Question 228 of 308

A child stands on the edge of the cliff $10m$ above the ground and throws a stone horizontally with an initial speed of $5m{s}^{–1}$ . Neglecting the air resistance, the speed with which the stone hits the ground will be _____ $m{s}^{–1}$ (given, $g=10m{s}^{–2}$ ).

A$20$B$15$C$30$D$25$

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Answer: (B) $15$

Given here, $u=5m{s}^{-1},h=10m$
Using ${v}_{y}^{2}={u}_{y}^{2}+2gh$ in vertical direction, we have the velocity of stone in vertical direction as ${v}_{y}=\sqrt{2gh}=\sqrt{200}m{s}^{-1}$ (since, ${u}_{y}=0$).
The final velocity along the horizontal direction is the same as the initial horizontal velocity as there is no acceleration along the x-axis.
The speed with which the stone hits the ground is
${v}_{net}=\sqrt{{v}_{x}^{2}+{v}_{y}^{2}}=\sqrt{25+200}=15m{s}^{-1}$