In his method the fluid is divided into small masses by taking volume elements. The masses are called fluid particles . The resultant forces on each particle are found. By solving for the position and velocity of all particles as functions of time, the fluid can be quantified. Since fluids flow through volumes and interact with areas, it is more convenient to use the intensities of these parameters.
Density is the intensity of mass and pressure is the intensity of force.
The density of a fluid is its mass per volume. This may be a problem if individual atoms have to be taken into account, but not in the normal macroscopic environment. Density is a scalar quantity and its SI unit is kilogram per cubic metre. The pressure exerted by a fluid on its surroundings is its force per area. In a fluid the force is always at right angles to the surface involved because the fluid cannot sustain a shear.