5 A small ball is thrown horizontally with speed \(5 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) from a point \(O\) on the roof of a building. At time \(t \mathrm {~s}\) after projection, the horizontal and vertically downwards displacements of the ball from \(O\) are \(x \mathrm {~m}\) and \(y \mathrm {~m}\) respectively.

1. Express \(x\) and \(y\) in terms of \(t\), and hence show that the equation of the trajectory of the ball is \(y = 0.2 x ^ { 2 }\). The ball strikes the horizontal ground which surrounds the building at a point \(A\).
2. Given that \(O A = 18 \mathrm {~m}\), calculate the value of \(x\) at \(A\), and the speed of the ball immediately before it strikes the ground at \(A\).