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A speedboat has a small hole in the front of the drive with the propeller that extends down into the water at a water depth of \(0.4 \mathrm{~m}\). Assuming we have a stagnation point at that hole when the boat is sailing with \(40 \mathrm{~km} / \mathrm{h}\), what is the total pressure there?

Short Answer

Expert verified
The total pressure at the stagnation point is 65652.44 Pa.

Step by step solution

01

Convert velocity to SI units

Convert the speed of the boat from km/h to m/s, as SI units are required for calculations. The conversion is done using the factor: \[1 \text{ km/h} = \frac{1}{3.6} \text{ m/s} \]Therefore, the velocity of the boat in m/s is: \[ v = 40 \times \frac{1}{3.6} \approx 11.11 \text{ m/s} \]
02

Calculate Hydrostatic Pressure

The hydrostatic pressure at the hole due to the water depth can be calculated using the formula: \[ P_{hydrostatic} = \rho \cdot g \cdot h \]where \( \rho = 1000 \text{ kg/m}^3 \) (density of water), \( g = 9.81 \text{ m/s}^2 \) (acceleration due to gravity), and \( h = 0.4 \text{ m} \). Substituting the values, we get:\[ P_{hydrostatic} = 1000 \times 9.81 \times 0.4 = 3924 \text{ Pa} \]
03

Apply Bernoulli's Equation for Total Pressure

At the stagnation point, the total pressure is the sum of the hydrostatic pressure and the dynamic pressure (due to velocity). According to Bernoulli's equation:\[ P_{total} = P_{hydrostatic} + \frac{1}{2} \rho v^2 \]Substitute \( \rho = 1000 \text{ kg/m}^3 \) and \( v = 11.11 \text{ m/s} \):\[ P_{dynamic}= \frac{1}{2} \times 1000 \times (11.11)^2 = 61728.44 \text{ Pa} \]Thus, the total pressure is:\[ P_{total} = 3924 + 61728.44 = 65652.44 \text{ Pa} \]
04

Conclusion

The total pressure at the hole where the stagnation point occurs is the combination of hydrostatic pressure due to water depth and the dynamic pressure from the boat's motion.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Bernoulli's Equation
Fluid mechanics often leverages Bernoulli's Equation to explain energy conservation within a flow system. This useful principle is established on the basis that the sum of three specific energies remains constant: potential energy due to height, kinetic energy from fluid velocity, and flow work or pressure energy. Bernoulli's Equation in its simplest form is expressed as:\[ P + \frac{1}{2} \rho v^2 + \rho gh = \, \text{constant} \]Here:- \( P \) is the pressure energy (Pa),- \( \rho \) is the fluid density (kg/m³),- \( v \) is the fluid velocity (m/s),- \( g \) is the acceleration due to gravity (9.81 m/s²),- \( h \) is the height of the fluid above a reference point (m).
This equation is fundamental when analyzing scenarios like a speedboat hitting a stagnation point. At this specific location, the velocity ceases, making Bernoulli’s equation incredibly useful. The pressure increases due to the compressed energy that was previously kinetic, a direct application of Bernoulli's theorem, making it critical for determining total pressure at the stagnation point during the boat’s movement.
Hydrostatic Pressure
Understanding hydrostatic pressure is essential when dealing with fluids at rest. It is the pressure exerted by a fluid due to gravity. Hydrostatic pressure can be calculated using the equation:\[ P_{hydrostatic} = \rho \cdot g \cdot h \]This equation reflects:- \( \rho \), the fluid's density (typically water, which is about 1000 kg/m³),- \( g \), the gravitational acceleration (9.81 m/s²),- \( h \), the height or depth of the fluid column above the point in question (m),As water depth increases, hydrostatic pressure also rises due to the greater weight of water pushing down.
In the context of the exercise, understanding this concept helps determine the portion of total pressure attributable to water column depth. It's a pivotal step in calculating total pressure at a stagnation point, especially when the hole is at a specific water depth, like the 0.4 meters that significantly influenced the pressure calculations in the problem.
Stagnation Point
A stagnation point occurs in fluid dynamics where a flowing fluid comes to rest at some boundary or obstruction. At this point, the fluid's velocity is zero, causing it to be an important aspect in pressure calculations.
For a speedboat cutting through water, a stagnation point can form at specific surfaces such as at a hole or obstruction where fluid flow halts. The kinetic energy of the moving fluid is transformed into pressure energy.
At stagnation points, the pressure reaches its maximum because all the velocity energy is converted into pressure energy. A crucial takeaway from this concept is that when fluid velocity decreases to zero, according to Bernoulli’s equation, all the dynamic pressure becomes static pressure. This is how you get stagnation pressure—an important factor in the exercise, computed as part of the total pressure where the boat meets these resistance points in the water.

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Most popular questions from this chapter

A large storage tank contains saturated liquid nitrogen at ambient pressure, \(100 \mathrm{kPa}\); it is to be pumped to \(500 \mathrm{kPa}\) and fed to a pipeline at the rate of \(0.5 \mathrm{~kg} / \mathrm{s}\). How much power input is required for the pump, assuming it to be reversible?

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