Math Problem Statement

Assuming that a 3 kg disk is initially at rest and a force of F = 69 N is applied to the thin, inextensible cable wrapped round the disk diameter d = 167 mm. Find the angular velocity of the disk after it has turned through 7.6 revolutions. [Answer in rad / s]

Solution

We are tasked with finding the angular velocity of a disk after a force is applied. Let’s break this into steps.

Given:

  • Mass of the disk, mm: 3kg3 \, \text{kg}
  • Force applied, FF: 69N69 \, \text{N}
  • Diameter of the disk, dd: 167mm=0.167m167 \, \text{mm} = 0.167 \, \text{m}
  • Radius of the disk, rr: d2=0.0835m\frac{d}{2} = 0.0835 \, \text{m}
  • Revolutions turned, nn: 7.67.6
  • Moment of inertia of the disk, II: 12mr2=12(3)(0.0835)2\frac{1}{2} m r^2 = \frac{1}{2} (3)(0.0835)^2
  • Convert revolutions to radians: θ=7.6×2π=15.2πradians\theta = 7.6 \times 2\pi = 15.2\pi \, \text{radians}

We are solving for the angular velocity ω\omega in rad/s.


Step 1: Calculate torque

The torque applied by the force τ\tau is: τ=Fr=690.0835=5.7615Nm.\tau = F \cdot r = 69 \cdot 0.0835 = 5.7615 \, \text{Nm}.


Step 2: Angular acceleration

The angular acceleration α\alpha is given by: α=τI.\alpha = \frac{\tau}{I}. Substitute I=12mr2I = \frac{1}{2} m r^2: I=12(3)(0.0835)2=0.010455375kg\cdotpm2.I = \frac{1}{2} (3) (0.0835)^2 = 0.010455375 \, \text{kg·m}^2. α=5.76150.010455375550.98rad/s2.\alpha = \frac{5.7615}{0.010455375} \approx 550.98 \, \text{rad/s}^2.


Step 3: Final angular velocity

Using the rotational kinematic equation: ω2=ω02+2αθ,\omega^2 = \omega_0^2 + 2\alpha \theta, where ω0=0\omega_0 = 0 (initial angular velocity), we have: ω2=0+2(550.98)(15.2π).\omega^2 = 0 + 2 (550.98)(15.2\pi). Simplify: ω2=2550.9815.23.1415952577.14.\omega^2 = 2 \cdot 550.98 \cdot 15.2 \cdot 3.14159 \approx 52577.14. ω=52577.14229.34rad/s.\omega = \sqrt{52577.14} \approx 229.34 \, \text{rad/s}.


Final Answer:

The angular velocity of the disk is approximately: 229.34rad/s.\boxed{229.34 \, \text{rad/s}}.


Do you need detailed clarifications or explanations for any of the steps?
Here are 5 related questions:

  1. How does the torque formula change if the force is applied at an angle?
  2. What happens if the disk is hollow instead of solid?
  3. Can we calculate the time taken for the disk to reach this angular velocity?
  4. How is this analysis different if the disk rolls without slipping?
  5. How would the result change if the force increases?

Tip: Always check unit conversions (e.g., mm to meters) carefully to avoid calculation errors.

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Math Problem Analysis

Mathematical Concepts

Rotational Dynamics
Kinematics
Torque
Angular Acceleration

Formulas

Torque: \( \tau = F \cdot r \)
Moment of Inertia for a Solid Disk: \( I = \frac{1}{2} m r^2 \)
Angular Kinematic Equation: \( \omega^2 = \omega_0^2 + 2 \alpha \theta \)

Theorems

Newton's Second Law for Rotation

Suitable Grade Level

Undergraduate Physics or Advanced High School (Grades 11-12)