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Tesladownunder

Mon Nov 06 2006, 06:24PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

It's Bismuth. That's how it crystallises with a pink/green sheen. It was not taken upside down, just lit from an angle that showed the colours the best.

Mystery photo 50

Electroholic

Mon Nov 06 2006, 07:25PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

project for 2nd year civil engineering?

Tesladownunder

Tue Nov 07 2006, 12:23AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

This was a school project (year 8) for my 13 year old. They each had to construct a "bridge" to span 80cm to support a weight using 100 kebab skewers, wood glue and cotton.
Picture shows it supporting 2 bricks (10kg) In the competition it supported 17kg with more distributed weights but the record was 23kg for the class.

Mystery photo 52 follows. The correct answer may need to give some figures for physical properties and some calcuations.

Peter

1162859014 10 FT10696 Mystery 50 Skewerbridgebricks

ragnar

Tue Nov 07 2006, 01:27AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Wee, bridges. A while ago when I was in year11 doing engineering studies, for a class assessment we had to construct a bridge and model a truss analysis. Mine was made from quarter-inch cross-section balsa wood, and weighed 24 grams. It spanned 40cm and, when tested to destruction (add weights, wait 1 minute, add more, etc) held 39kg. = ~1600x its own weight.

Yay for K-trusses and more cheap thrills. =)

Nik

Tue Nov 07 2006, 01:49AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

The photo is of a tuneign fork that should resonate at 256hz but you dipped it in liquid nitrogen making it resonate at 260.6 hz. Also the coil it is vibrating over looks like it came from a small motor (posibly a microwave turn table).

Tesladownunder

Tue Nov 07 2006, 02:21AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Correct, but can anyone work out how much stainless steel shortens with temperature and how much the bendy-ness (there is a better term for this) changes with temperatue to get an estimate of current temperature from the increase of resonant frequency?

Peter

Electroholic

Tue Nov 07 2006, 03:14PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

i think it highly depends on what kind of steel you have, although it is likely to be tempered high carbon steel.

Bored Chemist

Tue Nov 07 2006, 06:07PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Well, if anyone who really likes maths is interested, this site

gives the physics for a single cantilever. A tuning fork is just 2 stuck together and then you need to convert the expression for the static load into the expression for the motion and get the resonance (I bet it's density dependent too) then see what effect the temperature has on the length, density, and Young's modulus of the steel.

(Of course all that requires you to register with the site's operators (It's free), so I bet nobody bothers to get the explicit equation for the frequency of a tuning fork.)
Alternatively, you could just say "forget it- even dropping the temperature bu 200 degrees barely affects the resonant frequency".
If anyone is still reading at this point they might want to google "quartz crystal microbalances".

Tesladownunder

Tue Nov 07 2006, 06:26PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Looks like I'll have to work it out myself.....

Simple coefficient of expansion of steel would account for shortening of 0.000012/K for steel. Given room temp 24C and LN2 of -196C this gives a 220K range. Hence thermal expansion is .00264. This would on its own change frequency from 256 Hz to 256.67Hz. Clearly not the full explanation. Perhaps not even any explanation as shortening will be acompanied by increased density which may compensate ? fully.

So looking at the bendy-ness of steel. I guess this is Young's modulus and I suspect that this is the parameter that determines the vibration for a given force. The exact value is not important but the change with temperature is. This site gives the modulus for low carbon steel as being 29.5 at 70F increasing to 31.4 at -325F (=-198K). This is a change of 6.4% which would give a frequency rise to 272.4Hz.

Since the observed frequency rise is only to 260.6 from 256.2 (1.7%) and interpolating, this suggests that the temperature of the fork at the time has risen to -34C.

There are many assumptions, particularly about the steel type. Perhaps my logic is way off.
Nevertheless the results are plausible. Using Google to look up tuning fork and liquid nitrogen reveals many poorly documented brief physics demonstrations. The only one that gave figures was my own site!

Comments welcome from real engineers or people who know stuff.
(Edit looks like Bored Chemist just beat me to post about this)

Next Mystery photo - also an easy one.

Peter

Wolfram

Tue Nov 07 2006, 09:29PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Mercury switch frozen with (presumably) liquid nitrogen.

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