A2 Daily A Level Physics question

In a lab, a student uses a small-angle simple pendulum as a timebase inside a lift. When the lift is stationary the period is T0. The lift then descends with steady downward acceleration equal to 0.20 g. Air resistance is negligible. Which statement must be true about the period during the accelerating phase compared with T0, and why?

1. A It decreases by about 20%, because the bob falls with the lift so the restoring effect is stronger; in the limit a → g, the period would shrink toward zero.
2. B It increases by about 20%, because the period is inversely proportional to g, so reducing g by 20% makes T 20% larger; in the limit a → g, T would just double.
3. C It is unchanged, because for small angles the period depends only on length, not on the weight; even if a → g there would be no effect.
4. D It increases by about 11%, because the effective gravitational acceleration is g_eff = g − 0.20 g = 0.80 g and T scales as 1/√g_eff; in the limit a → g, the restoring force vanishes and oscillations cease. (correct)