Free fall

How long does the stone fall freely into a depth of 80m? What speed will it hit the bottom of the abyss?

Correct answer:

t =  4 s
v =  40 m/s

Step-by-step explanation:

$$\begin{gathered} s={\displaystyle \frac{1}{2}}g{t}^2 \\ g=10{\text{m/s}}^2 \\ s=80\text{ m } \\ t=\sqrt{{\displaystyle \frac{2\cdot s}{g}}}=\sqrt{{\displaystyle \frac{2\cdot 80}{10}}}=4\text{ s} \end{gathered}$$

$v=g\cdot t=10\cdot 4=40\text{ m/s}$

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