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Question: Answered & Verified by Expert
An ideal pendulum was oscillating with an angular amplitude θ = π / 3 inside a stationary elevator. When the pendulum was passing through equilibrium position, the lift suddenly starts moving upward with an acceleration a=g. Find the new angular amplitude of the oscillation.

PhysicsOscillationsJEE Main
Options:
  • A π 3
  • B π 6
  • C cos -1 3 4
  • D π 2
Solution:
1485 Upvotes Verified Answer
The correct answer is: cos -1 3 4
1 - cos  θ = 1 2 1 - cos  θ

          cos  θ = 1 2 + 1 2 cos  θ

⇒          θ = cos -1 1 + cos  θ 2

               θ = cos -1 1 + cos π / 3 2

                  = cos -1 1 + 1 / 2 2 = cos -1 3 4

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