# Today I learned

1. Jul 24, 2016

### OmCheeto

TIL that the U.S. distilled alcohol industry converted to the metric system in 1979.
One very common quantity, the "fifth", may have been widely accepted by consumers, as 1/5 of a gallon (0.2 gallons) is very close to the new 750 ml bottles (0.198 gallons).

2. Jul 24, 2016

### Stephanus

You tell me. I live in MKS, but I'm having trouble understanding my tyre (tire?) pressure in pascal rather than in psi.
1 pound = 0.453
1 inch = 2.54
$Pa = \frac{0.453}{2.54*2.54/100/100} = 702$
$Pa = 702 * 9.8 = 6881?$
$1 psi = 6881 pascal?$

3. Jul 24, 2016

### Stephanus

Ahhh, and there are gallons, barel, and all those things. And I also remember. I can picture AUXUSD (the price of gold in US dollar) in how many dollar per troy ounce, rather than how many dollars in gram/kg. Who is troy anyway?

4. Jul 24, 2016

### jim hardy

i gave up long ago
went to atmospheres , one of those is within 2% of 100kPa
and quarts, one of those is within 6% of a liter
but i still have to look up hogsheads .

5. Jul 24, 2016

### Astronuc

Staff Emeritus
When I studied physics, we did our work in MKS and cgs. When I did nuclear engineering, it was mostly British units with some SI.

When I started working professionally, we had clients throughout the US and the rest of the world, so we used both systems. I prefer SI, and many codes I have used are written with SI internally (with some models specifically in British units because experiments were done in BU), but the codes would have options to do input and output in British or SI/cgs. Since manufacturing uses dimensions in inches/mils/micro-inches or cm/microns, there were mixed systems.

6. Jul 24, 2016

### jim hardy

speaking of mixed systems....
the little diesel engine in 83-84 Ford Rangers is mixed , all the bolts in it have metric heads but inch threads...

it was a Perkins design built in Japan by Mazda.

7. Jul 24, 2016

### Staff: Mentor

One of the weird historical coincidences.

The second is 1/86400 of a day - that is older than the SI.

The two proposed definitions for the meter were:
(a) the length of a pendulum with a period of 2 seconds, fixing $g=\pi^2 \frac{m}{s^2}$ (~9.87) for the point where the definition is applied.
(b) 1/(10 million) times the length from a pole to the equator
Those two independent lengths agree to better than 1%, the latter got chosen.

The kilogram was based on 1dm3 of water, fixing its density (at a given temperature) to 1000 km/m3.

Combining those three units, kPa is fixed. 100 kPa (using the accidental $g \approx \pi^2 \approx 10$) corresponds to about 100,000 kg/m2 mass of the atmosphere, or (using the non-accidental density of water) to about 10 meters of water, which is about 1 millionths of the pole/equator distance. Which happens (another coincidence) to be within the small actual range of atmospheric pressure.

8. Jul 24, 2016

### jim hardy

I really like little memory aids like that, they give perspective.

A Newton is roughly 1/4 lbf, so i tell beginners "think of a hamburger not a cookie ".

9. Jul 24, 2016

### Ibix

That's nuts...

10. Jul 24, 2016

### Ibix

A year isn't far off $\pi\times10^7$s - about 0.5%.

One gravity is approximately one light year per square year, useful for those relativistic rockets.

11. Jul 24, 2016

### Stephanus

Today I learned that Americans use British Units, and the British' use metric unit??
That was what was confusing me all this time. I heard that Americans use different units than British.
American use miles, inch, pound.
But what about the British? Surely British would use something else than miles and pound.
But miles and pound are British units.
Argghhh...
@Ibix is right! But now I truly understand.
It's no wonder that a-200 millions dollars NASA's Mars Climate satelite crashed. We live in a crazy world, no matter what Carl said. We have to understand those units out of the blue in this pale blue dot!
I hope we'll be careful in our everyday life.

12. Jul 24, 2016

### Stephanus

Gosh, you're right!
But it's 9.8 km/s2 not m/s2

13. Jul 24, 2016

### Staff: Mentor

There are many different pounds and different miles. Three miles are still in use - the nautical one, the "normal" one, and my favorite weird unit, the US survey mile, which is about 3.2 millimeters longer than the normal mile.

@Stephanus: the 300,000,000 is meter/second. It is m/s^2.

14. Jul 24, 2016

### Stephanus

15. Jul 24, 2016

### DrGreg

The terms "British Units" and "English Units" are, nowadays, I think, used only by Americans. Here in the UK, if we want to refer to the old units, we call them "Imperial Units" (if anything).

Almost everything in the UK now is officially metric, with just a handful of exceptions, for example:
• draught beer and cider must be sold in pints;
• milk may be (optionally) sold in pints;
• distances along roads and railways are still in miles and yards (and chains on the railways);
• vehicle speeds are still in miles per hour.
Unofficially, imperial units may be used in everyday life by some people (especially older generations), e.g. stones and pounds for body mass, feet and inches for body height, Fahrenheit for air temperature on a hot day (but Celsius on a cold day!). Vehicle fuel consumption is often expressed in miles per gallon even though fuel is sold in litres!

I just hope that, post-Brexit, we don't abandon the metric system!

16. Jul 24, 2016

### DrGreg

And, of course, the speed of light is about one foot per nanosecond.

(I suppose, in the spirit of SI, that should really be one gigafoot per second.)

Last edited: Jul 24, 2016
17. Jul 24, 2016

### Ibix

You've slipped up somewhere - I agree that the answer should be c / year, but that should be $\simeq (3\times 10^8 \mathrm {ms^{-1}}) / (\pi \times 10^7 \mathrm s)\simeq 9.5 \mathrm {ms^{-2}}$, about 3% below g=9.81ms-2.

Last edited: Jul 24, 2016
18. Jul 24, 2016

### jim hardy

19. Jul 24, 2016

### jim hardy

British anvils are strangely marked .... i ran across a early 1800's " Mousehole Forge" anvil at the metal scrapyard.

20. Jul 24, 2016

### Astronuc

Staff Emeritus
And BTUs, BTU/hr, and BTU/ft2-hr. I've seen legacy code with steaming rates in BTU/ft2-hr. W/m2 is so much better

Or heat transfer coefficients in BTU/ft2-hr-°F. I much prefer W/(m2K).

Still it's better than horsepower/leagues or leagues/fortnight.