Gases and liquids are fluids, which means their atoms aren’t fixed relative to each other. They can’t resist shear forces, and instead flow when forces are applied. Fluids’ resistance to flow is called viscosity.Â
When fluids are confined in a container, they exert a pressure on the bounds. Pressure is the total force divided by the surface area. The SI unit of pressure is 1 N/m^2.Â
For gases, pressure is related to the volume of the container, and its temperature with the formula PV = nRT.
Liquids are incompressible, meaning the volume is nearly fixed independently of the pressure.
Pascal’s Principle
Net force of fluid must be zero in total, as on average fluid in a container isn’t moving. The vertical force must also be equal to the force on gravity.Â
Pressure must be the same in all directions, and doesn’t depend on direction in a fluid.Â
Pressure can also be resultant of external forces on a fluid. Any external pressure adds to the pressure in a fluid at all points. This is known as Pascal’s principle. Pressure in a fluid depends only on pressure at all points and pressure due to gravity. This principle is the basis of how hydraulic systems work, and the fact that liquids are incompressible.Â
Archimedes’ principle
Buoyant force is equal to weight of fluid displaced.Â
Volume of solid objects can be estimated from water displaced, and along with their masses can be used to determine density. Objects sink if their mass is greater than the mass of the water displaced, but the amount of water displaced is only due to the object’s volume.**