Explanation:

When a force is applied to a body, it creates a moment about a fixed point. The moment is a measure of the tendency of the force to rotate the body around that point. In the context of a bending moment diagram, the force applied at any point along its line of action will create the same moment about a fixed point P.

Key Points:
- Moment: The moment of a force about a point is the product of the force and the perpendicular distance from the point to the line of action of the force. It is given by the equation M = F * d, where M is the moment, F is the force, and d is the perpendicular distance.
- Bending Moment Diagram: A bending moment diagram is a graphical representation of the bending moments along a beam or a structural element. It shows the variation of the bending moment along the length of the element. The bending moment at any point is equal to the sum of the moments of all the forces on one side of that point.
- Line of Action: The line of action of a force is the straight line along which the force acts. It is the line that passes through the point of application of the force and is parallel to the direction of the force.
- Fixed Point: A fixed point is a point in space that does not move or rotate. In the context of a bending moment diagram, it is a point on the beam or the structural element that is chosen as a reference point for calculating the bending moments.

Explanation:
If a force applied at any point along its line of action creates the same moment about a fixed point P, it means that the force is creating a couple. A couple is a pair of forces that are equal in magnitude, opposite in direction, and parallel to each other but do not share the same line of action.

In the context of a bending moment diagram, a couple is formed when there are two equal and opposite forces acting at different points along the line of action of the force. The moment created by these two forces is the same about any fixed point P.

Therefore, the force in the bending moment diagram is said to be a sliding vector. A sliding vector is a force that can be moved along its line of action without changing the moment about a fixed point.

In conclusion, if a force applied at any point in its line of action creates the same moment about any fixed point, then the force in the bending moment diagram is said to be a sliding vector.