kg/kW could also be expressed as g/W
Pat Naughtin wrote:
Dear All (especially the Physicists),
Recently I posted a message (copy below) where I constructed a unit to show a ratio between power and mass of some common, and some not so common, cars. The unit I chose was watts per kilogram with W/kg as the symbol.
Since then, I have noticed references to another equivalent, but inverse unit to show the ratio between mass and power. This unit is given as kilograms per kilowatt with kg/kW as the symbol.
I suspect that, given mass in kilograms and power in kilowatts (as supplied by the car makers), that the latter unit, kilograms per kilowatt, is easier to calculate. I had to do an extra step to change kilowatts to watts before I could calculate the power/mass ratio in watts per kilogram.
I prefer my construction for a number of reasons even though the calculation is slightly more difficult.
One reason I prefer watts per kilogram is that the power unit, watts, is a derived unit 'with a special name', and as such it is of a more complex conceptual nature than the mass unit, kilogram, which as a base SI unit, is of a simpler concept and form.
Is there a preference in SI, or in ISO, for units that have base units in the denominator, or is there no preference, or guidance, either way?
The reason I ask this question is that I worked in the textile industry for some years and they had got into a real muddle by having 'direct units' and 'indirect' or 'inverse units' for many otherwise simple concepts with simple measures. As an example think of the linear density of a weaving yarn that might be described in metres per kilogram, or in kilograms per metre. Once you have these two choices it is a simple step to add (more or less randomly) some more prefixes to get millimetres per gram, metres per centigram, centimetres per gram, etc, etc, etc. Their life would be a lot simpler if: a they stuck to SI units and preferred SI prefixes (in 1000s), and b they had some guidance as to which way is up (when the choose to use a unit by division).
Cheers,
Pat Naughtin LCAMS
Geelong, Australia
