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In the photo above, we see the water line of a large cargo ship. For this cargo ship, as for any object submerged or partially submerged in water, the upward force of the water that is displaced by the ship, called buoyancy, is exactly equal to the weight of the displaced ship. This cargo ship is unloaded, which means that the ship sinks into the water about 13 feet and that the water displaced by the volume of the ship underwater will have a weight equal to the weight of the empty ship. As cargo, and therefore weight, is added to the ship, it will submerge further. This ship can submerge another 9 feet as it is loaded with cargo. As it submerges, it displaces more water so that the buoyant force can support more weight. In this chapter, you will learn about the forces involved in fluids, in addition to fluid expansion and the laws governing the behavior of gases.
Fluid pressure equations apply to all fluids, including liquids and gases. Confined liquids are subject to the same pressure throughout, and gaseous fluids exert equal pressures on all sides of an object. As such, objects partially submerged in water are acted upon by a buoyant force, equal to the submerged weight. Finally, gases are subject to laws regarding the relationships between pressure, temperature, and volume; these laws are compiled into the Combined Gas Law and the Ideal Gas Law.