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Contests/Gnit Sunday Open 002 (GSO002)/Problem 13 Watt Governor: Non-Linear Small Oscillations in a Rotating System
Problem 13

Watt Governor: Non-Linear Small Oscillations in a Rotating System

Finished
400 ptsLv.7 IntermediateMechanics
2026/03/22 19:30〜2026/03/22 21:00
Author: admin

Problem Statement

Let ggg be the magnitude of gravitational acceleration on a certain planet.

A thin vertical rotating axis is fixed. At a smooth hinge at point O on this axis, two massless rigid rods of length lll connect to two balls A and B (both of mass mmm, treated as point masses).

From each ball, another massless rigid rod of length lll extends, and the other ends of these rods are connected to a common smooth hinge at sleeve C (treated as a point mass of mass MMM).

Sleeve C can slide up and down the rotating axis without friction. Balls A and B are always in symmetric positions on opposite sides of the axis. The quadrilateral O-A-C-B always forms a rhombus, and these four points always lie in the same plane (which contains the axis) and rotate together around the axis.

The entire device is forced to rotate at constant angular velocity ω\omegaω around the axis. Sleeve C reaches a steady position at distance z0z_0z0​ from O (as seen in the rotating frame). At this point, let θ0\theta_0θ0​ (0<θ0<π/20 < \theta_0 < \pi/20<θ0​<π/2) be the angle between each rod and the rotation axis.

Then, in the rotating frame, sleeve C is given a small vertical displacement and released, after which it undergoes small oscillations about the steady-state position.

Find the period TTT of these small oscillations.

Air resistance and friction at all hinges are negligible.

Constraints

  • Gravitational acceleration: g=12 m/s2g = 12 \text{ m/s}^2g=12 m/s2
  • Mass of balls A and B: m=4 kgm = 4 \text{ kg}m=4 kg
  • Mass of sleeve C: M=3 kgM = 3 \text{ kg}M=3 kg
  • Length of rigid rods: l=1.4 ml = 1.4 \text{ m}l=1.4 m
  • Angular velocity of rotation: ω=5 rad/s\omega = 5 \text{ rad/s}ω=5 rad/s

Input Format

The period TTT of small oscillations can be expressed as T=ABπ [s]T = \frac{A}{B}\pi \text{ [s]}T=BA​π [s] with coprime positive integers A,BA, BA,B. Find the value of 100A+B100A + B100A+B and give the answer as a positive integer.

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