In this chapter we will mainly be interested in liquids and gases.

p = pa + ρgh.

The pressure in a liquid is due to the physical fact that any given part of a liquid must be supported in equilibrium by the rest of the liquid!

“A cylinder of air reaching to the top of the atmosphere is of equal weight with a cylinder of water about 33 feet high.” --- Isaac Newton

Normal atmospheric pressure is about 105 N/m2. The “electron Fermi pressure” inside an atom or molecule is about 1011 N/m2, which is the pressure with which matter resists being squeezed.


Pascal Principle...


Bernoulli Principle!

The table below shows the density of some common substances, in units of kilograms per cubic meter. Some of these values may certainly seem counter-intuitive ... one would not expect mercury (which is a liquid) to be more dense than iron, for example.

Notice that ice has a lower density than either water (freshwater) or seawater (saltwater), so it will float in them. Seawater, however, has a higher density than freshwater, which means that the seawater will sink when it comes in contact with freshwater. This behavior causes many significant ocean currents and a concern of glacier melting is that it will alter the flow of seawater, not just add more water to the ocean -- all from the basic functioning of density.

To convert the density to grams per cubic centimeter, merely divide the values in the table by 1,000.

Density of Common Substances

Material Density (kg/m3)
Air (1 atm, 20 degrees C 1.20
Aluminum 2,700
Benzene 900
Blood 1,600
Brass 8,600
Concrete 2,000
Copper 8,900
Ethanol 810
Glycerin 1,260
Gold 19,300
Ice 920
Iron 7,800
Lead 11,300
Mercury 13,600
Neutron star 1018
Platinum 21,400
Seawater (Saltwater) 1,030
Silver 10,500
Steel 7,800
Water (Freshwater) 1,000
White dwarf star 1010