We have developed a unique particle-separation technique based on size that uses kHz-band ultrasound irradiation in water. Dispersed-millimeter-size particles in dissolved-gases water form themselves into a spherically flocculated particle swarm (SFPS). With the changes of ultrasound irradiation properties, the particles are separated according to their sizes. We previously investigated the characteristics of an SFPS and elucidated its formation mechanism. To achieve an efficient and precise particle-separation technique, the forces acting on the particles during the transition state of the ultrasound modulation must be determined, but it is difficult to clarify the forces acting on each particle. Herein, we experimentally and systematically investigated the forces acting on a single particle trapped in the sound pressure field. We discuss nine types of forces acting on the particle and the bubble adhering to its surface. Under the static state, the particle buoyancy was counterbalanced with the acoustic radiation force acting on the acoustic cavitation-oriented bubbles (ACOBs). We examined two types of amplitude change: a gradual amplitude change of the input power of a transducer, and a step-like amplitude change. The results revealed that the particle motion depends on a subtle balance between the acoustic radiation force acting on the ACOBs and unsteady fluid-dynamical forces acting on the particle. We also demonstrate the classification of particles by diameter by controlling the transition state of the amplitude change of the ultra-sound input power. We conclude that the gradual amplitude change provided an efficient and precise classification of particles by their diameters. (C) 2019 Published by Elsevier Ltd.