Liquids and gases can flow and are therefore called fluids. It is this property that distinguishes liquids and gases from solids in a basic way. Fluids: Fluids are those substances which can flow when an external force is applied on it. Liquids and gases are fluids.
Fluids do not have finite shape but takes the shape of the containing vessel, the total normal force exerted by liquid at rest on a given surface is called thrust of liquid. The SI unit of thrust is newton.
In fluid mechanics the following properties of fluid would be considered
When the fluid is at rest – hydrostatics
When the fluid is in motion – hydrodynamics
The pressure is a scalar quantity. The definition of the pressure as “force per unit area” may give one false impression that pressure is a vector. The “force” in the numerator of the definition is the component of the force normal to the area upon which it is impressed.
As the temperature rises, the atoms of the liquid become more mobile and the coefficient of viscosity η falls. In a gas, the temperature rise increases the random motion of atoms, and η increases.