Understanding the concept of the centre of pressure is crucial when dealing with fluid pressure on submerged or immersed surfaces. Let's analyze the given options and define the centre of pressure.

Understanding Fluid Pressure and Forces on Immersed Surfaces

When a surface is immersed in a fluid, the fluid exerts pressure on that surface. This pressure is distributed over the entire area of the surface. The force due to this pressure acts perpendicular to the surface at every point. The sum of all these tiny pressure forces acting on the surface results in a single, equivalent force called the resultant pressure force.

What is the Centre of Pressure?

The centre of pressure is the specific point on the immersed surface where this single resultant pressure force acts. It is the point where if we were to apply a single force equal to the resultant pressure force, it would have the same effect (both in magnitude and moment) as the distributed pressure over the entire surface.

Analyzing the Options

Let's look at the options in light of this definition:

• Option 1: It is the point where the weight of body the acts. This point is known as the centre of gravity or centroid (if considering the geometry of the area). The weight of the body itself acts through its centre of gravity, which is generally different from the point where the fluid pressure force acts on the surface.
• Option 2: It is the point where the resultant pressure acts. As discussed above, this is the precise definition of the centre of pressure. It is the single point on the surface where the total force due to the fluid pressure can be considered to act.
• Option 3: It is the point where the weight of the body and resultant pressure acts. The centre of gravity (where weight acts) and the centre of pressure (where resultant pressure acts) are typically at different locations, especially for submerged surfaces where pressure varies with depth.
• Option 4: It is the point where the normal pressure acts. Pressure acts normally (perpendicularly) at every point on the surface. The centre of pressure is not just any point where normal pressure acts, but the specific point where the resultant of all these normal pressure forces acts.

Location of Centre of Pressure

The pressure exerted by a fluid increases with depth. Therefore, for a submerged plane surface, the pressure is higher at points deeper down. This non-uniform pressure distribution means the resultant force acts at a point below the centroid of the area. The centre of pressure is always below the centroid for a submerged plane surface unless the surface is horizontal (in which case pressure is uniform, and the centre of pressure coincides with the centroid).

Conclusion

Based on the definitions and analysis, the correct statement about the centre of pressure on an immersed surface is that it is the point where the resultant pressure acts.