Fluids are materials that flow. This includes both liquids and gases. The distinction between solid and fluid comes down to how they respond to shear stress. Solids deform a fixed amount and stop. Fluids keep flowing as long as the stress is applied. This continuous deformation under stress is what makes fluid mechanics rich and challenging.
Pressure in a fluid increases with depth. At the surface of a swimming pool, the pressure is atmospheric, about 101,325 pascals. Every 10 meters of water depth adds roughly another atmosphere of pressure. This is why your ears hurt at the bottom of a deep pool and why submarines need reinforced hulls. Our Pressure Calculator handles basic pressure calculations.
Buoyancy is the upward force exerted by a fluid on an immersed object. Archimedes’ principle states that this force equals the weight of the displaced fluid. A steel ship floats because its hull displaces enough water to match its weight, even though steel is denser than water. A helium balloon rises because the displaced air weighs more than the helium. Our Buoyancy Calculator computes buoyant forces for any combination of fluid and object.
Density is the key property connecting mass and volume. Water has a density of 1,000 kg per cubic meter. Air is about 1.225 kg per cubic meter. Gold is 19,300 kg per cubic meter. These numbers explain a lot. Why ships float, why hot air rises, why oil floats on water. Our Density Calculator computes density from mass and volume.
Bernoulli’s principle states that in a steady flow, an increase in velocity corresponds to a decrease in pressure. This is why airplane wings generate lift. The curved upper surface forces air to travel faster, reducing pressure above the wing compared to below it. It is also why a spray bottle works: the fast-moving air over the tube creates low pressure that draws liquid up. Carburetors in older cars use the same effect.
Viscosity is a fluid’s resistance to flow. Honey is viscous. Water is not. Motor oil is designed to have the right viscosity at operating temperature. Too thin, and it fails to lubricate. Too thick, and the engine wastes energy pumping it. The Reynolds number predicts whether flow will be laminar (smooth) or turbulent (chaotic). Laminar flow is orderly, with parallel streamlines. Turbulent flow is messy, with eddies and swirls. Most real flows are turbulent.