[LON-CAPA-users] Should we accept Nm as a unit of torque

Gerd Kortemeyer lon-capa-users@mail.lon-capa.org
Sun, 29 Jul 2007 17:01:02 -0400


On Jul 25, 2007, at 6:54 AM, Peter Riegler wrote:

> I am working with a colleague from the mechanical engineering  
> department here. As things are, they quite often have problems  
> related to torque und use numerical response expecting torque as an  
> answer.
> Now, the appropriate SI-unit is N*m which loncapa accepts, of  
> course. However, it does not accept Nm. From a syntax point of  
> view, this is probably o.k., since I guess the commonly practiced  
> rules are:
> o Product of units are to be written as products. The  
> multiplication operator can be replaced by an appropriate amount of  
> space.
> (So kg*m/s^2, kg m/s^2 will be accepted).
> o order of magnitude prefixes are to be attached in front (kg, mg,  
> km, MW, kW, etc.)

Yes, those are the rules.

> Now when we come to N*m I bet most of us will violate these rules.  
> Please test yourself and write this unit on a piece of paper.

Yes, that's because there is no SI unit for torque, and we are very  
used to saying Newtonmeter. Hmm ... sort of strange borderline case.  
I don't want the students to get used to not using the "*", since  
they might start to expect that behavior. And getting the unit wrong  
does not charge a try, and likely the next answer they would enter  
would be "N*m" anyway.

> Do you have inserted as many space between N and m as you would  
> have between kg and m in kg m/s^2? For all people I have tested the  
> answer is no.
> The very intention of this email is to trigger a discussion about  
> whether it is suitable to allow Nm as an equivalent to N*m in loncapa.


> PS: Thinking about it, Js for angular momenta is a related issue.

Hmm, Jouleseconds don't quite roll off the tongue like Newtonmeters,  
at least not for me. Also, "force times force arm" naturally results  
in Nm, while the connection of angular momentum to "energy times  
time" is not so obvious: kg*m^2*1/s=kg*m^2/s^2*s ...

I can see doing Nm, and while we are at it, also Wh (to immediately  
get kWh and MWh) ... but I am not sure if we should really set that  

- Gerd.