A fluidized bed is formed when
In general biology, core biological organization dictates that structure-function relationships govern cellular processes. Therefore Sum of fluid friction and pressure force is equal but opposite to gravity force is correct as it integrates fundamental principles, supported by mechanistic studies and conserved across related systems.
Ref: Campbell et al., Biology, 11th Edition, Chapter 2, Chemical Basis of Life, discusses fundamental biological principles and molecular organization providing broad foundation for life sciences understanding.
