stupid physics questions

[Dio]

Chimneys aren't closed systems. They are open to the atmosphere at the bottom, the hot gas feed is typically higher than the point at which cold feed air is taken. The bottom cold feed point is indeed at higher pressure than the top of the chimney and that pushes the air up it.

If a very strong updraft is a requirement the chimney bulges out after a constriction, which further expands the rising air and generates a higher pressure differential. Hence the classic shape of cooling towers.

[gumboot]

If a very strong updraft is a requirement the chimney bulges out after a constriction, which further expands the rising air and generates a higher pressure differential.

Not sure I can go along with that. In any other system a flare like that would be described either in terms of slowing the gas down (actually increasing its pressure, by Venturi), or it would be described as an impedance matching horn. In either case, it would be helping to maintain high overall flow rate by avoiding turbulence at the outlet.

Hence the classic shape of cooling towers.

And chimeneas.

[Dio]

Cooling towers - the hyperboloid shape aids in accelerating upward airflow.

The reason is different to the one I thought though, I forgot that C-D nozzles only operate if the throat flow is choked.

[gumboot]

Cooling towers - the hyperboloid shape aids in accelerating upward airflow.

The reason is different to the one I thought though, I forgot that C-D nozzles only operate if the throat flow is choked.

I think choking implies (not necessarily, but practically) that's a gas that has been compressed by the limits of the speed of sound against the flow rate, so it's unsurprising that it expands on its way out. I see the nozzle divergence as an impedance-matching and beam-forming matter, rather than promoting expansion itself -- as it will eventually have to expand no matter what shape beyond the throat.

[Portland]

Can't remember if the "too hard" Scottish exam earlier this year was posted on this thread, but this one didn't seem too bad for a Higher (A level equivalent)

[Dio]

Yeah, looks fine at A-level to me.

[Portland]

Easy formula plugging to get two end points and bit of calculus to get the optimum...

[gumboot]

I've forgotten the identities I need to complete the calculus. Would it come with a cheat sheet?

[pogo]

Quite a nice question. You don‘t need to derive the formula, you just need to understand it and plug in a couple of values. Then do some differentiation to find the minimum, but that is straightforward calculus using techniques that are presumably on the syllabus. Not really physics, more a very practical maths question.

[Dio]

It's related to an unspoken physical question in that least-travel-time analysis is useful in several areas of physics, particularly optics.

[gumboot]

Also, in navigating through shopping malls.

[Mayhem]

Not a very hard question really, only the final result of the derivation I couldn't quite remember the rules for to get correct. Then again, not done that sort of thing in 20 years since at university! If kids are struggling with the abstract concept of applying calculus and algebra to a "real life" problem (heh, crocodiles), then how are they going to fair in the real world?! Life and work isn't applying what you know and resolving things that are already known how to do, it's about applying them to best work out things that are not known. Do we chalk this through as another example of kids being taught how to pass the structured nature of exams rather than how to use what they've been taught?

[Portland]

Not a very hard question really, only the final result of the derivation I couldn't quite remember the rules for to get correct. Then again, not done that sort of thing in 20 years since at university! If kids are struggling with the abstract concept of applying calculus and algebra to a "real life" problem (heh, crocodiles), then how are they going to fair in the real world?! Life and work isn't applying what you know and resolving things that are already known how to do, it's about applying them to best work out things that are not known. Do we chalk this through as another example of kids being taught how to pass the structured nature of exams rather than how to use what they've been taught?

Possibly this, it's the same with people that have done a "stats course" they often don't know what sort of regression/distribution etc to use to make sense of a data set.

Last "too hard" question was also a pretty simple concept, but in a "real world" problem. My teachers always liked to show typical applications for things, may not be the case any more...

[Absolute_Zero]

Here will do... with possibly greater potential than graphene.

http://phys.org/news/2015-11-phase-carb ... ature.html

[Darkflame]

Heres one stackexchange didn't help much with;

http://physics.stackexchange.com/questi ... y-mutually