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# 16: Hydrostatics

This relatively short chapter deals with the pressure under the surface of an incompressible fluid, which in practice means a liquid, which, compared with a gas, is nearly, if not quite, incompressible. It also deals with Archimedes’ principle and the equilibrium of floating bodies. The chapter is perhaps a little less demanding than some of the other chapters, though it will assume a familiarity with the concepts of centroids and radius of gyration, which are dealt with in Chapters 1 and 2.

• 16.1: Introduction to Hydrostatics
This relatively short chapter deals with the pressure under the surface of an incompressible fluid, which in practice means a liquid, which, compared with a gas, is nearly, if not quite, incompressible. It also deals with Archimedes’ principle and the equilibrium of floating bodies. The chapter is perhaps a little less demanding than some of the other chapters, though it will assume a familiarity with the concepts of centroids and radius of gyration, which are dealt with in Chapters 1 and 2.
• 16.2: Density
There is little to be said about density other than to define it as mass per unit volume. However, this expression does not literally mean the mass of a cubic metre, for after all a cubic metre is a large volume, and the density may well vary from point to point throughout the volume. Density is an intensive quantity in the thermodynamical sense, and is defined at every point.
• 16.3: Pressure
Pressure is force per unit area. no particular direction associated with pressure – it acts in all directions – and it is a scalar quantity. The SI unit is the pascal (Pa), which is a pressure of one newton per square metre.
• 16.4: Pressure on a Horizontal Surface. Pressure at Depth
The pressure is the same at all points at the same horizontal level within a homogeneous incompressible fluid. This seemingly trivial statement may sometimes be worth remembering under the stress of examination conditions. Thus, let’s look at an example.
• 16.5: Pressure on a Vertical Surface
The total force on a submerged vertical or inclined plane surface is equal to the area of the surface times the depth of the centroid.
• 16.6: Centre of Pressure
If you were to replace all of these forces by a single force such that the (first) moment of this force about a line through the surface of the fluid is the same as the (first) moment of all the actual forces, where would you place this single force? You would place it at the centre of pressure.
• 16.7: Archimedes' Principle
If a body is floating on the surface, the hydrostatic upthrust, as well as being equal to the weight of fluid displaced, is also equal to the weight of the body.
• 16.8: Some Simple Examples
As we pointed out in the introduction to this chapter, this chapter is less demanding than some of the others, and indeed it has been quite trivial so far. Just to show how easy the topic is, here are a few quick examples.
• 16.9: Floating Bodies
We can start with an observation that we have already made that if a body is freely floating, the hydrostatic upthrust is equal to the weight of the body.

Thumbnail: Iceberg in the Arctic with its underside exposed. The volmune of ice below the surface pushed the ice above the surface; this is an example of Archimedes Principle. Image used with permission (CC BY-SA 4.0; AWeith).