views

### When is the Hamiltonian of a system not equal to its total energy?

I thought the Hamiltonian was always equal to the total energy of a system but have read that this isn't always true. Is there an example of this and does the Hamiltonian have a physical ...
7k views

### How am I able to stand up and walk down the aisle of a flying passenger jet?

The energy of a moving object is $E = mv^2\;.$ That is it increases with velocity squared. I walk at say 3 miles per hour, or lets round that down to 1 meter per second for a slow walk. I weigh less ...
10k views

### Pouring oil on choppy water to calm it , does it work and if so how?

Near where I live, local fishermen often bring cans of castor oil with them, to calm the water around their boats, if they feel bad weather is due. They claim this method of sea calming works, (...
49k views

### Where does gravity get its energy from?

I would like to know where gravity gets its energy to attract physical bodies? I know that the law of conservation states that total energy of an isolated system cannot change. So gravity has to be ...
17k views

### Amplitude of an electromagnetic wave containing a single photon

Given a light pulse in vacuum containing a single photon with an energy $E=h\nu$, what is the peak value of the electric / magnetic field?
7k views

### Why does moving through time not require energy?

Moving through the other three dimensions necessitates energy. But why doesn't moving through time necessitate energy?
5k views

### Can we run out of gravitational (tidal) energy?

I read an article on energy forms and sources that made me think. Energy comes from somewhere and is limited in various senses. It's most obvious for fuels: we burn coal and oil and at some point we'...
4k views

### Redshifting of Light and the expansion of the universe

So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light ...
10k views

### Why do we use the electron volt?

Why do we use the electron volt? Why did it come to be the electron volt and not, say, just a prefix of the joule, like the nanojoule? Does the electron volt represent anything particular as far as ...
8k views

### How is it possible that the energy needed to stop a train is the same as the (chemical) energy in a pack of chocolate cookies?

Today my friend told me something that blew my mind completely. He said: The energy necessary to stop a train is equal to the energy in a pack of cookies. How is that possible? Is he right? I'm ...
10k views

### Why does temperature remain constant when water is boiling?

As I understand it, during boiling the input of heat destroys or re-arranges the hydrogen bonds. It is used, in other words, against the potential energy of the intermolecular bonds. But if some ...
126k views

### Is two cars colliding at 50mph the same as one car colliding into a wall at 100 mph?

I was watching a youtube video the other day where an economist said that he challenged his physics professor on this question back when he was in school. His professor said each scenario is the same, ...
14k views

### Is plant photosynthesis more efficient than solar panels?

Is photosynthesis more efficient than solar panels? If so, by how much?
22k views

### How would a black hole power plant work?

A black hole power plant (BHPP) is something I'll define here as a machine that uses a black hole to convert mass into energy for useful work. As such, it constitutes the 3rd kind of matter-energy ...
4k views

### Is there an actual proof for the energy-time Uncertainty Principle?

As I understand, the energy-time uncertainty principle can't be derived from the generalized uncertainty relation. This is because time is a dynamical variable and not an observable in the same sense ...
31k views

### Why do solar panels not have focusing mirrors?

Most of the solar panels that I have seen do not have any mirrors, etc., but usually solar cookers have mirrors. What is the reason for solar panels not having focusing mirrors?
4k views

### Why do electrons fall from a high excitation to a lower one?

If when you shine a photon into an atom for example, and this excites an electron to a higher energy level, do the electron(s) keep going higher the more light you shine, and is there an energy limit, ...
My questions mostly concern the history of physics. Who found the formula for kinetic energy $E_k =\frac{1}{2}mv^{2}$ and how was this formula actually discovered? I've recently watched Leonard ...