Re: [Units] Incorrect definitions for torque, newton_meter

From: Family Schabel (family_at_[hidden])
Date: 2022-08-29 23:47:47

Hi Bert,

Maybe this link will clarify:

https://physics.stackexchange.com/questions/37881/why-is-torque-not-measured-in-joules

Boost Units explicitly keeps track of angular displacement for this (and similar reasons). The unit of torque really should not be energy (J or N.m) because a torque is a force applied tangentially, so, to compute the amount of energy required to exert a torque one needs to take the product of the force with the angular displacement. The nomenclature is a bit confusing because, as you note, newton_meter is not the same as newton*meter - the convention is that torque is measured in newton meters, but it is really newton meters per radian as encoded in the library. If that weren’t the case, you could equate torque with energy, which is the kind of mistake Boost Units is intended to help prevent. 

Matthias

> On Aug 25, 2022, at 5:27 PM, Albert Dvornik via Boost <boost_at_[hidden]> wrote:
> 
> It looks like my previous message wasn't visible by non-HTML readers
> (including the list archive) because I didn't configure my so-called
> mailer correctly.  Sorry for the duplication and the hassle.
> 
> bert
> 
> --------------------
> Hi, all.
> 
> The definitions of boost::units::torque_dimension,
> boost::units::si::torque, boost::units::si::newton_meter and
> boost::units::si::newton_meters seem broken. The most obvious symptom
> of this is that boost::units::si::newton_meter isn't the same thing as
> boost::units::si::newton * boost::units::si::meter (i.e. an actual N
> m), but is instead defined to be equivalent to *N m / rad* instead.
> 
> I'm attaching a source file that shows the problem. I've compiled it
> with Boost 1.80, g++ 10.2.1.
> 
> Am I confused in some way? If not, then I believe that the fix would
> be to modify boost/units/physical_dimensions/torque.hpp and remove the
> ", plane_angle_base_dimension,-1" in the current definition of
> torque_dimension.
> 
> Regards,
> bert Dvornik
> 
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