Question

A stone is dropped from a bridge and it reaches the ground in $4$ seconds. The height of the bridge is

• A. $78.4\text{m}$
• B. $64.2\text{m}$
• C. $98.6\text{m}$
• D. $116.8\text{m}$

Answer 1

The correct option is A $78.4\text{m}$

From the question, it is clear that $u=0$
Time to reach the ground is $t=4sec$
Applying $2^{nd}$ law of motion for constant acceleration, we have
$s=ut+\frac{1}{2}a{t}^2$
$\Rightarrow h=0+\frac{1}{2}\times 9.8\times (4{)}^2=78.4\text{m}$
Hence, height of the bridge is $h=78.4\text{m}$