1. The density of pure water (H2O, at 20oC) is r water = 1000 kg/m3 = 1.00 g/cm3.
2. An object will float in a liquid which is more dense than that object. The greater the density of the liquid the more volume of the object will be seen above the surface of the liquid. (For example, very little volume of an iceberg is seen above the surface of the water since the density of the water is not much bigger than the density of the ice.) We will be seeing how this works out mathematically later on in this lecture.
3. The average pressure exerted by the earth’s atmosphere (at sea level) is called the earth’s atmospheric pressure, denoted 1 atm or patm. It has a value in MKS units of patm = 1.01 x 10 5 Pa = 1 atm. This pressure is due to the weight of the earth’s atmosphere (per unit area) above sea level. (Be careful of the units here – the unit atm is not an MKS unit! It is used only for convenience in expressing the pressure in terms of a multiple of the earth’s atmospheric pressure. Thus, for example, a pressure of 2 atm is simply a pressure of 2.02 x 105 Pa.)
4. Gauge pressure, pg, is defined to be how much the given pressure is above atmospheric pressure. That is, if the pressure at some point is p, then the gauge pressure at that point is given by pg = p – patm. It is important, however, to keep in mind that, to find the force exerted on an object due to a given pressure, the absolute pressure (that is, the pressure p) must be used, and not just the gauge pressure, pg! That is, F = pA, not F = pgA !