This paper presents a theoretical analysis of the force and moment balance during incipient motion of a single particle. The analysis showed that large particles start their motion by rolling for spherical particles and by sliding for non-spherical particles. It also showed that fine spherical or near spherical powders start their motion by rolling. The theoretical analysis shown that the incipient motion velocity of individual particles for all fluid, system and particle properties might be presented as power relationships between the Reynolds and Archimedes numbers. (c) 2009 Elsevier B.V. All rights reserved.