Two-paper-screens-A-and-B-are-separated-by-100-m-distance-A-bullet-pierces-A-and-then-B-The-hole-in-B-is-10-cm-below-the-hole-in-A-If-the-bullet-is-travelling-horizontally-at-A-calculate-the-veloc

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Question Number 15599 by Tinkutara last updated on 12/Jun/17

$$\mathrm{Two}\mathrm{paper}\mathrm{screens}A\mathrm{and}B\mathrm{are}$$$$\mathrm{separated}\mathrm{by}100\mathrm{m}\mathrm{distance}.\mathrm{A}\mathrm{bullet}$$$$\mathrm{pierces}A\mathrm{and}\mathrm{then}B.\mathrm{The}\mathrm{hole}\mathrm{in}B\mathrm{is}$$$$10\mathrm{cm}\mathrm{below}\mathrm{the}\mathrm{hole}\mathrm{in}A.\mathrm{If}\mathrm{the}\mathrm{bullet}$$$$\mathrm{is}\mathrm{travelling}\mathrm{horizontally}\mathrm{at}A,\mathrm{calculate}$$$$\mathrm{the}\mathrm{velocity}\mathrm{of}\mathrm{the}\mathrm{bullet}\mathrm{at}A.$$$$(\mathrm{Neglecting}\mathrm{the}\mathrm{types}\mathrm{of}\mathrm{frictional}$$$$\mathrm{forces})$$

Answered by mrW1 last updated on 12/Jun/17

$$\mathrm{y}=-\frac{1}{2}{\mathrm{gt}}^2$$$$\mathrm{x}=\mathrm{ut}$$$$\Rightarrow \mathrm{y}=-\frac{1}{2}\mathrm{g}{\left(\frac{\mathrm{x}}{\mathrm{u}}\right)}^2=-\frac{\mathrm{g}}{{2\mathrm{u}}^2}{\mathrm{x}}^2$$$$\mathrm{y}=-10\mathrm{cm}\mathrm{at}\mathrm{x}=100\mathrm{m}$$$$-0.1=-\frac{\mathrm{g}}{{2\mathrm{u}}^2}\times {100}^2$$$$\mathrm{u}=100\sqrt{5\mathrm{g}}=100\sqrt{5\times 9.81}=700.4\mathrm{m}/\mathrm{s}$$

Commented by Tinkutara last updated on 12/Jun/17

$$\mathrm{But}\mathrm{answer}\mathrm{is}700\mathrm{m}/\mathrm{s}.$$

Commented by mrW1 last updated on 12/Jun/17

$$\mathrm{I}\mathrm{had}\mathrm{a}\mathrm{mistake}.\mathrm{now}{\mathrm{it}}^{\prime }\mathrm{s}\mathrm{fixed}.$$

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