The principle of virtual work is an essential concept in the field of mechanics and engineering. This is used to solve problems related to the equilibrium of a structure or system. It is based on the assumption that if a system is in equilibrium, the work done by all the forces during a virtual displacement is zero. This principle is applied by considering virtual displacements of the system and the corresponding work done by internal and external forces.

To apply the principle of virtual work, it is necessary to express the position coordinates of the internal and external forces in terms of a common coordinate parameter. This parameter is typically an angle, distance, or displacement that is relevant to the problem at hand.

Once the coordinates have been expressed, the virtual displacements corresponding to each of the forces are found by differentiating the coordinate expressions. The virtual displacements represent hypothetical displacements that the system could undergo, and they allow us to calculate the virtual work done by each force acting on the system.

The external force acting on the system does positive work when it is moving in the direction of its virtual displacement, while the internal forces do negative work when they are opposing their virtual displacements.

To solve a problem using the principle of virtual work, first, identify the unknown forces and moments that exist within the system. Then evaluate the work done by these forces and moments during a hypothetical virtual displacement of the system. The virtual displacement is very small and represents a hypothetical displacement that the system undergoes. This allows the determination of each force and moment acting on the system and the work done by them during this virtual displacement.

Next, write down the virtual-work equation for the system using the principle of virtual work. This equation considers the virtual displacements and corresponding work done by each force and moment acting on the system. Finally, solving the virtual work equation gives the values of the unknown forces and moments in the system.

The principle of virtual work is beneficial for solving problems related to complex systems involving many interconnected parts. It is widely used in engineering and mechanics to calculate a structure or system's unknown forces and moments.