When an object is immersed in a fluid, such as a liquid or a gas, it experiences an upward force known as buoyancy. Buoyancy is the upward force exerted on an object submerged in a fluid and is a result of the pressure difference between the top and bottom of the object.
The buoyant force is caused by the pressure exerted by the fluid on the object. The pressure increases with depth in a fluid due to the weight of the fluid above. Therefore, the pressure at the bottom of the object is higher than at the top, resulting in a net upward force.
According to Archimedes' principle, the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object. In other words, the buoyant force is proportional to the volume of the fluid displaced by the object. This principle explains why objects that are less dense than the fluid they are immersed in will float, while objects denser than the fluid will sink.
In summary, the upward force acting on an object immersed in a fluid is the buoyant force, which is a result of the pressure difference between the top and bottom of the object.