4 Two particles \(A\) and \(B\) are projected vertically upwards from horizontal ground at the same instant. The speeds of projection of \(A\) and \(B\) are \(5 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) and \(8 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) respectively. Find

1. the difference in the heights of \(A\) and \(B\) when \(A\) is at its maximum height,
2. the height of \(A\) above the ground when \(B\) is 0.9 m above \(A\).