Heres something thats been bothering me for ages. Not being a US native, I’ve done my studies uptil my BS outside the US. I’m a software engineer and have always been keen on scientific topics and courses. One of the most fascinating things for me was the use of the SI (metric) system of units which allows consistent and comparable formulas and calculations not matter what the domain of study is.
After coming here and realizing that there is a part of the world that doesn’t use the SI system, I was stunned! I simply cant comprehend how the math would work. Do the formulas remain the same or do you have to put in some conversion to account for the wildly inconsistent units of measurement. as a minor example, consider the formula for energy change in a uniform object due to change in temperature (ignoring all other factors)
E = mc(Δϴ)
where E = energy change measure in Joules (J)
m = mass measured in kg
c = specific heat capacity measured in Joules per kilogram Kelvin (J/Kg K)
Δϴ = temperature change measured in Kelvin or Centigrade (difference is equivalent in both units)
Notice that all units are related and the resulting formula is clean and simple and there is no need for conversion of any sort (or even a need to come up with a unit of measurement for the result). Even Joule is just shorthand for (kg.m^2)/s^2 or other equivalent metric units.
Maybe I’m overthinking this but my brain just BSODs when I even try to think how you would calculate this using pounds for mass, Fahrenheit for tempurature and god knows what for energy or specific heat capacity. Help me out here fellas! I’m going crazy!
Now in the US science is generally done in SI. But when the English system is used the units are determined so that everything works out. For example, torque is calculated feet x pounds and the resulting unit is the foot-pound.
It isn’t all that different. The conversion factor is built into the constant. So instead of heat capacity in J/kg-K it would be measured in calories/lb[sub]m[/sub]-ºF, or something to that effect, and the value would be different to account for the different units.
That constant, c, will just take on another value that accounts for all of the relationships between Δ°F, pound-mass, and BTUs (or whatever hogshead per fortnight bullshit you want to use).
ETA: But most science is done with SI. Some engineering, depending on the field, sticks with English units.
American here. First of all, you just get used to there being two systems.
The English system is used for most casual measurements. The news reports pretty much everything using English measurements (e.g. the weather report gives temperature in F and reports rain or snowfall in inches). Hard science uses SI. Construction work and other trades historically used English but may use SI depending on what is being built, what parts are being used, what the customer wants, and/or local codes.
And, generally, Dimensional Analysis will allow you to use all your favorite formulas. For example, F=ma is as true in English units as SI units, you just have to make sure that the values on BOTH sides of the equation are in applicable English units.
you could memorize constants in metric and english units. you could just remember the metric constants and use english-to-metric conversions which you have to know anyway in a dual system environment. actually if you used a few equations a lot you would know the english constants for those.
I can comment that the energy industry in the US uses English - btus, or mmbtus. And coal characterisitcs use English units as well, with heat content being given in btus/lb (or mmbtus/ton) and SO2 quantity given in lbs/mmbtu. Oil is usually priced by the barrel, but #2 heating oil is priced in $s/mmbtu. And when we talk about heat rates at power plants, we use mmbtus/megawatt-hour.
If I had to switch to SI, I would be really, really confused for a while, so I can certainly understand why someone going the other direction would ahve a hard time of it.
It’s not really that hard, so long as all your units are expressed in a self-consistent system. You might be interested in the Wiki pages on English Engineering Units and the FPS system. For example, in the latter system you would do the following:
E = energy change measured in foot-poundals
m = mass measured in pounds (or, for the nitpickers like robert_columbia, pounds-mass)
c = specific heat measured in ft-pdl/lbm*(°F)
Δϴ = temperature change in degrees Fahrenheit
It’s all self-consistent, and the units still all cancel out. Granted, every one of the numbers you’ll put into your calculator is different, but there’s no particular advantage of a system of units based on the meter and the kilogram versus one based on the foot and the pound-mass.
Now, if you were given a volume in (say) gallons and needed to figure out the mass of that volume of a particular liquid, and you knew its density in terms of base units (lbm/ft[sup]3[/sup]), you’d still need to convert it into cubic feet first. For that matter, if you’re given a volume in liters and you know its density in kg/m[sup]3[/sup], you need to convert liters into cubic meters too. Here, though, metric has the advantage, since 1 gallon = 0.133680556… ft[sup]3[/sup], while 1 liter = 0.001 cubic meters exactly.
Finally, as noted above, almost all scientific calculations are done in SI units. (The big exception in my field is the electron-volt for energy.) Some engineers still occasionally use FPS-flavored units, though.
That explains things a lot. Thanks. The main issue is, as you said, to use consistent but non SI units are inconsistent by nature (and there are so many of them!) where as SI are designed to be internally consistent. e.g take the water volume to mass conversion. 1 liter is 1 kg so conversions are easy. even using a factor like 0.001 is a lot easier than 0.133!
Never heard of foot poundals before! Also learned that pounds are a unit of force not mass! Shows how much I know. Still I’m relieved. (I mean that in the “oh good it’ll still be familier if I have to do it again” sense and not the “oh good there’s hope for you bloody yanks yet!” sense!)
Get either a programmable calculator or a software conversion tool or an online site like this one. You enter the result in one unit and get the result in the other unit. Do all calcuations first with the starting unit for accuracy - if your scale reads inches, then use inches per second instead of meters, and convert once you’re done.
The formulas are all the same after all - it’s distance over time, whether distance is feet or furloughs or meters. Just plug the numbers in.
Don’t worry about that…this is a topic that gets everyone confused.
A pound, as you normally hear it, is a unit of force. In some instances, however, it can be a unit of mass. These are typically differentiated as pound-force (lbf) and pound-mass (lbm). There is also the slug, another unit of mass.
The simple way to think about it is that a slug is the most direct equivalent of a kg in terms of mass. That is, 1 lbf accelerates 1 slug at 1 ft/s^2) the same way that 1 N accelerates 1kg at 1 m/s^2.
A lbm = 32.17 slugs (32.17 m/s^2 being the imperial value for gravity on Earth). It is thus defined so that, on Earth, 1lbm weighs 1lbf.
For me its become a part of my life. For the longest time, I internally converted units in my head in order to know what they meant. Kilometers to miles was easy since I drive a lot now but I still sometimes convert fahrenheit to centigrade in my head to get an actual feel of temperature. Even after 6 years, I still cringe when I hear the Mythbusters use a non SI unit on their show!
And likewise, rotational speed is measured in rotations per minute, or RPM. And power, which is just torque times rotational speed, is measured in foot-pound-rotations per minute.
Oh, wait, no it’s not. Power is measured in horsepower, and so the folks who use such units have to insert some crazy number into their calculations to convert from foot-pound-rotations per minute to horsepower.
It gets even worse. Some folks use slugs for mass, pounds for force, feet for distances, and seconds for time, and that works out OK. Other folks use pounds for mass, poundals for force, feet for distances, and seconds for time, and that works out OK, too, as long as you know which is which. But then there are some folks who use pounds for mass and for force, and still use feet and seconds, which means they need to cram an extra factor of something that resembles g all over the place into equations where it has no business being.
