Calculus Of One Real Variable By Pheng Kim Ving
Chapter 12: Applications Of The Integral Section 12.9: Force Exerted By A Fluid

 

12.9
Force Exerted By A Fluid

 

 

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1. Pressure And Force Exerted By A Fluid

 

 

 

 

 

 

According to Pascal's principle, the molecules in a fluid interact in such a way that the pressure of the fluid at any depth h
acts equally in all directions. For example, the pressure on a horizontal plane region and that on a vertical one at the
same depth are the same.

 

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2. Finding The Force Exerted By A Fluid

 

For a horizontal plane region submerged in a fluid, the pressure of the fluid on it is constant throughout the region. So the
total force F exerted by the fluid is found by multiplying the pressure p of the fluid by the area A of the region: F = pA.
For a non-horizontal plane region, the pressure isn't constant throughout the region, and thus the total force can't be
found so easily.

 

 

Fig. 2.1

 

Total force is approximated by sum of forces on strips.

 

 

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3. Total Force As Integral Of Differential Force

 

 

Noting that p dA is the differential force dF exerted by the fluid on the differential strip of area dA at depth h we see that
the total force is the integral of the differential force:

 

 

 

 

Remark that Eq. [3.1] also applies if the pressure is constant throughout the plane region:

 

 

where A is the area of the plane region.

 

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4. Practical Treatment

 

In practice, Eq. [3.1] allows us to find the force exerted by a fluid on a plane region by simply considering differential
elements and forming the appropriate integral, without going thru all the steps that lead to that equation. This aspect is
similar to that of work done by a force as discussed in Section 12.8 Part 2. For example, let's again find the force F in
the problem posed in Part 2, this time using the practical treatment.

 

Example 4.1

 

 

Solution

 

Fig. 4.1

 

Total force is integral of differential force.

EOS

 

Note that the differential rectangle is horizontal. For a vertical differential rectangle, the pressure isn't the same from 1
point on it to another at a different depth, so we can't use the formula (force on rectangle) = (pressure) x (area of
rectangle), thus the vertical rectangle can't be used. Remark that for it, the variable would be L and its width would be
dL.

 

Example 4.2

 

As shown in Fig. 4.2, a section of a dike has length of 100 m, vertical height of 20 m, and slanted surface inclining at an
angle of 30o from the vertical and holding back the water. The water comes up to the top of the dike. Find the total force
exerted by the water on the slanted surface.

 

Fig. 4.2

 

A Section Of The Dike.

 

Solution

Fig. 4.3

 

A Section Of The Dike.

 

Refer to Fig. 4.3. Let the vertical h-axis be directed downward and have origin at the top of the dike. Let h be an
arbitrary point in [0, 20]. A horizontal strip at h has width dh. The corresponding rectangle on the slanted surface has
length 100 and width say dw equal to the hypotenuse of the little solid black right triangle T, whose vertical side equals
dh and, by similar triangles, top angle equals 30o. In T, as cos 30o = dh/dw we have dw = dh/cos 30o. The area dA of
the rectangle is:

 


EOS

 

 

Fig. 4.4

 

A Section Of The Dike.

 

This approach yields the same answer. Remark that the depth of the water isn't measured along the slanted side. Of
course it's as always measured along a vertical axis.

 

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Problems & Solutions

 

1. A rectangular tank has a height of 2 m and a bottom measuring 4 m long by 3 m wide and is filled with water. Find the
force exerted by the water on the bottom of the tank.

 

Solution

 

The pressure of the water on the bottom of the tank is (1 ton/m3)(2 m) = 2 ton/m2. The force exerted by the water on the
bottom of the tank is F = (pressure) x (area of bottom) = (2 ton/m2)(4 m)(3 m) = 24 tons.

 

Note

 

Since the bottom of the tank is horizontal, the pressure is the same everywhere on it, so there's no need to do integration.
Let's do integration here to see if it yields the same answer.

 

 

The above figure represents the bottom of the tank. Let the horizontal l-axis be along a side of length 4, directed
rightward, and with origin at the left corner. Let l be any point in [0, 4]. A strip at l is a rectangle with length 3 and width
dl. The water pressure on it is the same everywhere on it and is (1 ton/m3)(2 m) = 2 ton/m2, and the water force on it is
dF = (2 ton/m2)(3 m)(dl m) = 6 dl tons. The force exerted by the water on the bottom of the tank is:

 

 

Indeed integration yields the same answer. Don't use it if you're not asked to do so; you'll waste your time if you do.

 

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2. A sheet of metal has the shape of a semi-disk of radius 3 m. It's immersed in water with its curved edge down so that
its straight edge is level with the surface of the water. The density of water is 1000 kg/m3. Find the force exerted by
the water on one side of the sheet. (As a matter of fact, the force on the other side is equal in magnitude but opposite
in direction. That's why the sheet doesn't move.)

 

Solution

 

 

 

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3. A circular plate of radius 2 dm is suspended vertically in water such that its highest point is at a depth of 4 dm. The
density of water is 1 kg/dm3. Find the force of the water on one side of the plate.

 

Solution

 

 

 

It follows that:

 

 

Therefore:

 

 

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4. A dam is 25 m high and is in the shape of a region bounded by the curves y = x2 and y = 25. The density of water is
1 ton/m3. Find the total force exerted by the water on the dam when the water comes up to the top of the dam.

 

Solution

 

 

 

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5. A swimming pool 30 m long and 10 m wide has a sloping plane bottom so that the depth of the pool at 1 end is 1 m and
at the other end is 3 m. Find the total force exerted on the bottom of the pool if the pool is full of water.

 

Solution

 

 

 

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