Flywheel Angular Acceleration and Work Done

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Rotational Motion Intermediate rigid body dynamics

Source: High school physics (Chinese)

Problem

A flywheel, which can be treated as a solid cylinder, has a diameter of 0.30 m and a mass of 5.0 kg. A rope is wound around its edge. A constant force pulls one end of the rope, causing it to accelerate uniformly from rest. After 0.50 s, its rotational speed reaches 10 r/s.

  1. Find the angular acceleration of the flywheel and the number of revolutions during this time.
  2. Find the pulling force and the work done by the pulling force during this time.

[Q1] $\alpha = 1.3 \times 10^2$ rad/s$^2$, $N = 2.5$ rev [Q2] $T = 47$ N, $W = 1.1 \times 10^2$ J

The problem involves rotational kinematics and dynamics. First, we find the angular acceleration and displacement from the given kinematic information. Then, using the moment of inertia for a solid cylinder and Newton's second law for rotation, we find the required torque and the pulling force. Finally, the work done is calculated from the torque and angular displacement.

The final angular velocity is $\omega_f = 2\pi f$. Since it starts from rest, the angular acceleration is $\alpha = \omega_f / t$. The angular displacement is $\Delta\theta = \frac{1}{2}\omega_f t$, and the number of revolutions is $N = \Delta\theta / (2\pi)$.

$$ \alpha = \frac{2\pi f}{t} $$ $$ N = \frac{(1/2)(2\pi f)t}{2\pi} = \frac{ft}{2} $$

The moment of inertia for a solid cylinder is $I = \frac{1}{2}MR^2$. The torque is $\tau = I\alpha$, which is also given by $\tau = TR$, where $T$ is the pulling force. The work done is $W = \tau\Delta\theta$.

$$ T = \frac{I\alpha}{R} = \frac{(\frac{1}{2}MR^2)(\frac{2\pi f}{t})}{R} = \frac{MR\pi f}{t} $$ $$ W = \tau\Delta\theta = (TR)(\pi ft) = (\frac{MR\pi f}{t})R(\pi ft) = MR^2\pi^2 f^2 $$

Substituting the given values: $M = 5.0$ kg, $D = 0.30$ m ($R=0.15$ m), $t = 0.50$ s, $f = 10$ r/s.

$$ \alpha = \frac{2\pi (10 \text{ r/s})}{0.50 \text{ s}} = 40\pi \text{ rad/s}^2 \approx 1.3 \times 10^2 \text{ rad/s}^2 $$ $$ N = \frac{(10 \text{ r/s})(0.50 \text{ s})}{2} = 2.5 \text{ rev} $$ $$ T = \frac{(5.0 \text{ kg})(0.15 \text{ m})\pi(10 \text{ r/s})}{0.50 \text{ s}} = 15\pi \text{ N} \approx 47 \text{ N} $$ $$ W = (5.0 \text{ kg})(0.15 \text{ m})^2\pi^2(10 \text{ r/s})^2 = 11.25\pi^2 \text{ J} \approx 1.1 \times 10^2 \text{ J} $$