5 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 \(7 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) and \(10.5 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) respectively.

1. Write down expressions for the heights above the ground of \(A\) and \(B\) at time \(t\) seconds after projection.
2. Hence find a simplified expression for the difference in the heights of \(A\) and \(B\) at time \(t\) seconds after projection.
3. Find the difference in the heights of \(A\) and \(B\) when \(A\) is at its maximum height. At the instant when \(B\) is 3.5 m above \(A\), find
4. whether \(A\) is moving upwards or downwards,
5. the height of \(A\) above the ground.