Hydrostatic Forces Decoded: An Advanced Insight

Deep Dive into Hydrostatic Principles Shaping Modern Engineering and Technology
January 25, 2024

Hydrostatics, the branch of fluid mechanics dealing with fluids at rest, has a fundamental place in engineering because it affects a vast array of principles and applications across multiple disciplines. 

To name a few in civil engineering, hydrostatics is vital for designing dams, aqueducts, and flood control systems. It influences how these structures are constructed, ensuring they can withstand the pressures exerted by static fluids. In mechanical engineering hydrostatics principles are employed in the design of hydraulic systems like pumps and pistons, which are essential in machines and vehicles.

What Are Plane Surfaces? 

Plane surfaces in the context of hydrostatics are flat, two-dimensional surfaces. These surfaces are characterised by having the same angle with respect to the horizontal in all points, meaning they do not curve or bend.

What Equation Is Used for Calculating Hydrostatic Forces for Plane Surfaces:

The key equation for calculating the hydrostatic force  exerted by a fluid on a plane surface is given by:


  • γ – is the specific weight of the fluid, which is the weight per unit volume (often expressed in units like N/m³ or lb/ft³).
  • h – is the depth of the fluid, or the vertical distance from the surface to the fluid’s free surface level.
  • A – is the area of the submerged surface upon which the force is acting.

This equation assumes that the fluid is incompressible and that the depth (h) remains constant across the entire surface area. For surfaces that are not horizontal, the calculation of the hydrostatic force can be more complex and may require integration to account for the variation of pressure over the surface.

What Are Curved Plane Surfaces and How Do We Calculate Their Hydrostatic Forces?

Curved surfaces are significant because the pressure exerted by a fluid at rest varies at each point due to changes in depth. Since hydrostatic pressure increases with depth, the pressure is not uniformly distributed across a curved surface.

The relevance of plane curved surfaces in hydrostatic calculations lies in the need to accurately determine the resultant force and its point of action on the surface. This involves integrating the varying pressure over the entire surface to find the total hydrostatic force. Additionally, engineers must locate the centre of pressure, which is the single point where the total hydrostatic force can be considered to act, causing the same moment as the actual pressure distribution.

Worked Example of Calculating Hydrostatic Forces on a Curved Plane Surface:

Let’s consider the example of a curved plane surface in the form of an arc of a circle submerged in water. This arc forms part of a cylindrical tank’s wall. To calculate the hydrostatic force on this curved surface, we’ll need to integrate the pressure across the surface.

Assume the following:

  • The radius of the cylindrical tank is 3 meters.
  • The width of the arc (into the page) is 1 meter.
  • The arc is fully submerged and centered vertically in the water.

We will need to do the following steps : 

  1. Pressure Formula: Pressure at a depth  is given by p = γh, where γ is the specific weight of water.
  2. Differential Force: The force on a small horizontal strip of the arc at depth  is dF = p x dA, where dA = w x dh is the area of the strip.
  3. Force Expression: Substituting the expressions for p and dA into dF, we get dF = γhwdh
  4. Integration: The total hydrostatic force  exerted on the arc is found by integrating  from the top depth  to the bottom depth  of the arc. The integration is performed over the height of the arc, which in this example ranges from the water surface (0 meters) to twice the radius of the tank (6 meters).
  5. Calculation: The integral of γhwdh from 0  to 6 meters is evaluated. For water γ = 9810 N/M3 and w =1m .

Result: The integration yields the total force , which calculates to approximately 176,580 N.

This calculation provides the hydrostatic force exerted by the water on the curved surface of the arc, taking into account the increasing pressure with depth.

What Are Layered Fluids

Layered fluids in hydrostatics refer to a scenario where multiple immiscible fluids (fluids that do not mix) with different densities are layered one above the other in a container. The concept is particularly relevant in natural settings (like oceans with layers of varying salinity or temperature) and in industrial applications (such as oil over water in a storage tank).

In hydrostatic calculations, the presence of layered fluids significantly impacts how the total pressure at a given depth is computed, as each fluid layer contributes differently to the pressure based on its density and depth. 

Example of Calculation of Hydrostatic Force on a Surface for a Layered Fluid:

Let’s consider an example where we calculate the hydrostatic force on a vertical wall in a tank that contains two immiscible fluids layered on top of each other, such as oil over water.


Water layer height = 4 meters

Oil layer height = 2 meters

Wall width = 5 meters

Specific weight of water = 9810 N/m³

Specific weight of oil = 8500 N/m³

To calculate the hydrostatic force of water: 


To calculate the hydrostatic force of oil :

Adding these 2 and you get the answer of 817400 

Application of Hydrostatics in Mechanical Engineering:

hydrostatics principles are employed in the design of hydraulic systems like pumps and pistons, which are essential in machines and vehicles. Understanding the transmission of force through fluids enables the creation of complex machinery that can perform heavy lifting and precise movements with hydraulic actuators.

Application of Hydrostatics in Environmental Engineering:

Environmental engineering too relies heavily on hydrostatics, particularly in the management of water resources, wastewater treatment, and the containment of hazardous materials. Calculating the forces exerted by stationary fluids helps in designing tanks and barriers to prevent leaks and spills, which is crucial for protecting the environment.

Applications of Hydrostatics in Naval Architecture:

The fundamentals of hydrostatics also extend into naval architecture and marine engineering, where the buoyancy and stability of vessels are paramount. Engineers must calculate the forces acting on ship hulls to design vessels that can float and move efficiently through water. In aerospace engineering, hydrostatic principles find a place in designing fuel tanks for rockets, where fluid behavior in low-gravity environments is a critical consideration.

Energy Harvesting Systems an Innovations Using Hydrostatics:

There’s ongoing research in using hydrostatic pressure differences in natural water bodies to generate renewable energy, which could open new avenues for sustainable energy resources.

The concept of generating energy from hydrostatic pressure differences is based on exploiting the potential energy available due to varying water depths. In simple terms, the greater the depth, the higher the hydrostatic pressure. This pressure difference can be harnessed to produce energy, similar to how hydroelectric power utilizes gravitational potential energy.

It involves 2 Technologies:

Pressure Retarded Osmosis (PRO):

This method involves using semipermeable membranes that allow water to pass through but block salt. When freshwater from a river meets seawater in an estuary, the osmotic pressure difference can be used to generate power.

Reverse ElectroDialysis (RED): 

RED uses membranes that allow ions to pass but not water. By alternating membranes that selectively allow positive and negative ions to pass, a potential difference is created that can be used to generate electricity.

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