Question

To estimate the height of a bridge over a river, a stone is dropped freely in the river from the bridge. The stone takes 2 seconds to touch the water surface in the river. Calculate the height of the bridge from the water level

CBSE Class IX Science SA 2 (3 Marks)

Answer 1

The stone is being dropped freely from rest hence, the initial velocity of the stone, u = 0. Again, the velocity of the stone increases as it comes down. Hence, the acceleration due to gravity 'g' is to be taken as positive.

Here,                          Initial velocity       u = 0 m/s

Time taken                                              t = 2.0 s
Let h be the height of the bridge            Apply 2nd kinematic equation,             
                                                     h = ut + (1/2)gt²           or        = (0.4 - 1/2) x 9.8 (2.0)² i m
or                    = 19.6 m 
 Hence, the height of the bridge is 19.6 m

Answer 2

Answer:

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