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From: Eric Lemings (lemings_at_[hidden])
Date: 2006-08-23 12:23:56


> -----Original Message-----
> From: boost-bounces_at_[hidden]
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Phil Richards
> Sent: Wednesday, August 23, 2006 8:58 AM
> To: boost_at_[hidden]
> Subject: Re: [boost] Boost Units library preview
> On 2006-08-23, Matthias Troyer <troyer_at_[hidden]> wrote:
> > - Nm would be a "dimension"
> Not really. It is the unit.

Correct. A newton is a derived unit composed of base units,
specifically kilogram, meter, and second. Meter is of course a
base unit.

> The dimension is: MASS * LENGTH^2 * TIME^2

Also correct. Every unit, whether a base unit or derived, has a
corresponding dimension.

> > - 1 Nm would be a "quantity"

No. This is a measure. It has a specific unit and a specific

Here's an example of a quantity. The distance from
here->| |<-to here is a quantity with a
dimension of length. You don't know what its value is exactly
but you do know that it has a value and can be measured. Until
you actually measure this length with a particular unit does the
"quantity" become a "measure". Converting a measure into a
quantity is essentially the same as stripping away the unit but
keeping the dimension.

Does that make sense?

> > - a torque measured in Nm or an energy measured in Nm
> would be two distinct units

Dimensions and units are C++ types. They have no "value" in
the conventional sense.

> > - 1 Nm as torque, 1 Nm as energy or 1 J as energy would be
> > three distinct measures
> I think as far as a dimensionality checking goes, they are
> all the same. As far as a units checking goes, Nm and J
> *could* be distinguished. It's an interesting question as to
> what, exactly, "energy" and "torque" are once you try to
> separate away the dimensionality and units bits... :-)

Yep, if the units are the same and the value is the same then
they are (or should be) considered the same measure. I think
it should be left up to users to interpret these measures and
how they are used.


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