Motion Under Gravity

Following NCERT Class 11, Chapter 3 (Motion in a Straight Line), an object in free fall under gravity starts with zero initial velocity and accelerates downward at g. The height fallen is h = (1/2)gt². Here the stone is simply dropped, so u = 0, and substituting g = 10 m/s² with t = 4 s gives h = (1/2) × 10 × 16 = 80 m. The option 40 m wrongly drops the factor of one-half twice or uses t = 4 with g halved improperly. The option 160 m results from forgetting the one-half factor entirely (10 × 16 = 160). The option 20 m comes from using t = 2 s instead of the given 4 s. Plausibility check: the final velocity on impact would be gt = 40 m/s, and the average velocity over the fall is 20 m/s, which across 4 s yields 80 m, matching our result and confirming the calculation is dimensionally and numerically sound.