A girl is swinging on a swing in sitting position. When the same girl stands up, the period of swing will

A playground swing acts like a pendulum; the time period of a pendulum depends on its effective length, not the mass of the rider [1]. When the girl stands on the seat her body’s centre of mass moves upward toward the pivot, reducing the distance from the pivot to the system’s centre of gravity — i.e. the effective pendulum length becomes shorter. The period for small oscillations is T = 2π√(L/g), so a decrease in L directly reduces T, making the swing oscillate faster (shorter period). Therefore, standing up on the swing shortens the period.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 8: Measurement of Time and Motion > THINK LIKE A SCIENTIST! > p. 110