5 A particle \(A\) of mass 0.5 kg is projected vertically upwards from horizontal ground with speed \(25 \mathrm {~m} \mathrm {~s} ^ { - 1 }\).
- Find the speed of \(A\) when it reaches a height of 20 m above the ground.
When \(A\) reaches a height of 20 m , it collides with a particle \(B\) of mass 0.3 kg which is moving downwards in the same vertical line as \(A\) with speed \(32.5 \mathrm {~ms} ^ { - 1 }\). In the collision between the two particles, \(B\) is brought to instantaneous rest. - Show that the velocity of \(A\) immediately after the collision is \(4.5 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) downwards.
- Find the time interval between \(A\) and \(B\) reaching the ground. You should assume that \(A\) does not bounce when it reaches the ground.