# American Engineering Units

1. May 12, 2009

Not quite sure if this is the correct place to post this, but I am having difficulty converting between AE and SI units. Having grown up using only SI units, the AE units are a little confusing.

What is the conversion formula for degrees Fahrenheit to degrees Celsius incorporating a degree change?

Secondly, what AE units do I use for the ideal gas law PV = nRT?

Many thanks,

2. May 13, 2009

### MATLABdude

3. May 13, 2009

### minger

Let me just say that get used to $$g_c$$. That is the gravitational constant and relates force and mass to each other in the american system. Get used to it. It is a pain in the ***, and make sure to always mark if a unit is force or mass.

Also, if you're going to do anything in aerospace, often times we use slugs for mass. Slugs are basically your normal pound-mass divided by gravity, e.g. to get density of a material in slugs/in³ you take lbm/³ and divide by 386.088

4. May 13, 2009

Why haven't americans switched to SI? Its so much easier.

5. May 13, 2009

### minger

Because 100 kph is WAY to fast to drive on the highway. I mean, there are THREE digits!

6. May 13, 2009

### mgb_phys

Found somewhere on the internet
The really annoying one is that they call Imperial 'English' then use different sized units to the the Brits!

7. May 13, 2009

Ok thats just weird. I'm from the UK, and we always use metric SI units over here now (well my parents don't), but what do they teach American kids in their high school physics lessons, SI or something else?

8. May 13, 2009

### PhanthomJay

In high school physics, they teach both, but with an emphasis on SI, because it is so much easier. However, in structural and civil engineering practice, and in elementary schools and in daily life, forget about the use of SI in the USA. The feds tried converting their highway layout plans to SI several years ago, but there were so many errors made that they are now back to the USA units. A psi will forever remain a psi and not some Paschal or GigaPaschal. Same in everyday life, they tried to get fancy by converting ball park distances to meters (those quickly disappeared), selling gas by the liter instead of the gallon (Shell Gas tried this many years ago and lost so much business, that they quickly changed their pumps back to the familiar USA system). And bank digital thermometer temperature readouts in degrees C are a thing of the past. And in grade school, after kids were being measured in centimeters (but still being weighed in pounds), and nobody could figure how tall they really were, that experimemt has also died, and SI is taught only in passing , as a footnote.

9. May 13, 2009

### Count Iblis

What they do wrong in schools is that they teach units (whatever units that may be) in completely the wrong way, http://insti.physics.sunysb.edu/~siegel/history.html" [Broken]

Last edited by a moderator: May 4, 2017
10. May 13, 2009

### mgb_phys

The UK of course manages to make everything a little more complicated.
Everything at work and official is SI.
Except speed limits and distances are in miles or yards, it's illegal to put distances in m.
Gas is sold in litres but everybody talks about mpg (except the gallons are different to US gallons).
Everything in stores must be priced in kg but you ask for pounds, beer is the only thing allowed to be sold in pints (which are different from US pints)
Clothes sizes are in inches and you give your height in feet and your weight in stone.

11. May 13, 2009

The problem in the UK is that people don't want to change. The EU made us switch most things (mass and volume of food etc) to metric, but replacing all our speed limits with m/s or kph would be expensive and confuse everyone. The biggest problem is most kids grow up listening to parents/grandparents telling them imperial units. As a physics and electronics student I always use metric, but if I asked my mum ingredients for something she would use pounds and ounces, so non science students never really use metric units. People won't change until they are forced to.

12. May 13, 2009

### TVP45

The US Customary System just about makes sense if you drop the silly pound force vs pound mass thingy and use slugs for mass (Though I noticed NIST really disapproves of that).

13. May 13, 2009

### mgb_phys

Metric on speed limits doesn't really help, except to avoid confusion for visitors, Km and Kph is no more SI than mi and Mph - unless they are going to switch to m/s!

For everyday use the customary units do make sense because they are a convenient size. That's why it's easier to say you weigh 10st rather than 160lb (as in america) or 70Kg (in Europe) 10 is a better sized unit to deal with. The same with inches, which is why c/g/s was popular for so long it's easier to picture a cm than a fraction of a m.
Even in France food is sold in markets in medieval pounds (ie 0.5kg) and timber might be labeled in mm but is sized in inches.

The same applies even in sciene, the Amp is really a factor of 1000 and the Farad 1,000,000 too large.

14. May 13, 2009

Yea but the best thing about SI is that you can say you mass in Kg, but a gram is just 1/1000th of a Kg when measuring smaller masses. Its all just 10 time tables rather than 12, 14, 16 and what ever else.

15. May 14, 2009

### minger

What does Europe use for cooking? I'd be aghast if I didn't have cups and tablespoons and had to measure everything in deciliters or whatever.

16. May 14, 2009

Food that you buy now is measured in Kg, L (with different SI suffixes). However most people still measure in old units. I was making gravy yesterday and the instrustions said 4*5ml (4 tsp).

17. May 14, 2009

### mgb_phys

At least they use mass. Ever tried to measure a 'cup' of butter?
It's a bit easier to get 250g onto scales

18. May 14, 2009

Yea except everyone says weight lol. When people as me how much I weigh I give them my weight in Newtons.

19. May 14, 2009

### D H

Staff Emeritus
Don't be so pedantic. Legally and colloquially, weight is a synonym for mass, not gravitational force. In a sense, it is we scientists and engineers who are abusing terminology here, not lay people. Keep your complaints about terminology to meaningful things, like the next time you overhear Joe Blow saying "I've got this theory" or "evolution is only a theory, after all".

20. May 14, 2009

### TVP45

That one's easy. Two sticks.