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Then why do we assume the motion of pendulum as linear with an angular displaement of 4 deg from equilibrium if its not a factor to be considered for a motion to be shm ?? Please clarify the reason for this

Question

Then why do we assume the motion of pendulum as linear with an angular displaement of 4 deg from equilibrium if its not a factor to be considered for a motion to be shm ?? Please clarify the reason for this

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Solution

The assumption of the motion of a pendulum as linear with an angular displacement of 4 degrees from equilibrium is based on the small angle approximation. This approximation is used in the study of simple harmonic motion (SHM) because it simplifies the mathematics involved.

Here's the step-by-step explanation:

  1. The motion of a pendulum is governed by the differential equation θ'' + (g/L)sinθ = 0, where θ is the angular displacement, g is the acceleration due to gravity, and L is the length of the pendulum. This is a nonlinear equation because of the sinθ term.

  2. However, if we restrict the motion of the pendulum to small angles (less than about 5 degrees), we can use the small angle approximation, which states that for small angles, sinθ ≈ θ (where θ is in radians).

  3. Using this approximation, the differential equation simplifies to θ'' + (g/L)θ = 0. This is a linear differential equation, which describes simple harmonic motion.

  4. Therefore, by assuming small angles, we can treat the motion of the pendulum as simple harmonic motion, which is much easier to analyze and understand.

  5. The reason we can make this approximation is that the error introduced is very small when the angles are small. For example, at 4 degrees, the error is less than 0.5%.

So, it's not that the angular displacement is not a factor to be considered for a motion to be SHM. Rather, we make an approximation to simplify the analysis, and this approximation is very accurate for small angles.

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