This paper presents an analysis of all forces affecting the void fraction (VF) and density of a particulate bed. The bulk density of many materials, both spherical and non-spherical, with a variety of properties, including particle size and density, were measured in air and various liquids. The Archimedes number (Ar) was found to be an appropriate representation of the particle behavior that considered the van der Waals forces. The effects of buoyancy and viscous forces, as components of Ar, on the VF were shown by experiments in various liquids. Additional forces, such as friction, impact, and tapping were discussed quantitatively and qualitatively. Furthermore, the effect of the particle shape on the VF was considered. Finally, general equations for VF and density were developed for practical use. The major effects of the moisture content and compression pressure on the VF will be discussed in a following paper. (C) 2020 Elsevier B.V. All rights reserved.Y