A mass weighting 56 lbs stretches a spring 3 inches. The mass is in a medium that exerts a viscous resistance of 207 lbs when the mass has a velocity of 6 ft/sec. Suppose the object is displaced an additional 3 inches and released. Find an equation for the object's displacement, u(t), in feet after t seconds.

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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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**Problem Statement:**

A mass weighing 56 lbs stretches a spring 3 inches. The mass is in a medium that exerts a viscous resistance of 207 lbs when the mass has a velocity of 6 ft/sec.

Suppose the object is displaced an additional 3 inches and released.

Find an equation for the object's displacement, \( u(t) \), in feet after \( t \) seconds.

\[ u(t) = \quad \text{\_\_\_\_\_\_\_\_\_\_\_\_} \]

**Explanation:**

This problem involves an object connected to a spring within a medium that provides viscous resistance. This forms a classic physics problem often modeled by differential equations.

- **Given Data:**
  - Weight of the mass: 56 lbs
  - Stretch caused by mass: 3 inches
  - Viscous resistance: 207 lbs at 6 ft/sec

- **Assumptions:**
  - The spring follows Hooke's Law.
  - The medium provides linear viscous resistance.

**Steps to Solve:**

1. **Convert Units:**
   - Convert the 3 inches stretch to feet (3 inches = 0.25 feet).

2. **Determine Parameters:**
   - Compute the spring constant \( k \) using Hooke’s Law: \( F = kx \)
     \( 56 = k \times 0.25 \)
     \( k = 224 \) lbs/ft

   - Compute the damping coefficient \( c \) using the given viscous resistance:
     \( c \times 6 = 207 \)
     \( c = 34.5 \) lbs-sec/ft

3. **Determine the Displacement Equation:**

   - The differential equation modeling the system is: 
     \( m \frac{d^2 u}{dt^2} + c \frac{du}{dt} + k u = 0 \)
     Here, \( m = \frac{56}{32} = 1.75 \) slugs (using \( g = 32 \) ft/sec\(^2\))

   - Plug in the values:
     \( 1.75 \frac{d^2 u}{dt^2} + 34.5 \frac{du}{dt} + 224 u = 0 \)

   - Solve the differential equation to find \( u(t) \).

**Note
Transcribed Image Text:**Problem Statement:** A mass weighing 56 lbs stretches a spring 3 inches. The mass is in a medium that exerts a viscous resistance of 207 lbs when the mass has a velocity of 6 ft/sec. Suppose the object is displaced an additional 3 inches and released. Find an equation for the object's displacement, \( u(t) \), in feet after \( t \) seconds. \[ u(t) = \quad \text{\_\_\_\_\_\_\_\_\_\_\_\_} \] **Explanation:** This problem involves an object connected to a spring within a medium that provides viscous resistance. This forms a classic physics problem often modeled by differential equations. - **Given Data:** - Weight of the mass: 56 lbs - Stretch caused by mass: 3 inches - Viscous resistance: 207 lbs at 6 ft/sec - **Assumptions:** - The spring follows Hooke's Law. - The medium provides linear viscous resistance. **Steps to Solve:** 1. **Convert Units:** - Convert the 3 inches stretch to feet (3 inches = 0.25 feet). 2. **Determine Parameters:** - Compute the spring constant \( k \) using Hooke’s Law: \( F = kx \) \( 56 = k \times 0.25 \) \( k = 224 \) lbs/ft - Compute the damping coefficient \( c \) using the given viscous resistance: \( c \times 6 = 207 \) \( c = 34.5 \) lbs-sec/ft 3. **Determine the Displacement Equation:** - The differential equation modeling the system is: \( m \frac{d^2 u}{dt^2} + c \frac{du}{dt} + k u = 0 \) Here, \( m = \frac{56}{32} = 1.75 \) slugs (using \( g = 32 \) ft/sec\(^2\)) - Plug in the values: \( 1.75 \frac{d^2 u}{dt^2} + 34.5 \frac{du}{dt} + 224 u = 0 \) - Solve the differential equation to find \( u(t) \). **Note
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