The h Bar - 2020-01-27

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5:41 AM

@JakeRose Your last integral is of the form $\int_{0}^\infty e^{-a (x-x_0)^2}\,dx$, which is very close to being a familiar integral

the last approximation to make is arguing that, under the conditions of interest, the difference between your integral and the familiar one is small enough that it may be neglected

3 hours later…

8:57 AM

morning

9:49 AM

0

Q: What happened to my account on Physics Stack Exchange?

Today when I clicked on Physics.Stackexchange tab on my chrome the site opened up normally but when I clicked on Ask the window the tab opened up saying “confirm your account” and then I saw that my account from P.SE was removed or something like that must have happened. I was suspended from ask...

bug user-accounts suspensions

2 hours later…

Novalium Company

11:23 AM

user image

12:15 PM

user image

12:51 PM

"A clock is defined to be any mechanism that induces an order preserving map from a given world-line to the real numbers"

1:11 PM

@JohnRennie thanks sir. That post of which mine was duplicate was really helpful.

What is the causal geometry of Minkowski space?

in this video it says it's like a hyperbolic saddle youtube.com/watch?v=1YFrISfN7jo

but I don't really understand

quotient of Minkowski space by its conformal group

@Slereah So are there clocks along CTCs? :P

@ACuriousMind No order along a CTC!

despite all the cartoons showing clocks when you travel back in time

so minkowski space is flat but it's causal geometry is not?

1:27 PM

I'm not watching that video but I would guess they are using a different meaning of the word

they said "causal geography"

How do they define it

user image

they don't really define it

they just give this image and don't really explain it well

I believe they just do it via the spacetime distance

$s^2 = x^2 - t^2$

Which is indeed the equation of a hyperbola

0

Q: Where did my edited Question go?

I had a question the was reviewed and now I can't locate it. It doesn't show up in my questions asked bin.

1:34 PM

okay I'll try to understand more

Yuvraj Singh...

Can anyone help me in highlighting the text copied from other sources. Like that yellow box.

this is probably easy, but when you use rectangular hyperbolas how do you write the metric?

I'm trying it on paper now

the metric is the same obviously, but the metric $s^2=x^2-t^2$ is in standard hyperbola form not rectangular form

would it just be $tx=s^2$?

oh no

since the axis are rotated it would not

@YuvrajSingh... Just put '>' before the text

@YuvrajSingh... If you ever want to learn from other posts, just click "edit" to see how the post is formatted. That is how I learn(ed) how to do a lot on this site.

1:51 PM

okay the metric is still $s^2=x^2-t^2$ because it remains invariant after a rotation

Yuvraj Singh...

@AaronStevens thanks!

@AaronStevens I have written an answer can you check that! Please.

Which answer

user image

Is this what it means by ‘equation’?

Yuvraj Singh...

2:09 PM

@AaronStevens you can see on the main site it is in top four for today..

2:49 PM

@JakeRose did you get that integral to work out

@Semiclassical ahh just seen your message. Gonna be reattempting later tonight. I can see the Gaussian, but not sure how to get the n under the square root from that fact?

pay attention to which Gaussian you get

it's not going to just be exp(-x^2/2)

1 hour later…

4:03 PM

where's ryan unger when you need him

4:23 PM

trying to decide which way of creating dilute solutions of known concentrations is better for an intro lab:

1) determine how much solute is needed for each desired concentration and in each case mix with water
2) create a solution of high concentration and dilute appropriately

based on the lab notes, it looks like it's the second.

I would think #2 teaches the most useful skills anyway, since things like acids and bases usually come high-octane concentrations and get added to water to dilute them down

for reference, this is a physics course. so it's a bit of a one-off

But I haven't taken a chemistry lab since... 2004. So maybe I have no concept of what is/is not useful.

yeah, I know what you mean

Yuvraj Singh...

@tpg2114 thanks sir!

4:29 PM

I'll check with other TAs and see what they're planning to do

Ugh someone just said this to me in an online tutoring session:

I am just trying to get through the class my major will never rely on physics. Personally it has no meaning to me. Even before we started this my teacher said if you are not majoring under physics all this will be useless to you

:/

This seriously makes me so upset

That person should not be teaching

@YuvrajSingh... For the edit? No problem. I was clearing out the comments and it was just easier to change it and get rid of them all

@AaronStevens Curiosity and problem solving are useful everywhere.

That's how I would respond to that

@tpg2114 I tried to tell them that

4:31 PM

What is their major?

They didn't say

Yuvraj Singh...

@tpg2114 I need some help regarding my answers!

I said While your major doesn't depend on the content of physics, you could still learn from it. I am sure in your own work you will need to understand the definitions of important concepts and how to apply them. How to take a problem, identify important concepts, and figure out how to use those concepts to come to an answer

Bummer. I bet you could take almost anything and ask them some questions related to what they are doing that would use some concepts

Except like... maybe literature

(this is intended as an osmosis lab: place potato slices in beakers containing various solute concentrations. if the solute concentration of the potato > solute concentration of solution, then water will flow from the solution to the potato to make the solute concentrations match. so the potato will take on mass.)

4:32 PM

@tpg2114 Yeah I should have tried. The session was already going long though, and it was obvious that they didn't really care for what I had to say

But even that, there's a whole world of great scifi out there

I blame the teacher

@AaronStevens It's like the people who hate math and just complain based on the super general idea "I'll never use this!" You might not even apply it exactly like you do in a math class, but that's quite different than never using it.

If they actually said that at the beginning of the class, then I can't blame them for having that mindset after going through problems like blocks on inclines

i mean if the person has no interest in physics, it's a bit silly they're required to take it

4:33 PM

@SirCumference At a lot of institutions everyone is required to take at least one lab science

learning the subject is a nontrivial amount of effort

Learning "physics" is nontrivial amount of effort, sure. But if you go into it learning "problem solving skills" then it is the same nontrivial amount of effort on the same material, but has payoffs later

@AaronStevens i know, just mumbling some complaints

@JMac Yeah... I just don't see why you wouldn't want to try and figure out how to get something out of it though. Like I don't study history, but if I needed to take a history class I would find a way to appreciate and learn new ways of thinking

Even if you aren't super excited about the particular problem being solved

@AaronStevens Even worse is when they are an undergrad and they are paying loads of money to take it and not get something out of it

4:35 PM

I am just more upset with the teacher than the student. I don't think it is the student's fault

@tpg2114 sure, though even then physics isn't the first subject i'd recommend if someone is unfamiliar with that kind of skill

well

they're paying loads of money to get a degree

Small question - What is the uncertainty in a 200 mL graduated beaker?

probably a (basic) math or comp sci course is an easier introduction to problem solving

@AaronStevens I had to take a whole years worth of history classes on the history of science and technology. I also took a course on Science Fiction in Film. Even when I thought "I won't need to know this"; I at least took the effort to try to take some interest in it... the way I saw it, the alternative of not caring and retaining nothing seemed more pointless...

But yeah, a teacher telling the students they wont need it amplifies the problem quite a bit, that's crazy.

4:37 PM

@JMac Yeah. During this session on each problem, in the middle of me trying to walk them through a line of reasoning they just said "I think the answer is this. That is what I am putting". And I said "Honestly, that isn't fully correct, and I would not give you full points if I was grading."

To which they said what I first quoted

The question was "Use physics to explain why air bags help prevent bodily harm during car accidents". They said "due to the force of the airbag and release of gas it help prevent a major bodily injury."

@AaronStevens they a freshman?

Which is basically just a restatement of the question

I am not sure

Ah, just got another session with them

that sounds like the kind of thinking of a freshman or a burnt out senior tbh

@AaronStevens It's especially crazy because they even have a (presumably hired) tutor and they still don't care. I had the attitude of "well screw it I'm just putting that down" with some homework; but that was typically when I already felt comfortable and just didn't want to do homework because it was worth peanuts in my actual grade.

Suggest they go watch episodes of Mythbusters -- they weren't terribly rigorous, but they made things interesting/exciting enough.

4:41 PM

@JMac I think the service is paid for by their instutition

@AaronStevens Ahh, that makes some sense. Still kinda weird for them to even bother if that's going to be their attitude (unless it's like mandatory).

@JMac Yeah idk. The other problem was about momentum conservation, and they kept just focusing on mass and not velocity

And I told them you needed both, but then they just said "I am going to put up the next question"

And said they needed to move on

So oh well

Well then. Must be a blast to tutor them.

you could advise them that the homeworks are practice for the exams

@SirCumference I tried that, but he said they don't have exams haha

4:46 PM

well then

Oh yikes, that's a strange program.

Student athlete by chance?

lack of exams pretty much cements the prof doesn't care

@Justin A beaker is not a volumetric instrument. It is not precisely calibrated and the scales serves only as approximate guide.

Yeah, I haven't heard of a physics class like that

2 hours later…

6:28 PM

It is called warp, not wrap. — Thomas Fritsch 5 mins ago

@rob You have never heard of a wrap drive?

The temptation to respond with "ducking autocorrect" is strong, but I know better

6:46 PM

- How would the torque about the axis of rotation differ from any coordinate axis other than the axis of rotation?

@JohanLiebert You need to be more specific by what you mean by 'differ'. In general you just know that they will be different

@AaronStevens do you think with some proper modification (either in this or a separate question) this part of the question might be answerable (as you say it's vague for now).

@JohanLiebert It is a tough one. You might need to specify a specific scenario, but just make sure it doesn't read like a homework / check-my-work problem

You might be able to answer your own question by actually doing that

Pick a simple system, and pick different axes to calculate torque about

Maybe you could start with a single particle falling near the Earth's surface

how does your choice of reference point influence the torque acting on the particle?

How does your choice of reference point influence the angular momentum of the particle as it falls?

Does it still hold true that the torque gives the rate of change of the angular momentum?

@AaronStevens I wanted to consider this qualitatively. Do you think it would be correct to say that the torque generated about an axis other than the axis of rotation is greater in magnitude?

@JohanLiebert Net torque?

6:52 PM

Yes

@JohanLiebert Maybe? I would have to do more thinking. For simple objects that just rotate about the same axis with torques always being parallel, perhaps. In general, I am not sure

@AaronStevens actually I wanted to qualitatively differentiate between an arbitrary axis and the axis of rotation, I thought about this for quite some time but couldn't get anywhere conclusive than this. Do you think I should modify my question to add this information?

@JohanLiebert I think a new question would be better. Your current question, at least to me, just seems to be asking if we can still use the definition of torque if we pick an axis that is not the axis of rotation

If you want to explore how that effects the analysis of a certain system, I think that should be a different question

But I would suggest picking a specific system to look at...

@AaronStevens OK then. I am going to delete this part of the question and ask a new one. Thanks for the help.

@JohanLiebert No problem. You seem to ask fairly insightful questions. Keep up the learning and good work :)

7:24 PM

0

Q: How to differentiate between the axis of rotation and an arbitrary axis?

I recently asked this question: What is meaning of torque about an axis other than the axis of rotation? In the previous form of the question I asked the following: How would the torque about the axis of rotation differ from any coordinate axis other than the axis of rotation? I discussed...

newtonian-mechanics rotational-dynamics coordinate-systems torque rotation

@AaronStevens what do you think of this one?

2 hours later…

Scientist Smith YT

9:01 PM

@Loong If I know the magnetic field strength of a yoke is 24.435 Tesla. How do I find the pulling or repelling strength based on the magnetic field measurement? I need to know this so I can weld a yoke together. And know if the iron plate thickness needed to saturate the magnetic field entirely will hold up against the force.

@Loong Or if I need a different material with more strength. The pull force\repelling force will tell me what I need to know

@Loong I'm making the magnetic field with permanent magnets then a cone on the top of the magnets to make the magnetic field intensity greater.

9:22 PM

@JohanLiebert What differences are you looking for? What do you mean by "how will they differ"?

0

Q: Confused over site policy regarding similar questions

I asked this question last night and it was closed as a duplicate earlier today, because it is similar to another question (this one). However, the (apparently automatically generated) closure note states the following: If this question doesn’t resolve your question, ask a new one. As far ...

10:04 PM

24 Tesla? That’s about as strong as a very heavy MRI machine. @ScientistSmithYT

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