Finding yR and xR - 4 | 6. Fluid Statics 2 Overview | Hydraulic Engineering - Vol 1
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Understanding Pressure and Resultant Forces

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0:00
Teacher
Teacher

Today, we're going to discuss how pressure behaves on submerged surfaces. Can anyone tell me how pressure changes with depth in a fluid?

Student 1
Student 1

Pressure increases as you go deeper into the fluid.

Teacher
Teacher

That's correct! The deeper you go, the higher the pressure due to the weight of the overlying fluid. We express this pressure as P = ρgh. Now, who can tell me what the resultant force FR is?

Student 2
Student 2

Is it the integration of pressure multiplied by the area?

Teacher
Teacher

Exactly! We calculate the resultant force on a surface by integrating the pressure over the area. So we can express it as FR = ρghA. Remember, this force acts through the centroid of the area. Do you all understand how we arrived at this?

Student 3
Student 3

Yes, because pressure varies with depth.

Teacher
Teacher

Great! Now let’s summarize: The pressure increases with depth, and the resultant force on a submerged surface can be calculated using the formula we derived.

Center of Pressure

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Teacher
Teacher

Let's move on to the center of pressure. How many of you know where the resultant pressure force acts on a submerged surface?

Student 4
Student 4

I think it acts at the centroid of the area.

Teacher
Teacher

That's a common misconception. The resultant force actually acts at the center of pressure, which is not the same as the centroid because the pressure increases with depth. How do you think we can determine the center of pressure mathematically?

Student 1
Student 1

Maybe using moments around an axis?

Teacher
Teacher

Exactly right! To find the coordinates of the center of pressure, we take the sum of moments around the x-axis and set it equal to the moment created by the resultant force. We have the equation: yR = (Ix / A) + yc. Practice this because it’s fundamental for fluid mechanics!

Student 2
Student 2

What does Ix represent in that equation?

Teacher
Teacher

Good question! Ix is the second moment of inertia about the x-axis and gives us an idea of how the area is distributed about the centroid. Let's recap: the center of pressure is distinct from the centroid due to varying pressure with depth.

Practical Applications and Understanding Forces

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Teacher
Teacher

Now, let’s bring in real-world applications. Can anyone give me an example of where understanding fluid forces is crucial?

Student 3
Student 3

Maybe in designing dams or bridges that deal with fluid pressure?

Teacher
Teacher

Spot on! Engineers must account for the forces acting on submerged surfaces in these structures to ensure safety and functionality. Let's think about buoyancy. What do we know about it?

Student 4
Student 4

It’s the upward force experienced by objects submerged in fluids.

Teacher
Teacher

Exactly! Buoyancy is a vital concept in fluid statics. Do you all remember Archimedes' principle related to buoyancy?

Student 1
Student 1

Yes! It states that the buoyant force on a submerged object is equal to the weight of the fluid displaced.

Teacher
Teacher

Great! Remember, understanding these principles not only aids in theoretical studies but is essential for practical engineering solutions too.

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section focuses on determining the resultant forces acting on submerged surfaces in fluid mechanics, including the concepts of pressure, buoyancy, and the significance of the center of pressure.

Standard

In this section, we explore the calculation of resultant forces on submerged plane and curved surfaces, emphasizing the role of pressure variation due to depth and how to locate the center of pressure. It touches on buoyancy and integrates foundational concepts of fluid mechanics necessary for further study.

Detailed

Detailed Summary

In this section, we delve into fluid statics specifically focusing on the calculation of resultant forces on submerged surfaces and determining the coordinates of the center of pressure, denoted as yR and xR.

Key Concepts:

  • Surface Forces: The section begins by explaining the concept of static surface forces acting on plane areas and curved surfaces and shows how to calculate these using pressure integration methods.
  • Pressure Variation: An essential point covered is how pressure varies with depth in a fluid. For any point in a fluid, the pressure increases linearly with depth, and resultant forces can be calculated by considering this pressure variation.
  • Resultant Force (FR): The section discusses how to derive the expression for resultant force as the integration of pressure over the area (FR = ρghA). This force acts through the centroid of the submerged area.
  • Buoyant Force: Buoyancy is briefly mentioned as a critical concept in fluid statics that helps understand fluid behavior around submerged objects.
  • Center of Pressure: A significant aspect is identifying the
    center of pressure (with coordinates yR and xR), which indicates where the resultant force acts. The relationship between centroid location and center of pressure is established through moments about axes.
  • Symmetry and Properties: The relationship between symmetric areas and resultant forces highlights how properties such as the location of centroid affect the behavior of submerged bodies.

These concepts are vital for understanding fluid interactions with surfaces as applied in various engineering fields such as hydraulics and structural engineering.

Audio Book

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Understanding Resultant Force (FR)

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So, the force resultant force is going to be the integration of pressure into area, So, p is constant so it comes out and that becomes pressure into area pA, where p is rho gh, okay, this is the gauge pressure. So, F = ∫pdA = p∫dA = pA. So, FR is actually nothing but the weight of the overlying fluid.

Detailed Explanation

In fluid statics, the resultant force (FR) acting on a surface submerged in fluid is found by integrating the pressure across that surface. The pressure (p) varies with depth and is given by the formula p = ρgh, where ρ is the fluid density, g is the acceleration due to gravity, and h is the height of the fluid column above the surface. If we assume p is uniform over the area A, we can simplify this integral to F = pA. This resultant force originates from the weight of the fluid above the surface acting downwards.

Examples & Analogies

Think of this as a column of water in a swimming pool. The deeper you are in the pool, the heavier the water above you, which pushes down. If you were to measure the force on the bottom of the pool, you'd notice it corresponds to the weight of all the water directly above that point.

Finding the Coordinates of Center of Pressure (yR and xR)

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So, we also need to find out X R. What is the logical way? We will do the summation of moment around y axis now. For X R we need to do the moment calculation around y axis so around this axis. How? See, X R into F R. Where is X R? This is X R.

Detailed Explanation

To calculate the x coordinate of the center of pressure (xR), we perform a moment analysis around the y-axis. The resultant force (FR) is multiplied by the x-coordinate of the center of pressure (xR) and this must equal the integral of x multiplied by the differential force (dF) across the area. Thus, we establish the relationship that xR can be found as an integral divided by the total area A. This concept is built upon understanding how distributed forces act on a surface and how they balance out at a point of interest.

Examples & Analogies

Imagine balancing a seesaw. The position where the seesaw balances represents the center of pressure. Each side of the seesaw exerts a force, and depending on how far you sit from the center, the seesaw either tips or stays level. This is similar to how the different pressures vary at depths lead to a resultant force acting not exactly at the centroid, but shifted based on the distribution of pressures.

Calculating yR

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For finding y R the moment equilibrium should be done around x axis. So, the way we do it y R, F R. So, c y R into F R, this will be equal to integral y dF.

Detailed Explanation

To determine yR, the y-coordinate of the center of pressure, the moment equilibrium around the x-axis is used. The equation setup involves the resultant force (FR) multiplied by the distance from the x-axis to the center of pressure (yR) being equal to the integral of each little force (dF) times its distance from the x-axis (y). This leads to a solution using the second moment of inertia about the x-axis and the area.

Examples & Analogies

Consider a large swing with kids sitting at varying distances from the pivot point. If you want to determine the exact point where the swing will balance, you need to consider not just how many kids there are, but their positions as well. The moments created by each kid need to sum up to zero for the swing to be stable, just like we balance moments in our calculations.

The Importance of the Centroid and Its Relation to Moments

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So, this is an important result to note down, F R is equal to gamma A h c. This equation of F R suggests that the magnitude of the resultant force is equal to the pressure at the centroid multiplied by total area.

Detailed Explanation

This highlights how the pressure at the centroid of the submerged area plays a critical role in determining the resultant force (FR). By establishing the equation FR = γAh_c, where γ is the specific weight of the fluid, A is the area, and h_c is the vertical distance from the fluid surface to the centroid, we comprehend how forces act on an area below the fluid's surface. Understanding where the centroid is and how deep it is submerged informs how we measure resultant forces in different orientations.

Examples & Analogies

Picture a boat floating on water. The deeper it sits (submerged more), the more water weight it must displace, leading to a greater upward buoyant force. This parallels how the depth of the centroid affects the force acting on underwater surfaces.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Surface Forces: The section begins by explaining the concept of static surface forces acting on plane areas and curved surfaces and shows how to calculate these using pressure integration methods.

  • Pressure Variation: An essential point covered is how pressure varies with depth in a fluid. For any point in a fluid, the pressure increases linearly with depth, and resultant forces can be calculated by considering this pressure variation.

  • Resultant Force (FR): The section discusses how to derive the expression for resultant force as the integration of pressure over the area (FR = ρghA). This force acts through the centroid of the submerged area.

  • Buoyant Force: Buoyancy is briefly mentioned as a critical concept in fluid statics that helps understand fluid behavior around submerged objects.

  • Center of Pressure: A significant aspect is identifying the

  • center of pressure (with coordinates yR and xR), which indicates where the resultant force acts. The relationship between centroid location and center of pressure is established through moments about axes.

  • Symmetry and Properties: The relationship between symmetric areas and resultant forces highlights how properties such as the location of centroid affect the behavior of submerged bodies.

  • These concepts are vital for understanding fluid interactions with surfaces as applied in various engineering fields such as hydraulics and structural engineering.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • A dam's design must consider the water pressure at various depths to calculate resultant forces accurately and ensure structural integrity.

  • A submerged object, such as a ship, experiences buoyancy, which is the upward force that allows it to float, calculated using Archimedes' principle.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • In waters deep and wide, pressure will surely guide, increasing higher with depth's stride.

📖 Fascinating Stories

  • Imagine a fish swimming deeper into the ocean. As it dives, the weight of the water above presses down, making the fish feel heavier at greater depths — just like how pressure changes.

🧠 Other Memory Gems

  • Remember 'B-P-C' for Buoyancy, Pressure, Center of pressure.

🎯 Super Acronyms

Use 'F-P-A' for remembering the factors of Resultant Force

  • Force
  • Pressure
  • Area.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Resultant Force (FR)

    Definition:

    The net force acting on a submerged surface, calculated as the integration of pressure over the area.

  • Term: Buoyant Force

    Definition:

    The upward force that opposes the weight of an object submerged in a fluid.

  • Term: Center of Pressure

    Definition:

    The point where the resultant force acts on a submerged area, differing from the centroid due to varying pressure distribution.

  • Term: Centroid

    Definition:

    The center of mass or geometric center of a shape or area.

  • Term: Pressure Variation

    Definition:

    The change in fluid pressure with depth.

  • Term: Second Moment of Inertia (Ix)

    Definition:

    A measure of an object's resistance to bending about an axis.