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Physics -- Units and Concepts: Bad Units |
I won't discuss Imperial units here, just bad would-be metric units. For a funny item on imperial units see "formats".
The metric system uses only factors that are 1 or powers of ten, such as 10, 100, 1'000, 1'000'000. Any time you see a different factor the unit is not a normal metric unit.
I list here a few commonly used units that are in fact not standard metric units.
Using km/h for car driving speed is very common. The "hour" however is 3600 seconds, so we are talking in terms of 1'000 m / 3600 s or 0.277 m/s instead of in m/s. Not metric: the factor 0.277 is not 1.
Expressing a force in "kilos" is not metric. A force is measured in Newton, N. A kg of mass weighs 9.8 N. A kgp is 9.8 N, the weight of 1 kg. When you say the roof has to withstand the forces of 10 tons of snow on it, you mean it has to stand up to the force with which 10 tons of mass are pulled down by the Earth's gravitational field. That force is 98'000 N or 10'000 kgp.
A calorie or 1cal is the heat needed to make the temperature of 1 g of water go up by 1 °C. The calories you sometimes still see on food packages are actually kilocalories, or 1'000 calories or kcal. Joule showed that heat and work are equivalent, the factor is: 1 cal = 4.18605 J
Energy or work is measured in Joules. If you have a 60W light bulb it consumes 60 Joules of energy every second. The electricity company charges you for the energy. There are 60 x 60 = 3600 seconds in an hour, if you leave your lightbulb on for an hour it will have consumed 60 J/s x 3600 s = 216'000 Joules or 216 kJ. You could also say that this is 60 Joule-hour. An electric heater of 1 kW consumes 1000 J per second and in an hour it burns 3'600'000 J. This is commonly called 1 kiloWatt-hour: the energy consumed by a device of 1kW power left running for an hour. The electricity company charges you by that unit: 1kWh.
1 kWh = 3'600'000 J = 3.6 MJ (three point six mega Joules)
It would be better to charge by the MJ instead, why anyone would think of counting kiloWatts during hours I don't know.
Many people confuse kW and kWh because the "hour" in kWh has nothing to do with a rate. In fact, a device of 1 kW power consumes 1 kWh per hour. Right? Right indeed: a kW is not an amount of energy but a rate of energy consumption; a kWh is an amount of energy.
An old unit of power was the "horsepower", the power needed to lift a weight of 75v kg at the rate of 1v m/s. Note once more that this is not work, it's not the energy needed to lift 75 kg over 1m, but the power to do this work in a single second. Any motor or person can move 75 kg up one metre: they may just have to take time and a lot of pulleys or levers, but eventually they will get there. Only a sufficiently powerful device can do it in a single second. A horsepower is not a metric unit for two reasons: (1) 75 is a factor that is not 1 and (b) the 75 kgp is not a metric unit either as we saw earlier. A horsepower is the work of 75 x 9.8 Nm/s delivered in 1s or 735 W.
A car engine rated at 200 hp can therefore deliver 147 kW.