# Hydrostatics

## 8. Hydrostatics

### 8.1 Hydrostatics as a part of mechanics

• Hydrostatics is the study of incompressible fluids that are at rest and in equilibrium.
• Mechanics also includes the study of fluids in motion, fluid dynamics, but this is one of the most complex branches of mechanics and will not be studied on this course.

### 8.2 Density

• Density = mass / volume ρ = m/V
• In the SI system, density is measured in units of kilograms per cubic meter (kg/m3).
• The density of water = 1000 kg/m3 and the density of air = 1.2 kg/m3 (atmospheric pressure, 20°C).

### 8.3 Pressure

• The application of a force over an area produces pressure.
• When the force is perpendicular to the surface area, the pressure equals the force divided by the area over which it acts.
• Pressure = Force / Area P = F/A
• Pressure is a scalar quantity measured in N/m2 or pascals (Pa) in the SI system
• A pressure of 1 pascal (Pa) is equivalent to a force of 1 newton acting on each square meter.
• Pressure can also be measured in atm, bars, millibars, mm Hg, psi ...
• Atmospheric pressure is surprisingly high: just over 100 kPa

### 8.4 Archimedes' and Pascal's Principles

• Archimedes' principle: any completely or partially submerged object experiences an upthrust or buoyant force equal to the weight of the fluid displaced by the object.
• Pascal's principle: The pressure applied at one point in an enclosed fluid under equilibrium conditions is transmitted equally to all parts of the fluid.