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 -- Dear All, This is an extension to my previous posting. I adapted some of the pounds and horsepower figures from the internet to produce this 'Rule of thumb'. Rule of thumb If you want a rule of thumb for buying cars, you can calculate the power-to-weight ratio for the cars you're interested in. For example the Australian model Ford Falcon that my wife drives is quite adequate around town and on the highway. It has enough acceleration to avoid most risky overtaking situations. Using this as a rough guideline, anything in the neighbourhood of 100 watts per kilogram should be worth considering as your next vehicle. Of course if money is no object, you might consider some of these other models. The solar model car is inserted for reference only � not for driving to work � yet! Model power mass power/mass Solar model car 6.6 W 1.2 kg 5.5 W/kg Dodge Caravan (4 cyl) 112 kW 1755 kg 64 W/kg Ford Escort 82 kW 1120 kg 73 W/kg Ford Falcon (Australia) 157 kW 1515 kg 104 W/kg Nissan Altima 130 kW 1385 kg 94 W/kg Mitsubishi 3000GT 239 kW 1700 kg 140 W/kg Porsche Carrera 224 kW 1315 kg 170 W/kg Chevrolet Corvette 257 kW 1470 kg 175 W/kg Lotus Esprit V8 261 kW 1380 kg 189 W/kg Shelby Series 1 239 kW 1160 kg 206 W/kg Ferrari 355 F1 280 kW 1350 kg 207 W/kg Dodge Viper 336 kW 1506 kg 223 W/kg Cheers, Pat Naughtin LCAMS Geelong, Australia -- on 2004-07-20 10.04, Pat Naughtin at [EMAIL PROTECTED] wrote: > Dear All, > > Recently, I read that the winning solar model car, with a chassis mass of > 1.2�kilograms, had a power to weight ratio of 5.5�W/kg. This referred to a > solar car challenge held in Melbourne in 2003. > > Does anyone know of equivalent figures for real cars, trucks and planes? > > The only figures that I could find on the internet referred to pounds and > horsepower. > > Cheers, > > Pat Naughtin LCAMS > Geelong, Australia
