Pressure

A fluid is a substance that offers no permanent resistance to deforming force. Fluids include liquids and gases.
Pressure is defined as force or thrust acting per unit area, the force being at right angles to the area i.e. Pressure=Force/Area. The SI unit for pressure is pascal (Pa) which equals one newton per square metre.
A pressure exerted on us and over the surface of the earth by air molecules in the atmosphere is known as the atmospheric pressure. The atmospheric pressure decreases with the increase in altitude and its unit is the atmosphere (atm) equivalent to 760mmHg or 101325Pa.
Pascal’s principle: “Pressure exerted on a contained fluid is transmitted undiminished throughout the fluid, acting in all directions and perpendicular to the walls of the container”. This has applications in hydraulic machines and jacks, syringe, drinking straws, etc.
Gauge pressure is the pressure relative to atmospheric pressure.
Absolute pressure is the sum of gauge pressure and atmospheric pressure.
Open-tube manometers have U-shaped tubes and one end is always open. It is used to measure pressure.
A mercury barometer is a device that measures atmospheric pressure.
Archimedes’ principle: “When a body is wholly or partly submerged in a fluid the upthrust equals the weight of fluid displaced (i.e. pushed aside)”. This principle is applied in ships designing, hot air balloon, submarine, etc.
Principle of floatation: “A floating body displaces its own weight of fluid.” A material floats in a fluid if its density is the same as or less than that of the fluid.
The pressure of fluids increases with depth, i.e. P=hgρ