Buoyancy. Photo 1 of 5. The upward buoyancy force on an object immersed in a fluid is equal to the weight of the displaced fluid (Archimedes' principle). It follows then that depending on the density of a solid object relative to the density of a liquid (such as water), the object will either sink or float on the surface. In this photo, polystyrene bits are floating but steel nuts are at the bottom of a beaker filled with water. See BJ3929, BJ3930, BJ3931, BJ3932 for others in the series.