A soil sample has a total unit weight of 125 lb/ft3. It is desired to find its total unit weight in kN/m3 and its density in kg/m3. Although the problem can be worked using a chain of conversion factors, a simpler approach is to consider that the unit weight and density of the soil sample have a constant ratio to the unit weight and density of water. Placing the unit weight or density of water in any system of units in both the numerator and denominator of a fraction forms an equality. (It is assumed that the problem is on the planet Earth and a mass of 1000 kg weighs 9.81 kN!) Thus,
