Question 156 of 308

A body falling under gravity covers two points $A$ and $B$ separated by $80m$ in $2s$. The distance of upper point $A$ from the starting point is _____$m$. Use $\left(g=10m{s}^{-2}\right)$

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Answer: 45

From $A\rightarrow B$, using the equation of motion, we have
$-80=-{v}_{1}t-\dfrac{1}{2}\times 10{t}^{2} \Rightarrow -80=-2{v}_{1}-\dfrac{1}{2}\times 10\times {2}^{2} \Rightarrow -80=-2{v}_{1}-20 \Rightarrow -60=-2{v}_{1} \Rightarrow {v}_{1}=30m{s}^{-1}$
From $O$ to $A$, it follows that
${v}^{2}={u}^{2}+2gS \Rightarrow {30}^{2}=0+2\times (-10)(-S) \Rightarrow 900=20S \Rightarrow S=45m$