Math Problem Statement

  1. The flow rate in a 7-in diameter pipe is 3.2 ft³/s. What is the average velocity in the pipe?

  2. A pipe delivers 239 gal of water a minute. If the velocity of the water is 27 ft/s, what is the diameter of the pipe?

  3. Water flows in a pipe of 23-cm diameter with an average velocity of 0.73 m/s. The diameter of the pipe is reduced and the average velocity of the water increases to 1.66 m/s. What is the diameter of the smaller pipe? What is the flow rate?

  4. Water in a 5.5-in diameter pipe has a velocity of 97 gal/s; the pipe splits in two to feed two systems. If after splitting one pipe has 3.2-in diameter and the other 1.8-in diameter, what is the flow rate from each pipe?

  5. A copper sphere is dropped from a building 273 ft tall. What will be its velocity on impact with the ground? Ignore air resistance.

Solution

Let's go step-by-step through each question:

1. Flow rate in a 7-inch diameter pipe with a flow rate of 3.2 ft³/s:

We need to find the average velocity of water in the pipe. The formula to calculate velocity based on flow rate is:

V=QAV = \frac{Q}{A}

Where:

  • VV is the average velocity (ft/s),
  • QQ is the flow rate (ft³/s),
  • AA is the cross-sectional area of the pipe (ft²).

For a circular pipe: A=π(D2)2A = \pi \left(\frac{D}{2}\right)^2

  • Convert the diameter to feet: D=712D = \frac{7}{12} ft.
  • Then, calculate the area AA: A=π(712×12)2=π(724)2A = \pi \left(\frac{7}{12} \times \frac{1}{2}\right)^2 = \pi \left(\frac{7}{24}\right)^2

Once you calculate the area, use the formula for velocity.


2. Pipe delivering 239 gallons/minute with a velocity of 27 ft/s:

We need to find the diameter of the pipe. Use the same flow rate formula:

Q=V×AQ = V \times A

Where:

  • QQ is the flow rate (ft³/s). First, convert 239 gal/min to ft³/s: 239gal/min=2397.481ft3/min31.94ft3/min239 \, \text{gal/min} = \frac{239}{7.481} \, \text{ft}^3/\text{min} \approx 31.94 \, \text{ft}^3/\text{min} Q=31.9460ft3/s0.532ft3/sQ = \frac{31.94}{60} \, \text{ft}^3/\text{s} \approx 0.532 \, \text{ft}^3/\text{s} Now, solve for the area AA: A=QV=0.532270.0197ft2A = \frac{Q}{V} = \frac{0.532}{27} \approx 0.0197 \, \text{ft}^2 Using the formula for the area of a circle, solve for the diameter DD: A=π(D2)2D=2×AπA = \pi \left(\frac{D}{2}\right)^2 \Rightarrow D = 2 \times \sqrt{\frac{A}{\pi}}

3. Pipe with water flow changing velocities:

The flow rate remains constant through the pipe. The formula for flow rate is:

Q=A1V1=A2V2Q = A_1 \cdot V_1 = A_2 \cdot V_2

Where:

  • A1A_1 is the area of the larger pipe,
  • V1=0.73m/sV_1 = 0.73 \, \text{m/s} is the velocity in the larger pipe,
  • A2A_2 is the area of the smaller pipe,
  • V2=1.66m/sV_2 = 1.66 \, \text{m/s} is the velocity in the smaller pipe.

First, calculate the flow rate QQ using the area of the larger pipe (with diameter 23cm23 \, \text{cm}): A1=π(232×100)2m2A_1 = \pi \left(\frac{23}{2 \times 100}\right)^2 \, \text{m}^2 Then solve for QQ: Q=A1V1Q = A_1 \cdot V_1 Now, use the same flow rate QQ to find the area of the smaller pipe: Q=A2V2Q = A_2 \cdot V_2 Finally, solve for the diameter of the smaller pipe.


4. Flow rate from two split pipes:

The total flow rate before the split is 97 gallons/s. Convert it to cubic feet per second:

97gal/s=977.481ft3/s12.97ft3/s97 \, \text{gal/s} = \frac{97}{7.481} \, \text{ft}^3/\text{s} \approx 12.97 \, \text{ft}^3/\text{s}

Now, for each pipe, use the formula for flow rate:

Q1=A1V1andQ2=A2V2Q_1 = A_1 \cdot V_1 \quad \text{and} \quad Q_2 = A_2 \cdot V_2

Where:

  • A1A_1 is the area of the 3.2-inch diameter pipe,
  • A2A_2 is the area of the 1.8-inch diameter pipe,
  • The velocities in each pipe depend on the area.

Use the continuity equation: Q=Q1+Q2Q = Q_1 + Q_2 Solve for Q1Q_1 and Q2Q_2.


5. Velocity of copper sphere dropped from 273 ft:

This is a free-fall problem where we neglect air resistance. The formula for velocity after falling a distance hh is:

[ v = \sqrt{2gh}

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Math Problem Analysis

Mathematical Concepts

Fluid Mechanics
Flow Rate
Velocity
Pipe Diameter
Continuity Equation
Free-Fall Motion
Kinematics

Formulas

V = Q / A
A = π(D / 2)^2
Q = V × A
v = √(2gh)

Theorems

Continuity Equation
Bernoulli's Principle (indirect application in fluid dynamics)
Kinematic equations of motion

Suitable Grade Level

Undergraduate Engineering, Advanced High School Physics