Pressure is defined as the effect of a force on a given area and can be calculated using Pressure = Force/Area. The unis for Pressure is Pascals, Pa.
Liquids and gases are both fluids, since they can change shape and flow. Fluids exert forces on any surface perpendicular to it. These forces cause a pressure to act on the given surface. Atmospheric pressure is the pressure that the atmosphere exerts on everything around us and it changes depending on the altitude. Liquids similarly exert pressure on objects which depends on the depth, caused by the weight of the column of liquid above the object and it acts in all directions. The pressure in liquids can be calculated using: Pressure = density of liquid × height of column × gravitational field strength. An object that is submerged in water experiences pressure from both above and below. Since the pressure from below is greater, this creates an upwards resultant force called upthrust, which can be calculated using Upthrust = density × gravitational field strength × Volume of fluid displaced.
Archimides' principle states that an object either partially or fully submerged in a fluid feels an upwards force called upthurst, which is equal to the weight of the fluid displaced by the object. If the upthrust is greater than the weight, the object will float. If it is not, the object will sink.