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With what minimum horizontal velocity (u) can a boy throw a rock at A & have it just clear the obstruction at B? Refer fig.

Refer to the following image

1 Answer
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Let A be the origin.

Hence $A = (0,0)$

Hence, $B = (40,- 4)$ (from the figure)

Hence, $x = 40; y = -4$

As seen from the figure, rock is thrown horizontally. Hence, $α = 0$

Using equation of path,

$Y= x \tan α – 4.9 x^2/ (u \cosα)^2 \\ -4 = 40 \times 0 – 4.9 \times 1600 / u^2 \\ U = 44.27 m/s$

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