Math Problem Statement

  1. A pipe leads from a storage tank on the roof of a building to the ground floor. The absolute pressure of the water in the storage tank where it connects to the pipe is 3.0 × 105

[Pa], the pipe has a radius of 1.0 [cm] where it connects to the storage tank, and the speed of flow in this pipe is 1.6 [m/s]. The pipe on the ground floor has a radius of 0.50 [cm] and is 9.0 [m] below the storage tank. Find (a) the speed of flow and (b) the pressure in the pipe on the ground floor.

Solution

To solve this problem, we will use the principles of fluid mechanics, specifically the equation of continuity and Bernoulli's equation.


Given Data:

  1. Pressure at the storage tank: P1=3.0×105PaP_1 = 3.0 \times 10^5 \, \mathrm{Pa}
  2. Speed at the storage tank: v1=1.6m/sv_1 = 1.6 \, \mathrm{m/s}
  3. Radius of the pipe at the storage tank: r1=1.0cm=0.01mr_1 = 1.0 \, \mathrm{cm} = 0.01 \, \mathrm{m}
  4. Radius of the pipe on the ground floor: r2=0.50cm=0.005mr_2 = 0.50 \, \mathrm{cm} = 0.005 \, \mathrm{m}
  5. Height difference: h=9.0mh = 9.0 \, \mathrm{m}
  6. Density of water: ρ=1000kg/m3\rho = 1000 \, \mathrm{kg/m^3} (standard assumption)

(a) Speed of Flow on the Ground Floor (v2v_2):

The equation of continuity relates the speed and cross-sectional areas of the fluid at two points:

A1v1=A2v2A_1 v_1 = A_2 v_2

The cross-sectional area of a pipe is:

A=πr2A = \pi r^2

Thus, the areas are:

A1=π(r1)2=π(0.01)2=3.14×104m2A_1 = \pi (r_1)^2 = \pi (0.01)^2 = 3.14 \times 10^{-4} \, \mathrm{m^2}

A2=π(r2)2=π(0.005)2=7.85×105m2A_2 = \pi (r_2)^2 = \pi (0.005)^2 = 7.85 \times 10^{-5} \, \mathrm{m^2}

From the continuity equation:

A1v1=A2v2A_1 v_1 = A_2 v_2

v2=A1v1A2=(3.14×104)(1.6)7.85×105v_2 = \frac{A_1 v_1}{A_2} = \frac{(3.14 \times 10^{-4})(1.6)}{7.85 \times 10^{-5}}

v2=6.4m/sv_2 = 6.4 \, \mathrm{m/s}

So, the speed of flow on the ground floor is:

v2=6.4m/s\boxed{v_2 = 6.4 \, \mathrm{m/s}}


(b) Pressure in the Pipe on the Ground Floor (P2P_2):

We apply Bernoulli's equation:

P1+12ρv12+ρgh=P2+12ρv22P_1 + \frac{1}{2} \rho v_1^2 + \rho g h = P_2 + \frac{1}{2} \rho v_2^2

Rearranging for P2P_2:

P2=P1+12ρv1212ρv22ρghP_2 = P_1 + \frac{1}{2} \rho v_1^2 - \frac{1}{2} \rho v_2^2 - \rho g h

Substituting the known values:

P1=3.0×105Pa,ρ=1000kg/m3,g=9.8m/s2,h=9.0mP_1 = 3.0 \times 10^5 \, \mathrm{Pa}, \quad \rho = 1000 \, \mathrm{kg/m^3}, \quad g = 9.8 \, \mathrm{m/s^2}, \quad h = 9.0 \, \mathrm{m}

P2=(3.0×105)+12(1000)(1.6)212(1000)(6.4)2(1000)(9.8)(9.0)P_2 = (3.0 \times 10^5) + \frac{1}{2} (1000)(1.6)^2 - \frac{1}{2} (1000)(6.4)^2 - (1000)(9.8)(9.0)

First, calculate each term:

  1. Dynamic pressure at the top:

12ρv12=12(1000)(1.6)2=1280Pa\frac{1}{2} \rho v_1^2 = \frac{1}{2} (1000)(1.6)^2 = 1280 \, \mathrm{Pa}

  1. Dynamic pressure at the bottom:

12ρv22=12(1000)(6.4)2=20480Pa\frac{1}{2} \rho v_2^2 = \frac{1}{2} (1000)(6.4)^2 = 20480 \, \mathrm{Pa}

  1. Gravitational potential energy term:

ρgh=(1000)(9.8)(9.0)=88200Pa\rho g h = (1000)(9.8)(9.0) = 88200 \, \mathrm{Pa}

Now substitute:

P2=(3.0×105)+12802048088200P_2 = (3.0 \times 10^5) + 1280 - 20480 - 88200

P2=300000+12802048088200=231600PaP_2 = 300000 + 1280 - 20480 - 88200 = 231600 \, \mathrm{Pa}

Thus, the pressure on the ground floor is:

P2=2.32×105Pa\boxed{P_2 = 2.32 \times 10^5 \, \mathrm{Pa}}


Final Answers:

  1. v2=6.4m/sv_2 = 6.4 \, \mathrm{m/s}
  2. P2=2.32×105PaP_2 = 2.32 \times 10^5 \, \mathrm{Pa}

Would you like further clarification or help with related problems? Here are some possible follow-ups:

  1. How does the height difference affect the pressure and speed?
  2. Why does the pressure decrease when the speed increases?
  3. What assumptions are made in Bernoulli's equation?
  4. How would using a fluid other than water (e.g., oil) change the result?
  5. What happens if the pipe radius on the ground floor increases?

Tip: The conservation of energy is key in fluid

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Math Problem Analysis

Mathematical Concepts

Fluid Mechanics
Continuity Equation
Bernoulli's Equation

Formulas

Continuity Equation: A1v1 = A2v2
Bernoulli's Equation: P1 + (1/2)ρv1^2 + ρgh = P2 + (1/2)ρv2^2

Theorems

Conservation of Mass
Conservation of Energy in Fluids (Bernoulli's Principle)

Suitable Grade Level

Undergraduate Level (Physics or Engineering)