Pressure in Forces on Submerged Surfaces

In summary, the discussion revolves around the concept of pressure and why we use gage pressure instead of absolute pressure in certain situations. The book mentions that the force on an element is equal to PgdA, where Pg is the gage pressure. However, the question arises as to why we don't consider the atmospheric pressure (Patm) in this calculation. The answer is that in certain cases, such as with a thin balloon wall, the pressures inside and outside must be equal for balance. This means that even in the sky, where the absolute pressure is lower, the gage pressure remains at zero. This is because gage pressure is referenced to ambient air pressure. It is noted that this is not a general principle and that the
  • #1
Est120
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Why does the pressure we take into account is the gage pressure and not the absolute pressure?

Reading Fundamentals of Momentum Heat and Mass transfer by Welty in chapter 2 it says "the magnitude of the force on the element dA is PgdA ,where Pg is the gage pressure"

my question is why the gage pressure ? it is supposed that the total pressure acting on that element of area dA is the absolute pressure (Patm+Pgage) why we don't take into account Patm?
 
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  • #2
Think of a thin wall such as a balloon. What keeps the balloon wall from exploding, or collapsing. The force inside pushing out must balance the force outside pushing in. That happens when the pressures inside and outside are equal. In other words, zero gauge pressure.

Now let the balloon float up into the sky. The absolute pressure outside is less. The balloon expands but does not burst. Even up in the sky, the gauge pressure is zero.

Gauge pressure is referenced to ambient air pressure. Zero gauge means ambient. In the balloon example, it is the outside pressure, both on the ground and in the sky.

It is no different for a pipe wall.
 
  • #3
Est120 said:
and not the absolute pressure?
This is not a general principle. It all depends on the situation; when the pressure difference between an experiment and the atmosphere then gauge is relevant. If you are studying how an enclosed gas will behave then it is absolute pressure that counts.
 
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1. What is pressure in forces on submerged surfaces?

Pressure in forces on submerged surfaces is the force per unit area exerted by a fluid on an object that is fully or partially submerged in the fluid. It is caused by the weight of the fluid above the object and is dependent on the depth and density of the fluid.

2. How is pressure in forces on submerged surfaces calculated?

The pressure in forces on submerged surfaces can be calculated using the formula P = ρgh, where P is the pressure, ρ is the density of the fluid, g is the acceleration due to gravity, and h is the depth of the object below the surface of the fluid.

3. What is the difference between pressure in forces on submerged surfaces and atmospheric pressure?

The pressure in forces on submerged surfaces is caused by the weight of the fluid above the object, while atmospheric pressure is caused by the weight of the air above the surface. Additionally, pressure in forces on submerged surfaces is dependent on the depth and density of the fluid, while atmospheric pressure is relatively constant at sea level.

4. How does pressure in forces on submerged surfaces affect objects?

Pressure in forces on submerged surfaces can affect objects in different ways. For example, it can cause objects to float or sink depending on the density of the object and the fluid. It can also cause objects to experience buoyant forces, which can be used to calculate the weight of an object.

5. What are some real-world applications of pressure in forces on submerged surfaces?

Pressure in forces on submerged surfaces has many practical applications, such as in hydraulic systems, scuba diving, and submarine design. It is also used in industries such as oil and gas exploration, where the pressure of fluids in underground reservoirs must be calculated to determine the potential yield of the reservoir.

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