The volume of an object is proportional to the third power of its dimension, while the surface area is proportional to the second power. As a result, the effect of surface force becomes larger than that of the body force in the microscopic world. For example, the dominant force in the motion of a microscopic object is not the inertial force but the electrostatic force or viscous force. Material properties of microscopic objects are also affected by the internal material structure and surface, and, as a result, characteristic values are sometimes different from those of bulks. Frictional properties in the microscopic world also differ from those in the macroscopic world. Therefore, those effects must be considered carefully while designing a micromachine.