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The balance wheel of an old-fashioned watch oscillates with angular amplitude \(\pi\) rad and period \(0.500 \mathrm{~s}\). Find (a) the maximum angular speed of the wheel, (b) the angular speed at displacement \(\pi / 2 \mathrm{rad}\), and \((\mathrm{c})\) the magnitude of the angular acceleration at displacement \(\pi / 4\) rad.

Short Answer

Expert verified
(a) \(4\pi^2\) rad/s, (b) \(2\pi^2\sqrt{3}\) rad/s, (c) \(4\pi^3\) rad/s².

Step by step solution

01

Understand the Parameters

The oscillation of the balance wheel is simple harmonic motion with an angular amplitude of \( \pi \) radians and period \( T = 0.500 \) seconds.
02

Calculate Maximum Angular Speed

The maximum angular speed \( \omega_{\text{max}} \) can be calculated using the formula \( \omega_{\text{max}} = \omega_0 \cdot A \), where \( \omega_0 = \frac{2\pi}{T} \) is the angular frequency and \( A \) is the maximum angular displacement. Here, \( A = \pi \) rad. So, \( \omega_0 = \frac{2\pi}{0.5} = 4\pi \). Thus, \( \omega_{\text{max}} = 4\pi \times \pi = 4\pi^2 \) rad/s.
03

Calculate Angular Speed at Specific Displacement

The angular speed \( \omega \) at a displacement \( \theta = \frac{\pi}{2} \text{ rad} \) can be found using \( \omega = \omega_0 \sqrt{A^2 - \theta^2} \). Substituting, we get \( \omega = 4\pi \sqrt{\pi^2 - (\pi/2)^2} = 4\pi \sqrt{\pi^2 - \pi^2/4} = 4\pi \sqrt{3\pi^2/4} = 4\pi \cdot \frac{\pi \sqrt{3}}{2} = 2\pi^2 \sqrt{3} \).
04

Calculate Angular Acceleration at Specific Displacement

The angular acceleration \( \alpha \) at \( \theta = \frac{\pi}{4} \text{ rad} \) is \( \alpha = -\omega_0^2 \cdot \theta \). Using \( \omega_0 = 4\pi \), we find \( \alpha = -(4\pi)^2 \times \frac{\pi}{4} = -16\pi^2 \times \frac{\pi}{4} = -4\pi^3 \) rad/s². The magnitude is \( 4\pi^3 \) rad/s².

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Angular Speed
Angular speed refers to how fast an object rotates around a central point. In the context of simple harmonic motion, such as the oscillation of a watch's balance wheel, angular speed varies as the object moves back and forth. It reaches its maximum at the equilibrium position - that’s when the restoring force or torque has pushed it to move the fastest.

For a balance wheel with a maximum angular amplitude of \(\pi\) radians and a period of 0.5 seconds, we can determine the maximum angular speed using the formula \(\omega_{\text{max}} = \omega_0 \times A\), where \(\omega_0\) is the angular frequency. The angular frequency \(\omega_0\) is found using \(\omega_0 = \frac{2\pi}{T}\).
  • In our case, \(T = 0.5\) which gives \(\omega_0 = 4\pi\).
  • The maximum angular speed, therefore, is \(4\pi^2\) rad/s when \(A = \pi\).
The angular speed can also be calculated at a specified displacement, such as \(\frac{\pi}{2}\) rad, by applying \(\omega = \omega_0 \sqrt{A^2 - \theta^2}\). This calculation reveals how the speed varies with position and showcases the dynamic nature of harmonic oscillations.
Angular Acceleration
Angular acceleration in simple harmonic motion is the rate of change of angular speed. In simpler terms, it's how quickly or slowly the speed of the object is changing as it swings back and forth. It is directly influenced by the angular position and ultimately determines how forcefully the object moves toward the equilibrium position.

Angular acceleration can be computed at any displacement within the cycle. For instance, at a displacement angle of \(\frac{\pi}{4}\) radians, the angular acceleration is determined using the equation \(\alpha = -\omega_0^2 \times \theta\). This tells us how fast the oscillating object is slowing down or speeding up at any given point.
  • Here, with \(\omega_0 = 4\pi\), we find that \(\alpha = -4\pi^3\) rad/s².
  • The magnitude of this acceleration is \(4\pi^3\) rad/s², indicating how intensely the object is decelerating at this particular displacement.
Understanding angular acceleration helps in visualizing the dynamic and continuous change in velocity as the object attempts to return to its resting position.
Oscillation
Oscillation refers to the repetitive motion back and forth around a central position. In a watch's balance wheel, this is the continuous to-and-fro movement, which ensures accurate timekeeping. The wheel's motion is an example of simple harmonic motion, characterized by oscillations that are sinusoidal in time and have a uniform amplitude across cycles.

The period of oscillation is the time taken for one complete cycle. In our case, the wheel completes an oscillation every 0.5 seconds. The maximum displacement from the central rest position is referred to as the angular amplitude, which is \(\pi\) radians in the exercise.
  • Oscillations occur due to the restoring torque that acts on the wheel, bringing it back to the equilibrium position.
  • The repetitive nature of oscillation implies a consistent transfer between potential and kinetic energy.
  • These oscillations maintain the watch's balances by evenly dividing time intervals, a crucial function for mechanical timepieces.
By mastering the concept of oscillation, students can comprehend the principles behind not just mechanical watches, but a vast array of harmonic oscillators in physics.

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