# Metric system problems

Hi guys. I come from a team in Israel where we use the metric system. While working on a few engine and gear ratio math I noticed stall torque was in lbin. I am used to work in Nm. This seemed weird since lb is weight and N is force. Obviously I can transform lb to kg and then multiply it by g but… That dosn’t sound right. Is there any other measurement refered to by lb? One to do with force maybe?

Also, does anyone have the engines table in metric?

I have always just used Google to change things from metric to standard units. Never really ran in to a problem with it. I have seen some plug in’s in chrome to auto change web pages values From metric to standard and the outer way as long as every thing is noted right.

You are correct, lb is a unit commonly used for weight. However, it is actually just another unit for force just like N (Newtons). With the metric system, Newton’s second law is commonly written out like this:

F=ma
(Newtons) = (kg) * (m/s^2)

In imperial units:

F=ma
(Pounds) = (slugs) * (ft/s^2)

As you can see, Pounds is analogous to Newtons. Kg is actually analogous to an uncommon unit slugs, which is the actual imperial unit for mass, not pounds.

Its easy to get weight and mass confused

R4: The ROBOT weight may not exceed 120 lbs.

FIRST assumes measurements in weight, not mass. This may become an issue in FRC’s inaugural Earth Orbit-based robotics competition. Or, ever so slightly, competition aboard a cargo aircraft

I would simply plug the numbers into google chrome and it will convert units for you (you may have to fiddle with the formatting of the way you type stuff in though).

Maybe the more important observation is that weight is force, by definition. It is never mass, but is always related.

However, it is not uncommon in some places (Japan and the U.S. in particular) to use the name of a unit of force as if it is a unit of mass (or vice versa), often without telling anyone. That’s stupid and wrong. Sometimes, they’ll identify this explicitly, by denoting the unit as lb[sub]m[/sub] (i.e. pounds mass), as opposed to lb[sub]f[/sub] (i.e. pounds force = pounds). That’s a valid workaround, but still a little bit wrong. I try to use the latter approach when communicating with an audience for which the weight-mass distinction is not demonstrably understood.

The distinction between lb[sub]f[/sub] and lb[sub]m[/sub] (or kg[sub]m[/sub] and kg[sub]f[/sub]) is the local gravitational acceleration, which you’ll have to factor in.

Google (in general—not just Chrome) will do unit conversions. And it understands “pounds force”, “pounds mass” and “lbf”, but not “lbm” for some reason. (Same for kg.) Be explicit about it, though, because it assumes “lb” is pounds force.

Welcome to our world.

We generally convert forces an torques to Newtons and Newton-Meters as a first step. We like to use Wolfram Alpha for this. Once your in metric, don’t look back!!!

Pounds can be used for force or mass. If you’re measuring force in pounds, you use slugs for mass. If you’re using pounds for mass, you use poundals for force:

This weirdness (plus the fact that a cubic foot of water weighs 62.4 pounds as opposed to some sensible round number) are the two big reasons why I prefer working in metric.

Interesting, and I thought slugs were uncommon. Never heard of a ‘Poundal’ before. Very cool.

But yeah, can we just use metric please? Its just easier to keep track of…

At Rose-Hulman, we’ve often had problems using lbf(pounds-force) and lbm(pounds-mass).

The conversation ratio was 1lbf = (32.2 ft/s^2)(1lbm).

The cause of many a headache when your error to a large dynamics problem is a small unit error buried deep inside your work…

This may be relevant to the topic, I use this “convert” tool allot, it converts just about anything. Just save it to any computer.

http://wscope.com/convert.htm

It’s a free, standalone executable file where you select what type of conversion you want, what you want for your input/output, and it converts it. There is even a place to create your “custom” conversion.

Hope this helps someone.

Uhh but people using the metric system use it just as wrongly.
When they say “the scale says my current weight is 80kg.”
In this case, weighing oneself in pounds force is more technically accurate. Why don’t the metric scales report weight in Newtons?

While that might be technically correct, Every metric laboratory analytical scale I have seen is calibrated in grams not newtons. They don’t even have a warning sticker to check to be sure gravity is at 1 G. Well just gee whiz.

Spring scales will usually have the words “not legal for trade” on them. Typical “legal for trade” scales are actually balance scales. They measure mass.

Not to mention when somebody builds a robotic blimp for competition.

Most modern analytical scale use some kind of force balance. Such as a strain gauge which is really just a fancy spring. They actually have to be calibrated in place to account for variations in gravity among other things. I remember the old analytical beam balances: little knob light & mirrors. Temperamental things.

I typically convert to SI units for calculations involving mass. It is just easier. As long as you don’t confuse Newton Meters with Joules. :]

Imperial measurments are so silly that The empire dosen’t even use them.

another great way to keep units straight is to use a freeware program called SMath Studio. The have a live version on their webpage (http://en.smath.info/) and try it out. I use it for FRC calcs as well as in my professional life to document calculations for clients. It’s a wonderful tool and functions much like MathCAD.

as a note, units can be inserted with the ’ (apostrophe) key.