Consider water flowing from a nozzle to a turbine vane. As the water hits the turbine vane, it exerts a force that causes it to move along the flow of direction. Force is an impact that changes an object's motion, shape, or orientation. Forces can be caused by physical contact, such as a push or pull, or through non-contact interactions, such as magnetic or gravitational forces. Force is a vector quantity with both magnitude and direction, and is measured in newtons (N) in the SI unit system. Consider an object with a mass of 1 kg that moves forward with an acceleration of 1m/s². In this case, the force that is applied to the object equals 1 N.

In mechanics, external forces and internal forces are two types of forces that act on a rigid body under consideration, causing external and internal effects. The external effects are the reactive forces that the body exerts on its surroundings due to external forces like body weight, forces applied by people, and frictional forces. The internal effects are the internal forces and deformations that occur within the material of the body. Consider an example of a bracket subjected to an external force. The external effects of this force on the bracket are the reactive forces that the foundation and bolts exert on the bracket. The internal effects of the force on the bracket are the resulting internal stresses and deformations that occur within the material of the bracket. These internal forces and deformations are distributed throughout the bracket's material and depend on the material properties and geometries of the body.

The principle of transmissibility is a concept that explains the effects of moving the point of application of an applied force along the same line of action. It states that if a force is applied to a rigid body and the point of application of that force changes but remains in the same line of action, the resultant force acting on the body will remain unchanged. Consider two equal forces acting on a rigid body from different points. According to the principle of transmissibility, the forces acting on the body are in the same line of action, making them equivalent to a single force acting at a particular point. When the point of application of these forces is changed, or moves along the same line of action, the resultant force on the body remains the same, since it is the same force acting on the same line of action.