Q 11-02-008NEETNEET 2023Top questionMedium
A horizontal bridge is built across a river. A student standing on the bridge throws a small ball vertically upwards with a velocity $4\ \text{m s}^{-1}$. The ball strikes the water surface after $4$ s. The height of bridge above water surface is (Take $g = 10\ \text{m s}^{-2}$) :
Answer: (C) $64$ m
Concept. Use $s = ut - \dfrac12gt^2$ with upward positive and the origin at the bridge.
Step 1. Displacement after $4$ s.
$$s = 4 \times 4 - \frac12 \times 10 \times 16 = 16 - 80 = -64\ \text{m}$$
Step 2. Interpret. The water is $64$ m below the bridge.
Answer. $64$ m (Option C).
Solution by Sreeraj P, M.Sc Physics