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29

Look at it this way: Suppose you are in a train travelling at 10 m/s. Somebody inside the train throws a ball at you in the opposite direction at 10 m/s. You feel the pain belonging to your first experiment. However, somebody looking at this experiment from outside the train would say that the ball is standing still and you are travelling towards the ball ...


20

A photon will travel "at the speed of light" until obstructed. From the speed, and elapsed time, you can calculate how far the light will travel. Laser light consists of more than one photon "in phase", which has exactly the same property in this respect, as a solitary photon.


14

It is true that general time-travelling violates conservation of energy. If you transport yourself into yesterday, you appear twice in the universe for that day, which means twice your rest energy, which is a lot of energy. It may mean that time-travelling is inconsistent and therefore impossible. But not necessarily. In general relativity, it is very hard ...


13

Yes, unfortunately. Because of the equivalence of inertial reference frames, the the physical laws are the same in both reference frames. However, another possibility, which is non abelian, is that instead of feeling the same amount of pain, you could be feeling the opposite amount of pleasure. It depends if pain (X) are fermions or bosons, that is, if ...


12

Theoretically, the photon (or the beam of photons, there really isn't a difference) can go an infinite distance, traveling all the while at a speed $c$. Since photons contain energy, $E=h\nu$, then energy conservation requires the photon to only be destroyed via interaction (e.g., absorption in an atom). There is nothing that could make the photon simply ...


12

First, you have system with some energy, named $U$ by physicists. You think you have all the information you need to characterize the system but then some guy comes near and says: "Whoa, that's bad, the volume of your system can change." You say: "No problem, we just add here $pV$. Our new energy is $H=U+pV$." "But hey," they say, "your temperature can ...


9

Short answer: Gibbs free energy $G = U + PV - TS$ combines internal energy $U$, pressure $P$, volume $V$, temperature $T$, and entropy $S$ into a single quantity that measures spontaneity. With that, I mean that processes that lower the Gibbs free energy of your system will spontaneously occur, and equilibrium is reached when the Gibbs free energy reaches ...


9

No, conservation of energy is for the entire system. If you can travel from time A to time B then both time A and B are parts of the same system as far as conservation of energy is concerned. Even if you assumed that despite travel being possible the times were separated, time travel would simply require the transfer of equal energy from in the reverse ...


8

Consider that most elevators have a counterweight to store energy. The counterweight isn't perfect, but it reduces the overall energy needed to move the carriage. As the elevator moves up, the counterweight moves equally down. Likewise, a time machine would have to overcome the energy deficit/surplus caused when moving from one point to another, but it ...


8

Let's take everything out of our scenario other than you and the ball. No baseball stadium, no Earth, no spherical cows, NOTHING in the entire universe but you and the ball. (Nope, not even microwave background radiation) Now the question has changed. Now you need to ask whether it's possible to decide whether you're moving towards the ball or vice versa.


8

Note that it is correct that a photon can travel an infinite distance in an infinite time, but it can not reach any desired point in the universe. This is caused by the expansion of the universe, which also leads to the fact that we can not receive information outside of the observable universe.


7

Now I am left wondering why does the heat become lost as if travels slightly. It is not lost. It is spread more out. If you stand so close to the heat source that you are hit by, say 1/10 of it's radiation (1/10 of all photons sent out hit you), then when standing further away you are maybe only hit by 1/100. The heat radiation sent from the source ...


6

Intensity has units of watts per area: $$ \left[I\right]=\rm\frac{W}{ m^2} $$ where the area is the surface area of the emitting source (in this case, the sun). This tells you the total amount of radiation present (over all wavelengths). The extra factor of 1/nm in your plot gives the spectral irradiance: $$ \left[\mathcal E\right]=\rm \frac{W}{m^2\,nm} $$ ...


6

How can the work energy theorem be valid in presence of non-conservative forces since conservation of energy is not there? The work-energy principle simply states that work is the net increase of KE The principle of work and kinetic energy (also known as the work-energy principle) states that the work done by all forces acting on a particle ...


5

Sanaris's answer is a great, succinct list of what each term in the free energy expression stands for: I'm going to concentrate on the $T\,S$ term (which you likely find the most mysterious) and hopefully give a little more physical intuition. Let's also think of a chemical or other reaction, so that we can concretely talk about a system changing and thus ...


5

When the block started to compress the spring, the restoring force was opposing its movement. And the spring ought to have kinetic energy otherwise the momentum of the spring-block system wouldn't be conserved. Ok, if I say the wall can't be neglected, then ok, if it had moved due to the force by the block, hadn't the KE of the block gone to the KE ...


5

Energy conservation stems from Noether's theorem applied to time (i.e., time-invariance leads to energy conservation, similarly to how spatial-invariance leads to momentum conservation). Since the universe is expanding (and accelerating at that), the state of the universe today is different than it was yesterday and will be tomorrow, hence energy ...


4

The H.E.S.S. Gamma ray observatory in the Namibian desert has sufficient collecting area that it is able to detect (via Cerenkov radiation) very rare high energy gamma rays that were simply not available to space observatories with comparatively low collecting areas. H.E.S.S. detects high energy gamma rays from all sorts of astrophysical objects and ...


4

One small addition to the other answers: While it is indeed true that the light will never stop if it doesn't hit anything, it will however get red shifted, and thus become less energetic, due to the expansion of the universe. For example, the cosmic microwave background consists of photons which were emitted back when the atoms formed. However, back then ...


4

No mater the inertial referential: The pain is linked to the energy dissipated by the change of speed of the 2 objects in any inertial referential. Remarque 1: a part of the energy can be absorbed by the ball by deformation or heating. So a soft ball would make less pain. Remarque 2: The pain depends on what is static behind you and what is static behind ...


3

Energy is the name physicists give to the Noether Charge that is conserved when a physical system's description through its Lagrangian is unchanged by time shifts. Or, in more everyday language, most physics does not depend on where one puts the $t=0$ time co-ordinate origin. The laws are invariant when we shift our time origin back and forth. Noether's ...


3

When one says that "kinetic energy is conserved in an elastic collision" that means that the total kinetic energy of the system of particles involved in the collision doesn't change. It does not mean that the kinetic energy of each particle is unchanged. For a two particle system, the kinetic energy of each will change, but the sum won't. Also, your ...


3

Not at first, but usually eventually. You should read the Wikipedia article on Annihilation, which is the name of the phenomenon of a particle's colliding with its anitparticle. Particles and antiparticles have opposite-signed, equal magnitude quantum numbers (such as spin and parity to name two). Collisions must conserve these quantum numbers, so the ...


3

To answer the question in your title - no, the kinetic energy of an electron is a function of its velocity, same as for any other particle. The charge of an electron is always $1.6\cdot 10^{-19}C$, and so the electron will pick up $1.6\cdot 10^{-19}J$ of energy for every Volt of accelerating potential. The key to answering the question (part d in the ...


3

There are two parts to this question: if the fans consume more power than the pump this depends mostly on the fan and pump, I don't have much of an answer here, but in any case the difference will be rather small relative to the total power consumed by the computer. if electrical components are more efficient at lower temperatures This is true, and ...


3

You're making this slightly more difficult than it needs to be with TW-h units, although there's nothing incorrect about the approach. Simply converting to averages makes it much nicer. To do this, divide your number by the number of hours in a day and the number of days in a year. You get around 17.1 TW. This is the average energy consumption of humans in ...


3

If the kinetic energy of both the ball decreases, how can their velocities be equal?? The front one ( B ),from the beginning, had low KE; if it decreases during the deformation, how can its velocity be equal to the velocity of the rear ball ( A )? In order to get a clear picture, let's consider the extreme case when the velocity of B = 0 Let's ...


3

The answer depends on many factors, but here are the basic bits of physics that play: The power and wavelength of the laser The reflectivity of the surface (function of wavelength of the laser) The size of the focal spot The thickness of the sheet The thermal conductivity of the sheet The reflectivity of the copper is a particularly important one. If you ...


3

The heat simply disperses and the further away from the heat source you get the more space the heat has to disperse into


2

The heat radiates away from the source equally in every direction (until it hits something). Imagine the sun. It is roughly spherical and radiates energy (some as heat) in all directions. The energy is radiated in a sphere. As the energy travels away from the source (the sun) the sphere expands but the amount of energy remains the same. When the sphere is ...



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