Just what exactly is a G?

[womble]

Historically, food was bought by mass; balance scales compare masses in the same gravitational field. The difference between mass and weight was entirely irrelevant until the concept of gravity was noted. It's only with the advent of spring balances that you actually measure the weight of something to determine its mass.

If we colonise, say, the moon and have such mundanities as grocery stores, they'll sell by mass since the nutritional content of the food doesn't change with the gravitational field it's sold in. Likewise, 'weightless' food in freefall.

 

[TJP]

Originally posted by Malenfant:
I'm fairly sure that the kg is the mass that is actually defined (I thought it was the mass of a certain lump of platinum in Paris, or something?), and gram is derived from that, but I could be wrong.

You're absolutely right. Kilogram is the base unit, not gram. Plus, the "lump" still exists (IIRC it's made of platinum and iridium) and is the measure of kg.

 

[Gruffty]

Originally posted by Malenfant:
I don't doubt that if the Yanks colonise another planet they'll be using feet and miles on it while they're driving around on it in their SUVs

.

And they'll *STILL* be on the *wrong side of the road* !!!

 

[Gruffty]

There's no such thing as pure white light.

Thanks for up-dating me on that

From 1960 to 1983, a meter was defined as 1,650,763.73 wavelengths of Kr-86. Currently, it's defined as the distance light travels in 1/299,792,458 of a second.

One second is 9,192,631,770 vibrations of Cs-133

Again, it's been a long time since I did any form of physics (not much call for quantum mechanics in nursing ). I had an idea that the length of a meter is fixed by another constant somewhere.

I take it Cs-133 is Caesium-133? Non? N'est pas?

 

[Malenfant]

Cs = Caesium, yes. Quite fun when you drop it in water

.

 

[aramis]

I think what you were referencing, Mal, is properly Gl (G'Local'), where said L is supoosed to be subscripted.

Metric units base from the degrees, gram, meter, second, liter, joule, newton, volt, watt, ampere, etc., at least logically. THe reference is that a cubic volume one decimeter on a side (1/1000 of a cubic meter) is 1 kilogram at standard temp, pressure, and content. THat the reference items (weights, lengths) for commerce are in fact not maintained in those reference methods is not a redefinition of the original unit, but a matter of legal practicality.

Most physics formulae I've seen have been in g/m/s/n/°k or °c/J/N/W/A.

The standard reference for desnity is, last I checked, still water at standard temp and pressure, is Specific GRavity 1. That means 1kg is 1L, 1Mg=1kL. 1cc (mL) of SG1 is 1g.

 

[GJD]

Originally posted by Malenfant:
That said, there's currently no good reason to change the cornerstones of the metric system in the future. A second can still be a second whether you're on Earth, Mars, Regina, or even near a black hole, because no matter where you are, it's still 9,192,631,770 vibrations of a Cs-133 atom. (now, why they chose that number of vibrations, I dunno. Probably because it's the same length of time as the traditional definition of a second, however that was defined).

Seconds are relative and subject to any perturbations of the local space-time, for instance the effects of large gravity wells or near luminal travel velocities. SO, if you are closer to a black hole a second will be different to an observer who is more distant from a black hole.

By the same measure, distance is also a relative concept, as can be observed by gravitational lensing whereby the path of an electromagnetic wave is affected by local fluctuations in spacetime, brought about by gravity.

However, these releative effects are either to small to notice exept as scientific curiosties (gravity ripples), side effects of a situation where you probably have more to worry about any way (reletivistic effects of a black hole) or too mind bending to easily explain (reletivistic time dilation).

G.

 

[BetterThanLife]

Originally posted by Gruffty:
</font><blockquote>quote:</font><hr />Originally posted by Malenfant:
I don't doubt that if the Yanks colonise another planet they'll be using feet and miles on it while they're driving around on it in their SUVs

.

And they'll *STILL* be on the *wrong side of the road* !!!

</font>[/QUOTE]Hey now, us Yanks drive on the "right side" of the road. Now if you mean the correct side of the road, that depends on whether you mean the slash at them with a sword as you pass or not slash at them as you pass side of the road. (The real reason the Brits drive on the left.)

 

[womble]

K, Kelvin, the SI measure for temperature, is not measured in degrees (as Farenheit and Celsius/Centigrade are), just plain Kelvin. F'rex: 0K ('absolute zero'), 273K ish is the freerzing point of water.

Though a second is relative, if you have a Cs clock with you when you're in the anomalous patch of space-time, it'll still oscillate the same number of times per local second. And a metre stick will still be a metre long.

 

[Malenfant]

Though functionally a difference in temperature of 1 Kelvin is the same as a difference of 1 degree C. The only difference is that the 0 degrees C is 273.13 K. But you're right, K is never measured in 'degrees', and it's a common mistake to do so.

A difference in temperature of 1 K however is not the same as a difference of 1 degree Fahrenheit. (the latter is for me the system I'd most like to see tossed in the garbage where it belongs. Utterly non-intuitive, nonsensical temperature scale. See here to see just how completely arbitrary the F scale is.

 

[Malenfant]

I think what you were referencing, Mal, is properly Gl (G'Local'), where said L is supoosed to be subscripted.

You're missing the point.

G is NEVER, EVER used for local gravity in science - that's reserved for g. G is a specific constant of the universe - the Universal Gravitational Constant, which does not change (there might be evidence that it was different in the early universe, but it's the same in our neighbourhood today at least).

Upper case G is not local gravity. Lower case g is local gravity.

 

[womble]

Ar. Farenheit is stoopid. Gud link.

And to amplify upon what Mlenfant has stated:
A drive ship rating in 'G's is not implying any gravity at all, but the rated acceleration in multiples of 10m/s/s. The fact that someone standing on a bulkhead perpendicular to the direction of thrust feels as if they are in a gravity field equivalent to the rating of the drive is down to action and reaction. Not gravity. Unless there are artificial gravity generators in operation. And then , is it really gravity?

[edit]
Oh, and symbols in equations aren't the same as the symbols for units of measurement. 'g' can quite happily and correctly be used to represent 'grams' when you're measuring something and 'local gracity' when you're describing, mathematically, the effect of two masses upon each other. It's all in the context.

 

[Malenfant]

Originally posted by womble:
Ar. Farenheit is stoopid. Gud link.

A lot of times when this comes up, people confuse 'familiar' with 'easy'. Sure, a lot of Americans are very familiar with the F scale - but that doesn't make it an easier one to use. To illustrate - what temperature on the F and C scale is halfway between the boiling and freezing point of water? For the C scale, it's easy - freezing is 0, boiling is 100, so midway is 50.

For the F scale, not so - freezing is 32 F, boiling is 212 F, so the difference is... 180 degrees, divide that by 2 to get 90, add that to 32 and you get 122 F as the halfway point.

I'm sure you'd agree that it's easier to figure out that half of 100 is 50, than do the somewhat convoluted calculation to find the same temperature using the F scale.

The same applies to other non-SI units. The metric scale is consistently defined in multiples of 10. Again, I'm sure you'd agree that it's a much more straightforward calculation to divide by 10s all the time than it is to remember all the different divisors for all the different non-SI units.

That said, if you're familiar with non-SI units then you're more likely to know what they feel like in practise. Because of that, you'd be more likely know how long a foot was, or how heavy an lb was, and you wouldn't necessarily know how long a metre was or how heavy a kg was. But that doesn't mean that imperial or US units are easier to learn or remember - it's just a matter of becoming familiar with the scales of the units.

 

[Anthony]

Originally posted by Malenfant:
A lot of times when this comes up, people confuse 'familiar' with 'easy'. Sure, a lot of Americans are very familiar with the F scale - but that doesn't make it an easier one to use. To illustrate - what temperature on the F and C scale is halfway between the boiling and freezing point of water? For the C scale, it's easy - freezing is 0, boiling is 100, so midway is 50.

How often do I need to know that? Yes, it's easier to compute the halfway point between water freezing and boiling, assuming pure water, but that's because you chose a specific case that the centigrade scale is optimized for.

Averaging, say, the melting points of lithium (181C/357F) and sodium (98C/208F) is about as easy in either system (139C/282F)

 

[Malenfant]

Well, maybe it's down to familiarity with what the units mean again. Having scales that aren't based on base-10 is just totally counter-intuitive to me. I can imagine that it's totally counter-intuitive to anyone else unfamiliar with it too. It's certainly vastly easier to calculate things in SI than in non-SI. I think anyone who wasn't familiar with either system would choose the SI/metric system over the non-SI/Imperial system any day because of its ease of use and elegance.

 

[BetterThanLife]

I personally don't understand the big deal between C and F. I personally don't like the C scale because the individual points are too far apart for an accurate reading on the temperature I am going to live with. For the normal human range of comfort the F scale works just fine. For other things, like wider variances than the extremes at which a Human is comfortable then perhaps the C scale is better. But to call something less than 0F is very cold and dangerous to go out ill prepared, however 0C is not all that bad. Human comfort is between 68F and 72F ideal temp, what is that in C about 39? 100F is uncomfortable and gets dangerous. How is that counter intuitive when it comes to normal human habitation. Seems to work well enough for me. (And about 260 million+ of us Barbarians.

It is a question of what you are measuring. If you are measuring water then the C scale is better, but how often does the average person worry about the temperature of water, when you aren't swiming or bathing in it?

Farenheight is base 10. It is the other stuff that isn't. And personally, even though we all grew up using base 10 I personally think base 12 is a better choice. Though Base 16 isn't bad either. But we have ten fingers and ten toes (Or at least most of us do.) so base 10 it is.

Originally posted by Malenfant:
Well, maybe it's down to familiarity with what the units mean again. Having scales that aren't based on base-10 is just totally counter-intuitive to me. I can imagine that it's totally counter-intuitive to anyone else unfamiliar with it too. It's certainly vastly easier to calculate things in SI than in non-SI. I think anyone who wasn't familiar with either system would choose the SI/metric system over the non-SI/Imperial system any day because of its ease of use and elegance.

 

[Anthony]

Oh, I agree that base 10 is preferable for units, but temperature is a bad example, since there's only one measure used.

Ideally, temperature would be measured as something along the lines of joules/mol for an ideal gas.

 

[BetterThanLife]

Back to temperatures. These are the useful numbers for the average person as far as temperatures. Do I need a coat, or a light jacket? Should I wear my short sleeve shirt or will I need a long sleeve one? Shorts or pants? Average person on water temp: Is it frozen, cold, cool, luke warm, hot, scalding or boiling? Who cares what the actual water temp is? If the water is less than body temp then it is cool to cold if it is more than body temp then it is warm to hot. And if it is boiling then it is boiling and I can use it to cook. (And I am likely to add salt to raise the boiling temp so it cooks better.

)

 

[Gruffty]

What makes me laugh is that during WW2, US servicemen had problems with our old £, s and d (pounds, shillings and pence) system, which was based on the same mathematical system as feet and inches (i.e. 12 pence to the shilling, 24 shillings to the pound). Yet the dollar is 100 cents in a dollar.....

 

[mike wightman]

The answer to these questions can tell you a lot about our mixed up measurement systems.

What units do you give your own height in?

Feet and inches, inches, metres or centimetres?

Most Brits answer in feet and inches still... even the children who have been brought up with metric measurements for the last thirty years.

What units do you quote your own weight as?

Stones and pounds, pounds, kilograms ( )?