Concurrent force system:
It state that if force acting at a point on a rigid body, it can be considered to act at any other point on its line of action, given this point is rigidly connected with the body. The external effect of force on body remains unchanged. The problems which are based on concurrent force system are solved by application of this principle.
For example, consider force 'F' acting at a point 'O' on a rigid body as shown in fig(a). On this rigid body," There is another point O1 in the line of action of force 'F' Suppose at this point O1 two equal and opposite forces F_{1} and F_{2} (each equal to F and collinear with F) are applied as shown in fig(b).The
force F and F_{2} being equal and opposite will cancel out each other, leaving a force F_{1} at point O_{1} as shown in fig(b).But force F_{1} is equal to force F.
The force F acting at point O has been transferred to point O1 that is along the line of action of force F without changing effect of the force on rigid body. Thus any force acting at a point on rigid body can be transmitted to act at any other point along its line of action without changing the effect of it on the rigid body. This proves principle of transmissibility of forces.