Short, plain-English meanings of the terms used in the lessons.
- Absolute pressure
Pressure measured from a perfect vacuum. Absolute pressure equals gauge pressure plus atmospheric pressure.
- Archimedes’ principle
An object in a fluid is pushed up by a force equal to the weight of the fluid it displaces.
- Atmospheric pressure
The pressure of the air around us, about 101.3 kPa at sea level.
- Buoyant force
The upward push a fluid exerts on an object placed in it, equal to the weight of the fluid the object pushes aside.
- Capillary action
Liquid rising or falling in a narrow tube or gap because of surface tension and the attraction between the liquid and the wall.
- Contact angle
The angle a liquid surface makes where it meets a solid wall. Below 90° the liquid wets the wall; above 90° it does not.
- Density
Mass per unit volume, ρ = m ÷ V, measured in kg/m³. Fresh water is about 1000 kg/m³.
- Fluid
Anything that flows and takes the shape of its container: liquids and gases. A fluid keeps deforming as long as a sideways force (shear) acts on it.
- Gauge pressure
Pressure measured above the surrounding air pressure. A tyre gauge reads gauge pressure.
- Hydrostatic pressure
Pressure in a fluid at rest caused by the weight of the fluid above, p = ρ g h.
- Kinematic viscosity
Dynamic viscosity divided by density, ν = μ ÷ ρ, measured in m²/s. It describes how easily a fluid flows under its own weight.
- Newtonian fluid
A fluid whose viscosity stays the same however fast it is sheared, such as water and most oils.
- Non-Newtonian fluid
A fluid whose viscosity changes with how hard or fast it is pushed, such as ketchup, paint or a cornflour and water mix.
- Pascal’s law
Pressure added to an enclosed fluid is passed on equally to every part of the fluid and the walls of its container.
- Pressure
Force per unit area, p = F ÷ A, measured in pascals (Pa). 1 kPa is 1000 Pa.
- Reynolds number
A number, Re = ρ v D ÷ μ, that tells whether a flow is smooth (low Re) or turbulent (high Re).
- Specific gravity
Density of a substance divided by the density of water. Steel is about 7.85, so it is 7.85 times denser than water.
- Specific weight
Weight per unit volume, γ = ρ g, measured in N/m³. Fresh water is about 9810 N/m³.
- Surface tension
The pull that makes a liquid surface act like a stretched sheet, measured in N/m. Water is about 0.073 N/m at room temperature.
- Terminal velocity
The steady speed a falling object reaches when drag and buoyancy balance its weight.
- Viscosity
Internal friction of a fluid, its resistance to flowing. Dynamic viscosity μ is measured in pascal seconds (Pa·s).
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