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Blue whales, tennis courts and other non-SI units

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Dr. Slack

Fri Mar 02 2018, 10:04AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Hey, no problem. Thanks for the apology. Even with your name, I hadn't realised that language may be a consideration, take that as a compliment on the quality of your English. That criticism of your grammar/spelling was pretty cheap of me, and for which I apologise.

This is an interesting reference from the wikipedia page, reference #8

The National Standard of Canada, CAN/CSA-Z234.1-89 Canadian Metric Practice Guide, January 1989:

5.7.3 Considerable confusion exists in the use of the term "weight." In commercial and everyday use, the term "weight" nearly always means mass. In science and technology "weight" has primarily meant a force due to gravity. In scientific and technical work, the term "weight" should be replaced by the term "mass" or "force," depending on the application.

5.7.4 The use of the verb "to weigh" meaning "to determine the mass of," e.g., "I weighed this object and determined its mass to be 5 kg," is correct.

AndreiRS

Fri Mar 02 2018, 02:15PM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

I don't really got the idea of the first post. Maybe because english is not my first language... You mean the people who uses "everyday objects" to compare size and these things? Here people love to use the area unit called football fields. For height they use eiffel towers stack. Weight is measured in whales or cars. The biggest confusion comes with gun calibers in the news, since they don't even imagine that .38 means a fraction of an inch. Then sometimes they say 38mm. Can you imagine a revolver with a 38mm bullet. They even call 380mm caliber sometimes. Which is a very big gun to carry around.

DerAlbi

Mon Mar 05 2018, 05:51PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Sorry for the late response, i am currently doing a major overhaul of my work space. I wanted to get back to the original topic. Although i just quit my job at the university, being a teacher there wasnt only a job, but a passion.
Given the english fuck up with the word "weight" (scientific vs every-day use) one could now ague again (with confidence this time) if explaining forces to someone would benefit from removing this word from such a context. But this you already stated in your quotes. Maybe teaching about "force" outside the context of gravity is much more fruitful since everyone can relate to acceleration in some way (car, elevator, ...) this would be better.
Specially when talking about blue whales, one could argue, since its lives basically weightless, its "weight" is not intuitive at all ^_^ This brings me to another point: i think equivalences are nice for a start, but actually using them as thought model can become complicated. I dont think thats a major issue, but it might be one after all: i lived through a money system change (to EURO) and i can observe a lot of people even after >15 years who still relate to prices in the old currency so they are constantly converting back. Relating to Forces with masses in gravity can seed a bad thought pattern.

Talking about energy is very complicated and tricky and a topic on it own. The problem here is, that energy is not a thing but ONLY a mathematical model or a relation. Nothing has energy; its a mathematical transfer helper or whatever you call it. You cant define energy (the same problem have "fields"), this is why there are a lot of energy types such as heat, electro static, magnetic, potential, kinetic and actually.. well and Einstein added "mass" to this list

While every energy is different, they are all the same and can be converted.
Thinking about energy as a thing is somewhat wrong, so assigning equivalences related to objects is even worse ^_Â°. Also one should consider if one chooses to teach with lossy energies like friction heat, or energies that can convert back and forth like potential <-> kinetic in a pendulum. I think specially in the non-recoverable energy conversions should be delayed until the concept of energy conservation is understood.
Recoverable energy might be "One Joule can lift 1kg of mass 10cm up." and relating to the heat capacity of water "like 4.2J can heat up 1g of water 1K." Why the first equivalence is easy for smaller energies, the water-equivalence is easier for higher energies like "you could boil x amount of water with the heat dissipated of the breaks of a train when stopping from 100 km/h". The interesting part about the heat energy is that it is somewhat unrecoverable, but since you have warm water to wash your hands with in the end, you kind of think about it using this energy again ^_^ Very strange. [and yes, i know one can re-use heat energy with special machines that actually have over 100% efficiency - strange enough]

Again, its always better to know more about the thing one teaches as its necessary, so consider watching

more than once

Btw: i have now installed a spell checker in my firefox. I had one in Chrome and i had better text quality. It always bothered me that the german distribution of firefox has no spell checkers buit-in due to copyright reasons.. thats now fixed

I hope my text hurt less now.

AndreiRS

Tue Mar 06 2018, 03:30AM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

I like all that text but... How can a machine have over 100% efficiency? That is just perpetual motion which cause perpetual discussion lol...

Bjørn

Tue Mar 06 2018, 09:22AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

A machine can have more than 100% efficiency but a system as a whole can't. So a heat pump that uses 100W of power can give off 400W of heat at one end. If you include the whole machine and the environment it is in then no energy has been created.

AndreiRS

Tue Mar 06 2018, 03:40PM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

Yes but it is a pump. I don't think that shoul be read as over 100%.

If I have a truck loaded with 10 tons of charged lithium batteries and move them 100 meters downhill while using an alternator as brake, these refrigeration companies would say it is a billion % efficient. But it is not. It just moved the energy somewhere else. It didn't create it.

DerAlbi

Tue Mar 06 2018, 06:12PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

You are arguing with the argument of conservation of energy. Think about it: it is always true. How then, can anything have _less_ than 100% efficiency.
While conservation of energy is a law of nature, efficiency is a man made engineering definition.
Consider my coilgun: its efficiency is defined by me (and others that agree) as KineticEnergy / ElectricalEnergy. Of course there is heat generated - thats exactly what i consider as loss.
Now in a refrigerator you have Eff = HeatOutput / PowerInput. This is lager than 100% because we consider the Heat output as gain - in winter at least ^_^ .

AndreiRS

Tue Mar 06 2018, 08:11PM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

Ok I got it from this point, the way this term is used is what doesn't sink. Because even knowing it will move a lot more energy than it uses, the electric bill only increase. I think we should only use over 100% if the air system was throwing back energy into the line which is impossible. At least giving more than it is using is. But thats seems to be a semantic problem I'm having.

Uspring

Thu Mar 08 2018, 11:26AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

DerAlbi wrote:

The problem here is, that energy is not a thing but ONLY a mathematical model or a relation. Nothing has energy; its a mathematical transfer helper or whatever you call it. You cant define energy (the same problem have "fields"), this is why there are a lot of energy types such as heat, electro static, magnetic, potential, kinetic and actually.. well and Einstein added "mass" to this list While every energy is different, they are all the same and can be converted.

I agree with most of this, but I wouldn't go as far as energy not being definable. It's a value in the mathematical description of a physical system. In classical mechanics it is the sum of kinetic and potential energy, in quantum mechanics the Hamiltonian operator.

Classical mechanics is not a complete theory, so if you want to add e.g. electric and magnetic forces you have to include electric potentials (=energy) and vector potentials for the magnetic forces. Beyond this, there are nuclear forces with their own form of energy.

You might even want to add human "energy" as a form of energy to this. But due to the lack of a quantitative description of human behaviour, this attribution is vague.

Ash Small

Mon Mar 12 2018, 11:59PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I used to have an engineering shop, in a previous life in my distant past, and while most of my clientele were lovely people, you always get a few difficult customers who consistently argue the toss.....anyway, to cut a long story short, I got so fed up with this bloke, and just wanted him out of my workshop, I started talking in Arbits......Eventually he stopped me, and asked what an Arbit was, to which I replied "It's the arbitary unit'......He turned and walked out

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