* They can flow: This is the most fundamental property of a fluid. They can change shape easily and take the form of their container. Unlike solids, which have a fixed shape, fluids can flow around obstacles and fill any space they occupy.
* They can't resist shear stress: When a force is applied tangentially (like pushing on the surface of a fluid), fluids deform continuously. Solids resist this deformation.
* They are relatively incompressible: While not perfectly incompressible, both air and water resist changes in volume when pressure is applied. This is more pronounced in liquids like water than in gases like air.
Here's a breakdown of how each characteristic applies to air and water:
Air:
* Flow: Air moves freely as wind, flowing around buildings and trees.
* Shear Stress: Air can be easily moved by wind, and it deforms continuously as it moves around objects.
* Incompressibility: Air is compressible, meaning its volume can change significantly under pressure. However, compared to solids, air is still relatively incompressible.
Water:
* Flow: Water flows freely through rivers, pipes, and over waterfalls.
* Shear Stress: Water readily deforms when stirred or moved, exhibiting the characteristic flow of a fluid.
* Incompressibility: Water is much less compressible than air. While its volume can change slightly under pressure, it generally maintains its shape.
Key differences:
* Density: Water is much denser than air. This means that water has a higher mass per unit volume.
* Compressibility: As mentioned above, water is much less compressible than air.
* Viscosity: Water is more viscous than air. This means it resists flow more strongly.
In conclusion, both air and water exhibit the defining characteristics of fluids, which allows them to flow and deform under shear stress. However, they have important differences in density, compressibility, and viscosity.
