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June 2013 p41 q3
3866
The top of a cliff is 40 metres above the level of the sea. A man in a boat, close to the bottom of the cliff, is in difficulty and fires a distress signal vertically upwards from sea level. Find
the speed of projection of the signal given that it reaches a height of 5 m above the top of the cliff,
the length of time for which the signal is above the level of the top of the cliff.
The man fires another distress signal vertically upwards from sea level. This signal is above the level of the top of the cliff for \(\sqrt{17}\) s.
Find the speed of projection of the second signal.
Solution
(i) Using the equation of motion: \(0 = u^2 - 2gs\), where \(s = 45 \text{ m}\) (40 m + 5 m) and \(g = 10 \text{ ms}^{-2}\), we have:
\(u^2 = 2 \times 10 \times 45\)
\(u = 30 \text{ ms}^{-1}\)
(ii) Using \(s = ut - \frac{1}{2}gt^2\) with \(s = 40\), \(u = 30\), and \(g = 10\):