What are your favourite physics/mathematics misconceptions?

[DaveE]

That reminds me, here's a test to see whether you're a physicist or a mathematician at heart...
if $f(x,y) = xy$, what is $f(r,\theta)$?

Exactly. The difference between mathematics and physics is not so much in what is taught, and even less in the instruments, they use. The main difference is simply that it is another language.

Yes, a perfect illustration. Once again the Mathematicians are correct, exactly, precisely correct. But the people that need to use the math to solve problems leave out the tedious rigour only retaining the essence that is useful and tailored to their particular field. Then they add in, but don't talk about, some jargon or assumptions that are always made by their peers. Which is why none of us find it easy (or possible) to read the papers from the "other guys". I think it's actually a pretty efficient system. There are good reasons that people take shortcuts, you know, the good ones anyway.

This is often what leads to misconceptions. Someone threw out some useful stuff because it was easier that way. Like me; I do this all the time.

 

[vela]

Favorite misconception: The short version of mathematics is maths.

 

[vela]

Actually, I was going to go with favorite doesn't have a u in it.

 

[Eclair_de_XII]

thinking too hard

Oh, I get it. In the case of $\tilde{\mathbf{f}}$, you use the substitution $x=r\cos\theta,y=r\sin\theta$, but in the case of $\mathbf{f}$, you just substitute $x=r,y=\theta$ without making any allusions whatsoever to polar coordinates.

 

[fresh_42]

Actually, I was going to go with favorite doesn't have a u in it.

My spell checker is set on "American". You cannot imagine how many "u"s I've deleted thru the years. I'd say there is a u in it, at least as long as you don't clean up the more important mess. Otherwise, we may start calling it Pittsborough.

And here is my legitimation.

Favorite misconception: The short version of mathematics is maths.

You mean there are no shortcuts to mathematics?

 

[jedishrfu]

Interesting that for the f(x,y)=xy question, when I saw the r, $\theta$ I immediately and implicitly thought polar coordinates as that was our training. Certain variable names x,y,z,t or $r,\theta,\phi$ or m,a,f,e, … all had physical meanings in physics.

Whereas a mathematician may see patterns beyond the meanings.

 

[Jarvis323]

Possibly interesting reading...
https://bridge.math.oregonstate.edu/papers/bridge.pdf

Bridging the Gap between
Mathematics and the Physical Sciences
Tevian Dray and Corinne A. Manogue

How about $f(\pi, \tau)$? I think some will assume $\pi$ is just a symbol.

 

[OCR]

"Farcebook?"

Well, of course. . . . Damn, I now feel so foolish. . .

.

 

[valenumr]

Exchanging the roles of $\phi$ and $\theta$ in my use of spherical coordinates was a seminal moment in my transition from amateur hack mathematician to amateur hack physicist

So wait, is the spherical coordinate system I learned in engineering calculus different than what physicists use? We learned $\rho$ $\phi$ $\theta$.

 

[Office_Shredder]

This is madness. The x-y plane is defined by $r$ and $\theta$, adding a third dimension doesn't change that. If you switch polar coordinates to $r$ and $\phi$ then I have no problem with this convention.

 

[valenumr]

This is madness. The x-y plane is defined by $r$ and $\theta$, adding a third dimension doesn't change that. If you switch polar coordinates to $r$ and $\phi$ then I have no problem with this convention.

Well, what about the order of evaluation? I learned it a certain way, and have never thought differently about it. But there is certainly more than one way to skin a cat.

 

[rsk]

As a high school teacher you get to hear loads of these. (Although I teach Physics to the 16-18 yr olds I'm also sometimes required to teach general science lower down the school)

Random question from Y8 (prob G7 to many of you) at the end of a lesson on the heart and circulation: "What's the second fastest thing in the world miss after light?". While I'm pondering what to say, another one pipes up: "Miss I've heard that there are things that go faster than light, they're called quarks and they're the smallest thing you can see in a microscope". And while I'm wondering which misconception to correct first, he goes on ..."and I heard that the male testicles can squeeze the quarks and shoot them really fast so they go so many times around the body".

I went home to drink wine.

 

[DaveE]

Well, what about the order of evaluation?

I prefer whichever way makes the solution easier. Mind that you do have to do it correctly. But you really ought to be comfortable with multiple ways so you can choose the best.

 

[jedishrfu]

@rsk looks like your kids were trolling you.

Also it looks like you lived through that popular American jack-in-the-box burger commercial where the son asks a profound question to his dad and as the father is partway through the answer the kid switches topics asks another completely different profound question.

 

[rsk]

Another one from me. You'll think I'm making it up, but I blame the IPC.

These ones were Grade 6 in a small private international school (Y7 in UK) - but I suspect that some of them were a bit younger than they should have been. Anyway, we were talking about types of energy. We get energy from food, that's chemical energy. What kind of energy do we turn it into? Movement energy. One little girl pipes up: But miss, what about skeletons?
Holy crap. I can't even remember what I said to that (she wasn't joking) but it didn't end there. Later another one said:"Miss you know sometimes when someone dies and they put them in their tomb or whatever and they come back to life again? How does that happen?"

I repeat. I blame the IPC.

 

[fresh_42]

"Miss you know sometimes when someone dies and they put them in their tomb or whatever and they come back to life again? How does that happen?"

This only happens to famous skeletons, and only if someone who is alive starts it!

Spoiler

Newton would rotate in his grave.

 

[berkeman]

I repeat. I blame the IPC.

Sorry, what's that? (maybe the first one on the list below?)

https://acronyms.thefreedictionary.com/IPC

 

[fresh_42]

Sorry, what's that? (maybe the first one on the list below?)

https://acronyms.thefreedictionary.com/IPC
View attachment 293509

I had the same question. But then I decided that International Plumbing Code perfectly fits.

 

[rsk]

Plumbing code might have served them better.

It's the International Primary Curriculum. I'm not sure whether it's really to blame or whether it's just coincidence, but the schools (most international schools are 3-18 all through) I've worked at where it's been implemented have invariably been those which describe their eduation as 'innovative' and who are bigger on gimmicks than on substance.

The kids arriving in secondary from those systems have been noticeably less well prepared, both in terms of knowledge and in terms of thinking/reasoning skills. They can make a mean video on an ipad though and always with that same background music.

We are spared some of that nonsense in secondary because of the need for qualifications to access university places, yet still I've known more than one HT who spoke about ditching formal qualifications in favour of the more 'innovative' approach they claimed worked so well in primary. Those HTs just happened to be HTs in the two schools I mention where these shocking misconceptions arose and where the IPC was the system in primary.

 

[hutchphd]

Back at the main subject: For a long time (before I thought about it ) I was worried that shaking a bottle of soda or home-brew would raise the pressure and cause it to explode. Simple physics but it absent thought it seems reasonable....

 

[fresh_42]

As a kid, I thought we could get rid of our nuclear waste by shooting it into the sun only using the sun's gravity. Now I know, that this plan has a little momentum problem. But on the other hand, I have a new idea: why not use the waste for generating electric energy for a small magnetic field so that the radiation can be turned into an ion thruster?

 

[ergospherical]

Better yet - if you have a Kerr black hole handy...

Source: MTW.

 

[Frabjous]

As a kid, I thought we could get rid of our nuclear waste by shooting it into the sun only using the sun's gravity. Now I know, that this plan has a little momentum problem. But on the other hand, I have a new idea: why not use the waste for generating electric energy for a small magnetic field so that the radiation can be turned into an ion thruster?

I actually saw a presentation about launching waste into orbit in the late nineties. If memory serves, putting the stuff in an orbit inside of Venus is “reasonable.” The big problem is Earth’s gravity well. Large amounts in a single launch has safety problems in case things go wrong. The proposal I saw proposed launching small volumes to get around this. Another issues is getting the material to a launch site. A lot of the waste is a sludge in decaying barrels. The total amount is extremely large in the context of moving or “processing” it for launch.

 

[jedishrfu]

This only happens to famous skeletons, and only if someone who is alive starts it!

Spoiler

Newton would rotate in his grave.

What kind of angular momentum would he have?

 

[fresh_42]

What kind of angular momentum would he have?

About 2 Nms.