A fluid is defined as a material which does not have a crystalline lattice, like solids do. This allows fluids to be able to flow, to attempt to fit whatever container they are in.
A solid that experiences force can deform. However, fluids instead flow when experiencing force.
In fluids, we use Density and Pressure instead of mass and Force, because in most cases, the flowing nature of fluids means they don’t have a defined mass and force everywhere like rigid solids do.
Fluids at Rest
Because a fluid does not have a rigid structure, and can easily flow (accelerate) the only way a fluid can be at rest is if it does not have any forces parallel to it’s surface.
Properties
Compressibility
Unlike rigid solids, fluids can be compressed. Compression is defined as the increase in the Density of the fluid, and rarefaction is the opposite. Since the formula for density is given by:
In a closed system, the conservation of mass applies, meaning we cannot change . However, we can change .
If a fluid is compressible, then we can change it’s pressure by changing the volume. Else, the fluid is incompressible.