Total electromagnetic force and distributed electromagnetic force density acting on electromagnetic materials in electric machinery dynamic systems are fundamental forces that cause physical phenomena such as movement, structural deformation, vibration, and noise. Therefore, they are essential in electromagnetic coupling analysis. As a conventional method for calculating the electromagnetic force and force density, several methods have been suggested, such as the Korteweg-Helmholtz force density method (KH), the Maxwell stress tensor method (MX), the equivalent magnetic charge force density method (MC), and the Kelvin force density method (KV). 

In cases of a relevant material separated by air or empty space, the results of total force by the conventional methods are satisfactory, resulting in the same values. However, when two materials are in contact, the electromagnetic contact force and electromagnetic force distribution are obtained differently in each method, shown as surface force. Therefore it was observed that there is no consistency among the electromagnetic calculation methods. To apply these conventional methods to specific problems where two different materials are in contact with each other, the virtual airgap scheme was recently introduced. In this method, a thin airgap was inserted between materials on the basis of the concept that electromagnetic force was applied as a body force to the material; satisfactory results were obtained. If the virtual airgap scheme is applied, the force on two contacted bodies can be more precisely calculated.