# Force (in lbs) to speed?

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russ_watters
Mentor
Enormous cost of conversion is one reason, but that is secondary to the major reason: Familiarity with the USA units.
This is true, but not for the reason you think (skip to the end for that...):
Every layperson, engineer, construction worker, etc. knows how much an inch or a foot is( About the length of an adult male's shod foot) . But how much is a meter? Not many know. So the construction super calls me the engineer on the phone and asks 'say there, Phantom, how much should I stick this foundation above the ground? It doesn't say on the drawings.' And I answer 'oh, sorry, stick it up about 6 to 12 inches" and the super says thanks and that's it. Now if using SI, and the same question is asked, I'd have to mentally or with calculator respond after several minutes 'about .15 to .3 meters' , hoping I did the conversion right without slipping a decimal point, and the duper would respond' huh? How the heck much is that?' And I would respond 'about 6 to 12 inches' and he is very happy and so am I, relieved that I didn't have to recheck my metric calc. Same goes for building materials be it steel or wood or concrete ; we all as engineers or fabricators know instantaneously that A36 steel has a yield strength of 36,000 pounds per square inch....I can't imagine specifying it as 200 MPa or whatever it is, hoping I didn't mean 200 Gpa, both of which are wrong anyway.
Neither construction industry engineers nor construction workers are stupid. But they are stubborn. And more importantly in your particular industry, they are cloistered. But in other parts of the construction industry (I'm in HVAC, pharma specific) we don't have that luxury, so I'm fluent in both. The phase-in would take time, but I learned SI in elementary school in the 1980s, so based on that the majority of today's workers should be capable of both.

Converting back and forth is indeed a pain, but the pain would go away if you go to all SI. There is no issue of intuitive familiarity because you aren't eyeballing the measurement anyway, you use your SI tape measure without conversion!
On the contrary, in civil engineering, its moving backwards. 20 years ago, the government mandated SI units on state and federal highway plans...
The most difficult place for the conversion that I can see is with speed limit signs, since they don't have units on them. As a result, you'd have to replace signs multiple times over a generation to do the phase-in otherwise there is a safety risk. That's the type of "why bother?" effort that I think doomed the conversion to failure. But it is short sighted.

mfb
russ_watters
Mentor
In mechanics, the fundamental requirement for consistency is that Newton's Second Law be satisfied with no fudge factors included.

For those who are familiar with Greek and Latin prefixes, the whole SI system is inconsistent if you think about it. The "kilogram" is essentially (by the meaning of the kilo-prefix) 1000 grams. Thus, like it or not, the gram is the fundamental unit of mass for SI, not the kilogram. With that understanding of what is really fundamental, Newton's 2nd Law is again not satisfied in the SI system. Look at the units in Newton's Second Law:

1 lb = 1 slug * 1 ft/s^2 --> FPS system is consistent
1 lb = (1 lb-s^2/in) * 1 in/s^2 --> IPS system is consistent
1 N =/ 1 gram * 1 m/s^2 --> SI system is inconsistent

Prefixes such as milli-, kilo-, centi-, mega-, etc are essentially fudge factors inserted into the equations. The fact that they are powers of 10 makes them somewhat easier to work with, but it does not change their basic nature as fudge factors.
This is silly. Units are arbitrarily chosen for a variety of reasons, among them convenience. The SI prefixes are no more "fudge factors" than 12 inches per foot or 16 oz per lb (and obviously easier to remember since there are fewer conversion factors to memorize).

jerromyjon
russ_watters
Mentor
If one is trying to "lose weight", one is trying to lose mass. In medicine (leaving podiatry to one side) and in commerce, it is mass that is relevant. The kilogram is the right unit to use.
Agreed. Flying to the moon doesn't fix the problem.

Prefixes such as milli-, kilo-, centi-, mega-, etc are essentially fudge factors inserted into the equations.
No, they are a multiplicative factor used to rescale large or small numbers into something close to unity i.e. 1000000 bytes = 1Mb or 0.000001 Farads = ##1\mu##F. Their is no "fudging" here.

The fact is that imperial units have no place in most science and engineering. Not only do imperial units have random scaling factors everywhere, there are scenarios where there is no imperial unit for something. There is no US unit of electrical charge for example and it would be exceedingly painful to work with US units in electronics where things are nicely defined with the Volt, the Ampere, the Farad, the Ohm, etc.
Now if using SI, and the same question is asked, I'd have to mentally or with calculator respond after several minutes 'about .15 to .3 meters' , hoping I did the conversion right without slipping a decimal point, and the duper would respond' huh? How the heck much is that?' And I would respond 'about 6 to 12 inches' and he is very happy and so am I, relieved that I didn't have to recheck my metric calc.
Really? I am from the US and have no problem estimating how many meters something is or what the temperature is outside in Celsius.

mfb
This is silly.
An interesting comment, Russ. I note in passing that you did not undertake to rebut a single word of what I have written, but essentially said that you simply don't like it. Well, that's certainly your privilege, but not it is not much of a rational argument.

The inconsistency with the SI system (which I pointed out above) came when the wizards defining the SI system chose to name the mass unit in such a way as to incorporate a prefix. If they had just given it a new name, maybe call it a "K" that just incidentally happens to be equivalent to 1000 times the older mass unit called a gram, then that flaw would do a way.

I learned SI in elementary school in the 1980s
You are ahead of me in that respect, I suppose, Russ. I did not learn the SI system until I was in college in the late 1950's, although it was called MKS then. The SI system was not published officially until 1960, by which time I was a grown man. I've been quite fluent in both USC and SI for over 60 years, but I have yet to see that one system is any better than the other for work in mechanics (electricity and magnetism is another matter, and SI certainly works better there). I use both equally well, although like @PhanthomJay I hate to have to convert back and forth.

The fact is that imperial units have no place in most science and engineering.
Wow! That's a shock!! I've been practicing engineering for over 50 years, using both SI and USC units, and never noticed when I departed fro the straight and narrow. What was I thinking? Fortunately, my superiors must have been just as confused as I was, since they always thought we were doing engineering.

russ_watters
Mentor
An interesting comment, Russ. I note in passing that you did not undertake to rebut a single word of what I have written, but essentially said that you simply don't like it. Well, that's certainly your privilege, but not it is not much of a rational argument.
Um...there was more to my post than just those words...

(electricity and magnetism is another matter, and SI certainly works better there)
You forgot about high performance computing, chemical engineering, metallurgy, food process engineering, medicine, pharmaceuticals, forensics, .... and a lot more. Mechanical engineering seems to be the one field that persistently wishes to hold on to USC units. I assume they are just to lazy to update some the tables they use to SI units.

Khashishi
The fact that the world uses SI is reason enough to use it over Imperial units. Also, I'm sure more Americans know what a kilogram is than a slug. People associate pounds with mass. Yes, it's possible to educate people to use the proper units, but it's perhaps easier to just go metric.

jerromyjon
mfb
Mentor
For those who are familiar with Greek and Latin prefixes, the whole SI system is inconsistent if you think about it. The "kilogram" is essentially (by the meaning of the kilo-prefix) 1000 grams. Thus, like it or not, the gram is the fundamental unit of mass for SI, not the kilogram.
The US units don't even have such a prefix system. You are complaining about tiny nuisances in one system while ignoring that the other system is not even coherent enough to discuss this question there.
The kilogram is the SI base unit. Don't misrepresent the SI please.
but I have yet to see that one system is any better than the other for work in mechanics (electricity and magnetism is another matter, and SI certainly works better there). I use both equally well, although like @PhanthomJay I hate to have to convert back and forth.
You avoid conversions within the unit system.
The only way to avoid conversions between unit systems is a single standard used worldwide. In other words, adopting the metric system in the US everywhere, because I can guarantee that the rest of the world won't go back to imperial units. It is not an accident that the SI got adopted nearly everywhere.

lekh2003
lekh2003
Gold Member
Regarding "weight" expressed in kilograms, it is NOT TRUE that 1 kilogram is 1 kilogram * 1 meter/s^2, so this is an inconsistent usage. It is essentially the same mistake made by expressing both force and mass in pounds, only reversed. Who is to say they are more interested in their mass than in their weight? Are they more interested in the pull of the earth on their bodies and the resultant ache in their ankles and the soles of their feet? Or are they really thinking about how their momentum will be affected in a collision?
I beg to differ with what you are saying.

Firstly, you are saying that the SI system is arbitrary due to the usage of fudge factors to quantify and make sense of the units. This logic is inherently flawed since everything we observe about the universe is then a fudge factor. We make sense of different things with prefixes and units, because there is no universal system to make everything magically work. The US system and SI system are equally as arbitrary and useless as each other, but if the SI system is easier to use, then let us use it.

Secondly, these so called "inconsistencies" are not a product of the SI system, but a misconception created by the common public without any technical understanding. From a purely technical point of view, I understand the differences in weight and mass, and I'm sure anyone on this forum definitely does. If somebody doesn't, it isn't a fault in the SI system, it is a fault of the people and their linguistic assumptions.

Any units system is a construct of our imagination. This is the way humans understand the world. If one of the methods is easy to convert with, regardless of it's fudge factors, why shouldn't we use it?