Question

A boy drops a stone from a cliff, which reaches the ground in $8s$. Calculate the height of the cliff? [Take $g=9.8m{s}^{-2}$]

Answer 1

Step 1: Given data

Initial velocity of stone, $u=0$

Time taken, $t=8s$

Acceleration due to gravity, $g=9.8m{s}^{-2}$

Height, $h=?$ (To be calculated)

Step 2: Calculating the height of the cliff

Applying equation of motion for the freely falling object,

$$\begin{gathered} h=ut+\frac{1}{2}g{t}^2 \\ \Rightarrow h=0\times 8s+\frac{1}{2}\times 9.8m{s}^{-2}\times {(8s)}^2 \\ \Rightarrow h=313.6m \end{gathered}$$ [ '$g$' is positive when a body falls downwards]

Hence, the height of the cliff is $313.6m$