Pressure is a force per unit of area exerted over the surface of an object (as in 35 psi in the tires on your car). When an object is immersed in water, or some other liquid medium, the fluid pressure exerted on that object varies with the depth at which it is submerged. For example, the volume of water pressing down on an object submerged 10 feet is twice that of an object submerged half that depth.

Fluid Pressure (force per unit area) can therefore be defined as follows:

p=wh ,where w = weight density of the fluid h = depth at which the object is submerged

Fluid Force (total force exerted on object) is therefore given by

F=pA ,where A=total area of surface object in question F=whA

According to Pascal’s Law (principle), an object submerged in a fluid is subjected to equal pressure in all directions (at any given depth). For a sheet of metal submerged in water and resting horizontally at a given depth, the fluid force is constant over its entire surface. If, however, the submerged sheet is resting vertically, the entire force exerted over this object by the water varies with depth; the bottom of the sheet will experience more fluid force than its top. In order to determine the total force acting on this vertically oriented sheet, the force exerted on each rectangular cross-section of infinitesimal width (Δy) will be calculated and summed over the object’s entire vertical span; enter integration. With a well-chosen location for the y-axis in our model, the length of each rectangular cross-section can be easily expressed as some variation of f(y).

 

Rectangular Plate

Fluid ForceRectangularPlate

 

The example above was relatively simple since f(y) was a constant throughout its vertical span. With shapes whose widths are not constant, the mathematical model can once again vary depending on the perspective chosen. In the examples below, circular plates have been introduced since they provide opportunities for multiple forms of substitution in the integration process, thereby maximizing learning opportunities for students.

 

The fluid force acting on the ends of a cylindrical water tank is the subject below. I felt that this would be more interesting than imagining the force exerted on a circular plate submerged in a body of water. To simplify matters here, atmospheric pressure and other factors such as sliding forces have been ignored.

Cylindrical Tank: Half-full (Perspective 1)

tank2

 

Cylindrical Tank: Half-full (Perspective 2)

tank

 

Cylindrical Tank: Filled to Capacity

tank3

 

It is worthwhile drawing attention to the forces acting on the ends of top half of the tank filled to capacity (451.34 lb) and the bottom half filled to half capacity (332.8 lb). This type of thoughtful comparison can add to the students’ understanding of this topic.

 

The two images directly below once again show the two perspectives of fluid forces acting on the ends of our water tank. They are included here to illustrate the contents of the interactive link that follows.

Tank Centered at (0,0)

CircularPlateSubmerged2

 

Tank Centered at (0,-2)

CircularPlateSubmerged2b

Click on the link provided here to interact with fluid force on the ends of a cylindrical tank.

 

In the image and link below, the circular end has been removed from the tank and submerged on its own. Once again, various mathematical models could be used to describe this scenario; the one chosen here has placed the origin at the circle’s center.

CircularPlateSubmerged3

Click on the link provided here to interact with fluid force on a submerged vertical plate with center at origin.

 

The following links will be of interest to some:

Pressure and Buoyancy and Hydrodynamics

 

Thanks for reading.

 

Reference
Larson, R., Hostetler, R. P., & Edwards, B. H. (1995). Calculus of a Single Variable: Early Transcendental Functions. Lexington, MA: D.C. Heath.

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