Today I Learned

[Stephanus]

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.

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...

That's nuts...

@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!

We would have way more accidents like this if every country would use their own units.

I hope we'll be careful in our everyday life.

 

[Stephanus]

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.

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

 

[mfb]

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.

 

[Stephanus]

There are many different pounds and different miles. Three miles are still in use - the nautical one...

Yep, 40000km/360/60

 

[DrGreg]

Today I learned that Americans use British Units, and the British' use metric unit??

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!

 

[DrGreg]

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

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.)

 

[Ibix]

View attachment 103759
Gosh, you're right!
But it's 9.8 km/s² not m/s²

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.

 

[jim hardy]

Almost everything in the UK now is officially metric, with just a handful of exceptions, for example:

No more Whitworth bolts ?

http://britishfasteners.com/threads/bsf.html

 

[jim hardy]

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

Anvils-6: Marked Weight of Anvils
Anvils are marked in a variety of methods but most English anvils were marked using the hundredweight system. American made anvils are marked in pounds. Anvils made in other places (including many Swedish anvils) are often marked in pounds. A few are marked in kilograms and some cast anvils are marked in pounds rounded to the nearest 10 pounds (250# = 25). Cast markings are easy to identify as they are usualy raised figures rather than stamped into the anvil. Then there are the many unmarked anvils. . . If you are not sure and you really need to know then weigh it. English Hundreds Weight (hundredweight) System:
Typically the hundreds weight markings are separated by dots but not always. These figures were stamped into the finished anvil and are often not very deep. The first figure to the left is hundred weights which equal 112 pounds. The next figure is quarter hundred weights which equal 28 pounds and the last number is whole pounds. The three are added together for the total weight. Examples:

1 · 0 · 16 = 112 + 0 + 16 = 128 pounds

2 · 1 · 3 = (112 x 2) + 28 + 3 = 255 pounds

2 · 2 · 25 = (112 x 2) + (28 x 2) + 25 = 305# The second place is never over 3 so if you have difficulty reading the number it is a 1, 2 or 3. The last place is never over 28 and most often is a single digit or less than 20.

The weight of these is usualy +/- 1 pound when measured on an accurate modern scale.

SO. . . how did the English weigh anvils and figure the marking?

EASY, on a balance scale using hundred weight, quarter hundred weight and pound weights.

 

[Astronuc]

American use miles, inch, pound.

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.

 

[DrGreg]

No more Whitworth bolts ?

http://britishfasteners.com/threads/bsf.html

That does appear to be one of the exceptions according to Wikipedia: British Standard Whitworth § Current usage

 

[jim hardy]

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

I'm still kinda stunned by that thought.
Are photons affected by gravity?
If so, do they arrive here with a wavelength different than they started out with ?

Have a laugh at that question i don't mind
i'm still struggling with modern physics
as Leonard Cohen says, 'waiting for the miracle to come... 'old jim

ps i kept my Whitworth wrenches .

 

[OCR]

That is an amazing story.

Yeah, it is an amazing story...[COLOR=#black].[/COLOR]

Here is the video of The Gimli Glider incident ... http://www.dailymotion.com/video/x28kvkj_air-disasters-gimli-glider-season-1-episode-2_tv .

 

[Astronuc]

TIL Russia is doing simulated nuclear attacks on Sweden, and has apparently been violating Swedish airspace. Sweden is a neutral country.
http://nationalinterest.org/blog/th...raft-killer-missile-now-service-its-not-17100

We do know that one test off Scotland sent the missile “well in excess of 100 kilometers,” an MBDA engineer told AINonline.

But Sweden is a major weapons exporter — the 12th largest according to 2014 data from the Stockholm International Peace Research Institute.

Sweden works closely with the NATO alliance during military exercises. And in May, the Swedish parliament ratified an agreement allowing NATO more room to conduct exercises in the country.

 

[Stephanus]

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

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.

Yep, you're right. My mistake. I'm off by 000 .

 

[Stephanus]

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

Talking about Brexit...
http://www.dailymail.co.uk/news/art...ridlock-Indonesian-junction-named-BREXIT.html
I live in Indonesia, Brebes is some 200 km east of me. Every year in Idul Fitri celebration, millions people from Jakarta are coming to their hometown. And most of them pass the north route. The jam in Brebes Exit Tol is so severe. Cars can get stuck for days in 40 km long traffic jam.

 

[Ibix]

I'm still kinda stunned by that thought.

There isn't any interesting physics lurking in this one - it's just a change of units. A light year is a distance and a year is a time, and a distance divided by a time divided by a time is a unit of acceleration. However this particular choice of units is very handy for thinking about relativistic rockets for a number of reasons. A light year is an appropriate scale for interstellar distances; a year is an appropriate time scale, and useful for "how old will I be when I get there"; the speed of light is exactly 1 by definition so all the annoying factors of c in relativistic equations drop out; the acceleration due to gravity at Earth's surface is very close to 1, and most people want to accelerate their rockets at 1g so it feels like home inside.

Are photons affected by gravity?

If so, do they arrive here with a wavelength different than they started out with ?

This is unrelated to the above. But yes, light is most certainly affected by gravity. Sometimes astronomers see multiple images of a galaxy because a nearer galaxy pulls the light out of a straight line path. https://en.m.wikipedia.org/wiki/Gravitational_lens. Also, black holes wouldn't be black if their gravity couldn't trap light.

Yes, light can arrive with a different wavelength compared to when it was emitted if the emitter and receiver are at different heights (more formally, different gravitational potentials). Pound and Rebka were the first to verify this experimentally. https://en.m.wikipedia.org/wiki/Pound%25E2%2580%2593Rebka_experiment

I'm drifting pretty far off topic.

 

[Stephanus]

I'm still kinda stunned by that thought.

There isn't any interesting physics lurking in this one - it's just a change of units. A light year is a distance and a year is a time, and a distance divided by a time divided by a time is a unit of acceleration.

It stuns me, too. Even the units match. Gravity = Distance/time square.
Light year per year square = distance per time square.
[Edit: the unit and the number matches]

 

[jim hardy]

Thanks ibix ! I saw the convenience of the units ,

There isn't any interesting physics lurking in this one - it's just a change of units. A light year is a distance and a year is a time, and a distance divided by a time divided by a time is a unit of acceleration. However this particular choice of units is very handy for thinking about relativistic rockets for a number of reasons. A light year is an appropriate scale for interstellar distances; a year is an appropriate time scale, and useful for "how old will I be when I get there"; the speed of light is exactly 1 by definition so all the annoying factors of c in relativistic equations drop out; the acceleration due to gravity at Earth's surface is very close to 1, and most people want to accelerate their rockets at 1g so it feels like home inside.

to make things so simple is the mark of genius.

The pound rebka link returns a "bad title " error,
is this it ?
https://en.m.wikipedia.org/wiki/Pound–Rebka_experiment

Fascinating, thank you,
my old mentor said that would happen , but we were not aware of the experiment.

I think your post is right on topic for "today i learned"

 

[Stephanus]

...I'm drifting pretty far off topic.

I think your post is right on topic for "today i learned"

Far off?? I'd say deep into.

 

[epenguin]

I'm still kinda stunned by that thought.
Are photons affected by gravity?
.

Yes. That's why black holes are black.

I'm sure you really knew that.

 

[jim hardy]

I'm sure you really knew that.

yes,
you know how something rattles around in the back of your mind and you can't even phrase the question ?
I'm still trying to weigh that with respect to Hubble's Red Shift

plodder that i am a lot of things just never have clicked together.

old jim

 

[Stephanus]

I'm still trying to weigh that with respect to Hubble's Red Shift

Hubble red shift??
Don't tell me.
Today I learned that constant can change??

The universe has expanded and the Hubble Constant has changed its value since then.

 

[jim hardy]

Don't tell me.

Tell ?? I'm only in a position to ask...

 

[fresh_42]

Tell ?? I'm only in a position to ask...

Sokrates?

 

[Stephanus]

No, Jim. It's just that you guys are discussing about Hubble, and just several hours ago I received that reply, that can't help my hand to comment. Frankly, I didn't read all your and Mr. epenguin's comment. It's just that red shift comes up and I recall the answer I had this afternoon (my time).

 

[NascentOxygen]

Yes, light can arrive with a different wavelength compared to when it was emitted if the emitter and receiver are at different heights (more formally, different gravitational potentials). Pound and Rebka were the first to verify this experimentally. https://en.m.wikipedia.org/wiki/Pound%25E2%2580%2593Rebka_experiment

Browsers don't like that URL.

 

[mfb]

Hubble red shift??
Don't tell me.
Today I learned that constant can change??

Hubble red shift probably means "the redshift due to the Hubble constant".

But the Hubble constant is actually changing, it decreases slowly. It is constant in terms of the distance of objects (if they are not too far away), but not constant in time.

 

[Astronuc]

TIL that a third of claims for lightning strikes on boats in Florida!
http://www.boatus.com/seaworthy/swlightning.asp

Thirty-three percent of all lightning claims are from the Sunshine State and the strike rate there is 3.3 boats per thousand. Surprisingly, the second most struck area in the country is the Chesapeake Bay (twenty-nine percent), and those who boat there in the summer can attest to the ferocity of the sudden thunderstorms. Not surprisingly, the majority of strikes are on sailboats (4 per 1000), but power boats get struck also (5 per 10,000); Trawlers have the highest rate for power boats (2 per 1000) and lightning has struck houseboats, bass boats, and even PWCs.

 

[Cdz]

Today I learned that I registered on PF more than one year ago!

 

[Pepper Mint]

Today I experienced or understood not knew or learned how young fresh girls' acts and words can change a straight man's mind completely. It's the power of lust or sexual urge not love. And that does occur in life as always.
So you! all physics ladies should always be aware of how and what your spouses are thinking and want.

 

[Ibix]

The pound rebka link returns a "bad title " error,
is this it ?
https://en.m.wikipedia.org/wiki/Pound–Rebka_experiment

Yes.

TIL two things. One, that snipping the URL out of the super-long URL that Google provides doesn't always reduce to the valid URL. Two, I should always check my links...

 

[jim hardy]

It's the power

While raising teenage daughters i realized they are not very aware of it.
It is very potent. Please be careful with it.

 

[ProfuselyQuarky]

Today I learned what Wellie Wanging is--pretty much a sport consisting of throwing boots as far as possible.

 

[fresh_42]

Today I learned what Wellie Wanging is--pretty much a sport consisting of throwing boots as far as possible.

I'll have to scale up my font sizes. I misread this.