@PM2Ring I think I'd argue that what you're doing is assigning a temperature to whatever the photons are in equilibrium with. I know it comes down to the same thing in the end, but you need to be careful about assigning statistical properties to photons.
@Akash.B light doesn't interact with itself, so you cannot affect photons by shining laser beams at them.
It is possible to bring photon to rest by making them interact with certain types of matter.
Though strictly speaking when a photon interacts with matter it is no longer just a photon. The photon wavefunction becomes entangled with the matter wavefunction.
@JohnRennie I totally agree. But pretending that the photons in an oven are in thermal equilibrium with oven walls does lead to interesting & useful results.
That's why the photons slow down. The photon/matter entangled system behaves as a quasiparticle that has a non-zero mass. This particle is called a polariton.
What is the way to develop calculus in theoretical physics articles?
Most of the time, we find in our way when reading articles expressions that had been put by the author(s) without expansion and I wonder how to prove such expressions?!
@Student404Mus The author assumes that if you're reading their article then you have the necessary physics & mathematics pre-requisite knowledge to follow their text & their equations. But it's frustrating when you stumble on an article with an interesting sounding title and you don't know what those pre-requisites are. Or you kind of follow what they're saying, but their notation is unfamiliar, so you're unsure how to interpret their equations.
Suppose you apply a small impulse at a distance $r$ from the centre of mass of the body. (Impulse is force times time, and it's equal to change in momentum).
The linear momentum must increase. Since momentum is equal to $mv$ and the mass is constant the linear velocity must increase my an amount $dv$ given by $Fdt = mdv$.
Now for angular motion we have an angular impulse that is torque times time. And this is equal to the change in angular momentum. Since we apply the force at a distance $r$ from the centre of mass the torque is $Fr$, so we get an increase in the angular velocity given by: $Frdt = I d\omega$.
So the force both accelerates the object and increases its rotation rate.
Hi, I have a question I cannot figure out: You wind a string around the outer rim of a solid disc with a radius of 10 cm and a mass of 2.8 kg. The disc sits at on a frictionless horizontal surface. You pull on the string with a force of 10 N for 3 s. What is the final kinetic energy of the disc? This includes both translational and rotational kinetic energy.
I have tried to simply do K = 1/2mv^2 + 1/2I(\omega)^2 but numerically I'm getting half of the correct answer. The moment of inertia for a solid disc, as far as I know, is 1/2mr^2 about the z-axis. Is this not correct?
@user8718165 there, I answered just this type of question this morning
@user8718165 yes, but the way to solve it is to calculate the linear velocity $v$ and angular velocity $\omega$, then the KE is $\tfrac{1}{2}mv^2 + \tfrac{1}{2}I\omega^2$.
What exactly expands when we say the universe is expanding? Spacetime? The magical fabric that makes space? Why aren't we expanding as well? (our bodies, eyes...)
@ACuriousMind dammit, I was about to post the exact same link, I'm blaming you for letting me waste those 20 seconds searching (and an additional 40 seconds making a rant)
I'm reading A Brief History of Time and there is one more thing I dont understand called light cone. I understand it describes the delay of the light reaching and so on but how?
@EmilioPisanty I actually upvoted your question and answer for this one > physics.stackexchange.com/questions/100864/… because a different answer came into a queue, so you can take a bit off the upvote chain as legitimate upvotes. I got hit with what looked like some serial votes last week too
@EmilioPisanty Not me, although I do tend to upvote your answers when I bump into them. If I understand the topic well enough to know that you're right. ;)
Yeah that one's a lot worse than last week for me. I got hit with 3 in a row on all my top rated answers though. Two of them within 6 seconds of each other. Seems like whoever did mine isn't the same as yours though; since they clearly don't care about having the votes reversed
I guess Floris earned the bulk of his rep over ~four years, while Qmechanic's is spread over twice as long, and that tips it over into a vastly reduced number of days at the cap
@EmilioPisanty Qmechanic only has 8 max rep days!? That's pretty wild, it would take a very careful balance of consistent quality contributions, while somehow not having many answers hit like HNQ and stuff. I have less than 10% of his rep, and 11 full rep days, but that's because I pretty much only answer classical physics questions, which are usually easier to reach a broad audience quickly
@JMac indeed, it would take a substantial amount of extremely high-quality contributions spread out over a very long period, making sure that you're answering every day so that they're evenly spread and they don't bunch up
Well what gets me is he somehow managed to do that while still avoiding getting into HNQ one way or another enough times to just start piling up rep caps. Gotta be a strange mix of the types of questions he answers and some pure chance sprinkled in.
Plus he doesn't seem to give answers as easily digestible by the broader audience; I think that's a big part of it. Like all the people with the most rep cap days seem to be the ones who answers questions in a more qualitative way (while still trying to be accurate). Qmech seems to take a more technical approach, which is a lot harder for drive by users to read and go "yeah that makes sense I'm going to upvote".
@EmilioPisanty : Thx, Floris collected all his rep in just 3.5 years. If he becomes active again he could presumably easily get back in front :) Anyway, we're not really here for the rep. We're here for the physics!
@OliverWatkins: In this community, you gotta make the rep first. Then when you get the rep, you get the power. Then when you get the power, then you get the women. — PM 2RingOct 27 '14 at 10:16
In case it's not obvious, that comment is a parody of a quote from a movie (and a rap song, which samples the movie). If ROs or mods think it's in poor taste, please remove it.
