Question

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

Answer 1

Step 1: Given data

The initial velocity of stone, $u=0$

Time taken, $t=8s$

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

The final velocity of the stone, $v=?$ (To be calculated)

Step 2: Calculating the final velocity of the stone

Applying the equation of motion for the freely falling object,

$$\begin{gathered} v=u+gt \\ \Rightarrow v=0+9.8m{s}^{-2}\times 8s \\ \Rightarrow v=78.4m{s}^{-1} \end{gathered}$$ [ '$g$' is positive when a body falls downwards]

Hence, the final velocity of the stone is $78.4m{s}^{-1}$.