Re: [Boost-users] [Units] Can't assign equation

Subject: Re: [Boost-users] [Units] Can't assign equation
From: Matthias Schabel (boost_at_[hidden])
Date: 2009-01-21 11:50:02

> Why does first work without a conversion and second doesn't?  Also,
> how do I get second to work?  I assume this will require some type of
> explicit conversion, but wrapping the right-hand side with
> quantity<velocity>(...) doesn't work.
>
> quantity<velocity> nextVelocity(quantity<velocity> old)
> {
>    quantity<velocity> first = (pi / 6.0 * pow<3>(particleDiameter) *
> gravity * (densityWater - densityGold)) * deltaTime / particleMass;
>    quantity<velocity> second = (pi / 8.0 * densityWater *
> dragCoefficient * old * root<2>(pow<2>(old))) * deltaTime /
> particleMass;
>    ...
> }

The second equation is dimensionally incorrect; i.e. the library is  
doing what it should be doing : telling you that you've made a mistake  
in your equation. In particular, the first equation gives

m^3 (m s^-2) (kg m^-3) s kg^-1 -> m s^-1

while the second gives

(kg m^-3) (m s^-1)^2 s kg^-1 -> m^-1 s^-1

Matthias