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Question

A pendulum of length \(L\) oscillates with an angular amplitude of \(\theta = 60^\circ\) and time period \(T\). Let \(T_0 = 2\pi\sqrt{\frac{L}{g}}\) be the time period for small angle of oscillations, where \(g\) is the acceleration due to gravity. If air resistance is negligibly small and the string remains straight, then which one of the following is correct?

This question was previously asked in
NDA 2 2026 GAT Question Paper (13-Sep-2026)
The correct answer is

\(T\) will be slightly greater than \(T_0\).

To understand the behavior of the pendulum for larger angular amplitudes, we need to consider how the time period \(T\) of a pendulum oscillating with a larger amplitude compares to the time period \(T_0\) calculated for small angles.

The time period of a simple pendulum for small angles (less than \(15^\circ\) is given by:

\(T_0 = 2\pi\sqrt{\frac{L}{g}}\)

However, for larger angular amplitudes, the assumption that the motion is exactly simple harmonic motion (SHM) is invalid. The time period is affected and can be calculated more precisely using an elliptic integral, which results in an increased time period due to a larger arc of travel in higher angles.

For angles up to about \(60^\circ\), as specified in this problem \((\theta = 60^\circ)\), the time period \(T\) is slightly greater than \(T_0\). This is because:

  1. As the amplitude increases, the pendulum has to travel a longer arc.
  2. The restoring force, while still primarily gravitational, is less effective at larger angles as it deviates from the linear restoration seen in small angles.
  3. The deviation from SHM leads to the pendulum taking slightly more time to complete a cycle.

Thus, when \(\theta\) is increased to \(60^\circ\), \(T\) becomes greater than \(T_0\). Given the options, the correct answer is:

 

\(T\) will be slightly greater than \(T_0\).

Let's rule out other options:

  • The option that \(T\) is exactly equal to \(T_0\) is incorrect because this assumes perfect SHM, which is a valid approximation only for very small amplitudes.
  • The proposition that \(T\) will depend upon the mass of the bob is incorrect for simple pendulums, as the time period does not depend on the mass of the bob, as per the pendulum time period formula.
  • The claim that \(T\) is smaller than \(T_0\) is incorrect since increasing the amplitude leads to an increase in the time period, as reasoned above.
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