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STEP II 1992 question 11 solution

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TSR Wiki > Study Help > Subjects and Revision > Mathematics > STEP > STEP II 1992 question 11 solution


if the purse scrapes the edge of the cliff, it reaches its max point at 0.5d from edge of cliff. vertical movement: u=VsinA, v=0, a=-g \implies s = \dfrac{(V\sin A)^2}{2g}

horizontal movement after max point: v=VcosA, s=0.5d \implies t = \dfrac{d}{2V\cos A} vertical movement after max point: s=\dfrac{(V\sin A)^2}{2g} - h, u=0, a=g, t = \dfrac{d}{2V\cos A}

\displaystyle \frac{(V\sin A)^2}{2g} - h = \frac{0.5gd}{2V\cos A}

\displaystyle \implies 8V^4\sin^4A - 8V^2(2gh+V^2)\sin^2A + 2g(gd^2+8V^2h) = 0\\ \\
\sin^2A = \frac{8V^2(2gh+V^2) \pm \sqrt{}(64V^4(V^4+4gh+4g^2h^2) - 64gV^4(gd^2+8V^2h))}{16V^4}

for the purse to be able to be thrown in the situations given, sinA must exist so:

V^4+4ghV^2+4g^2h^2 \geq g^2d^2 + 8ghV^2\\
V^4-4ghV^2+4g^2h^2 \geq (gd)^2\\
V^2-2gh \geq gd\\
V^2 \geq g(2h+d)

Solution by *bobo*.

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