If T is the time period of an oscillating pendulum, which one of the following statements is NOT correct ?

The time period (T) of an oscillating pendulum is defined as the time taken to complete one full oscillation. By definition, periodic motion repeats itself after every fixed interval of time T [2]. Therefore, the motion repeats after time T, 2T, 3T, and generally nT, where n is a positive integer [2]. Statement 1 is incorrect because it suggests the motion repeats only once after time T, whereas periodic motion is continuous and repeats indefinitely in an ideal system. Statement 2 is correct as T is the fundamental (least) period of repetition. Statement 4 is also correct because the 'isochronism' of a pendulum—where the period remains constant regardless of amplitude—is an approximation that holds true only for small angular displacements [3]. For larger angles, the restoring force is no longer strictly proportional to displacement, causing the period to lengthen [3].

Sources

1. [2] Science-Class VII . NCERT(Revised ed 2025) > Chapter 8: Measurement of Time and Motion > In a Nutshell > p. 118
2. [3] Science-Class VII . NCERT(Revised ed 2025) > Chapter 8: Measurement of Time and Motion > THINK LIKE A SCIENTIST! > p. 111