If you have tried to push a beach ball under the water you have experienced how the water pushes back with a strong upward force. This upward force is called the buoyant force , and all fluids apply such a force to objects that are immersed in them. The buoyant force exists because fluid pressure is larger at greater depths. An object immersed in water generates a downward force , where is the area of its top surface. The object also generates an upward force , where is the area of its bottom surface. (For clarity, the object has same top and bottom areas.) Since the pressure is greater at greater depths, the upward force exceeds the downward force. Therefore, the liquid applies to the object a net upward force, or buoyant force, whose magnitude is : This is true because is equal to the downward force plus the weight of the liquid affected by and . Substituting ; is the volume of liquid that the cylinder moves aside or displaces in being submerged. denotes the density of the liquid, not the density of the object. Therefore gives the mass of the displaced fluid, so that the buoyant force equals , the weight of the displaced fluid. In conclusion, any fluid applies a buoyant force to an object that is partially or completely immersed in it ; the magnitude of the buoyant force equals the weight of the fluid that the object displaces. This is called the Archimedes' Principle and it is written in an equation : 1. Magnitude of the buoyant force. 2. Weight of the displaced fluid.