Question

Pebbles are dropped freely for every half second into a river from a bridge above it. When the first pebble is about to strike the surface of the water, the fifth pebble is dropped. Then the height from the surface of the water from which the pebbles are dropped is (Acceleration due to gravity = 10 m/s²).

• A. 20 m
• B. 31.25 m
• C. 125 m
• D. 80 m

Hint:

Objects in free fall follow the equation \( s = \frac{1}{2} g t^2 \).

Answer 1

The Correct Option is A

Step 1: Using Free Fall Equation

\[ s = \frac{1}{2} g t^2 \] For the first pebble: \( t = 2 \) sec \[ s = \frac{1}{2} \times 10 \times (2)^2 = 20 \text{ m} \]
Step 3: Conclusion

Since the height is 20 m, Option (1) is correct.