The h Bar

M.N.Raia

1:42 AM

Good morning/afternoon/night for everyone!

Hmm, I know that this chat isn't the place for that, but is there any function on this site which allows you to save some question which you find interesting?

danielunderwood

2:00 AM

You can star them if you don't mind it being public. They'll be in your profile under the (hard to find) favorites tab

dmckee

2:28 AM

I was also going to mention the favorites feature. It is not, however, particularly featureful: no indexing or organization, just a pile of links.

danielunderwood

3:26 AM

Is there an ideal solution? Just bookmarks?

Secret

3:54 AM

11 hours ago, by John Rennie

The Earth is an oblate spheroid precisely because the mantle flowed from the poles to the equator.

That is a naive sounding question that turns out to be quite profound. I really underestimate where profoundness can pop up

But then, with so much actual dead end questions out there, is there a good way to hunt for the hidden profound ones

David Z

4:52 AM

@M.N.Raia That's a perfectly fine thing to ask in chat, by the way.

Emilio Pisanty

7:17 AM

@Danu have you discussed this with him?

Akash. B

9:20 AM

my answer?

John Rennie

9:32 AM

@Akash.B answer to what?

Danu

@EmilioPisanty What? That I know you on PSE? I wouldn't call it "discussed" but yeah he's the one that mentioned it to me actually, when we saw each other for the first time in a while some time ago, and PSE came up somehow.

Akash. B

@JohnRennie the question that i asked yesterday

John Rennie

@Akash.B start by considering the Earth as a perfect sphere and not rotating. The gravitational force always point towards the centre of the Earth.

Akash. B

@JohnRennie okay

Anonymous

@DanielSank superconducting-quantum-computing What is the charging energy in relation to a superconducting transmon?

John Rennie

9:36 AM

@Akash.B so the force on the ball is always normal to the surface i.e. there is no sideways force on the ball so it won't roll sideways.

Akash. B

@JohnRennie oh i see

I was burning my head too much

dmckee

2:02 PM

@Secret High scores are a hint, but no guarantee. For more technical questions 10+ is "high".

PM 2Ring

2:12 PM

@EmilioPisanty Party pooper! ;) Seriously though, I can't complain. I got 2 days of rep cap from that question. Originally, I posted a 1 sentence comment + a Wikipedia link to the Equation of Time, but I figured I ought to write a proper answer, since the mods tend to harass me for answering in comments. :)

Lucas Henrique

2:30 PM

How can I claim that 3 bodies, initially in contact, upon action of a horizontal, constant force, will stay together with the same acceleration?

Assuming no dissipating force (non-native speaker, looks wrong)

I came from Math.SE and I’m used to formal stuff, but I can’t seem to deduce the solution (description of the position of each body) only with Newton’s Laws.

I tried to assume general facts like “normal $\leq$ external force”, and that the positions (I assumed they’re in the same line) are increasing, always; otherwise they’d have collided and broken.

Professor told me something about Jacobi coordinates but I know nothing about it nor I think it’d be necessary.

dmckee

@LucasHenrique Pushed in line with the external force? You set up Newton's 2nd law for each body including inter-body normal force (which are equal and opposite by the 3rd law).

That gives you a system of three equations with three unknowns (common acceleration $a$, and two inter-body forces $N_1$ and $N_2$)., which solves in a straight forward manner.

Though few books emphasize it the whole point of freebody diagrams is helping you to write such systems of equations so that the physics problem can be reduced to a math problem.

Lucas Henrique

My problem is the supposition that the accelerations are the same.

The bodies are initially in contact but I can’t see why they’d keep this contact.

For example, the equations could (absurdly) tell us that the first body goes backwards (normal > external force) and the other two go forward.

dmckee

2:48 PM

What would cause the one in the front to accelerate more than the others? You need a force for that, and it can't be the normal force because that relies on contact (unless you are willing to admit dynamic compression to get spring-like behavior but then you are in a much harder regime).

PM 2Ring

@LucasHenrique Generally, they won't stay collinear. Sure, if you have 3 square blocks, it's easy. But try it with 3 balls. ;)

dmckee

@LucasHenrique Normal forces are (in the physics 101 understanding) constraint forces. They take on exactly the value needed to enforce a rule. For normal forces the rule is "material object don't co-occupy the same space".

Lucas Henrique

It seems that these constraints to the problem do always rely on some kind of intuition.

dmckee

I'm still working on my wording for that rule. It really applies for one or more solid objects, but of course gases and liquids can mix.

@LucasHenrique You can define the deformation behavior of solids and observe what is going on in detail, but you that is a much harder computational realm.

Lucas Henrique

Prof. told me those are bond equations and they can’t really be deduced generally. They always rely on your intuition on the problem.

dmckee

2:52 PM

Still I would not call this a "intuitive" theory, it is an effective theory in which the Hookian deformation of sufficiently rigid bodies is abosrbed into a simpler set of rules.

Lucas Henrique

I was sad to know differential equations but not high school physics (not literally, because I knew how to solve it, but not how to formally deduce it). It’s not a “linear complexity” problem and I’m fine with it. Thank you all :)

Emilio Pisanty

3:25 PM

@PM2Ring Serious question: do you think that the question deserves to be at score +24? do you think that it would achieve any meaningful score (say, above net score 5) from the core site community alone?

Lozansky

3:38 PM

How do I go about finding the distance $d_{hkl}$ between $(hkl)$ planes in a hcp structure?

Ah

It is the same as the distance from origin to the $(hkl)$ plane in the reciprocal lattice space?

It is

Still seems difficult:((

PM 2Ring

@EmilioPisanty No. And I agree it lacks sufficient prior research, which is why I originally intended to just leave a Wiki link comment. But then I figured that many people these days don't know about the equation of time, and a short answer about it may be useful & interesting.

Secret

5:08 PM

5

Q: Can a single, larger black hole be split into multiple smaller black holes?

Essentially, my question is, can one larger black hole be split into a system of smaller black holes? I'm assuming energy requirements will be prohibitive, and that it may not be possible without exotic matter with negative mass. Can, if energy requirements are not a problem, and exotic matter i...

27

Q: Is there a way to split a black hole?

Classically, black holes can merge, becoming a single black hole with an horizon area greater than the sum of both merged components. Is it thermodynamically / statistically possible to split a black hole in multiple black holes? If the sum of the areas of the product black holes would exceed th...

thermodynamics general-relativity black-holes entropy black-hole-thermodynamics

Surprisingly interesting question. It seems that even if you pump a lot of energy, a splitting may not occur

We don't tend to think of blackholes as "chunky" like lego blocks though, but perhaps that will change as LIGO continue to bring more insights into them

PM 2Ring

5:39 PM

Coincidentally, a few hours ago, this question got me wondering if it's possible to split a neutron star. Of course, trying to blow it apart with a nuke is about as likely to succeed as attacking it with a plastic spoon. But maybe if you had a young neutron star with high spin you could increase its spin even further by accretion.

But could that disrupt it, or would it take so much energy that it'd just collapse into a black hole? On a related note, it's suspected that in some magnetars, the interaction between spin & magnetic field distorts the shape into a prolate spheroid, which is pretty weird, but IIRC, the ratio between the polar & equatorial radii isn't much greater than 1.

Physics Meta

6:17 PM

0

Q: Should a rude comment be deleted if the addressed user doesn't mind?

On the linked post, a comment was made addressing me with words that might be considered mildly rude by some. I had absolutely no issues with the comment (and thus, I didn't flag it). And yet, the comment has been deleted--it might have been deleted by the commentator themselves but I suspect the...

discussion comments moderation best-practices

thermomagnetic condensed boson

6:57 PM

ok I have another QM question about the wavefunction

say that you see a system and write down its wavefunction as a superposition of eigenstates. then you make 1 measurement. my question is whether the wavefunction necessarily collapses into a single eigenstate (I guess not) or whether it can be modified but still in a superposition of eigenstates (I believe so)

?

Sito

7:38 PM

Interested in some opinions concerning the following post on academia SE related to physics/career in physics.
https://academia.stackexchange.com/questions/112512/the-utility-of-graduate-math-study-for-theoretical-physics
Anyone familiar with categories (2) and (3) here and willing to give some advice?

Curio

8:02 PM

Is deltaG of N2 <0?

thermomagnetic condensed boson

8:27 PM

nevermind i got the answer to my own question at physics.stackexchange.com/questions/359465/… great!!! exactly what I thought.

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