4 A particle is projected vertically upwards from a point O at \(21 \mathrm {~ms} ^ { - 1 }\).
- Calculate the greatest height reached by the particle.
When this particle is at its highest point, a second particle is projected vertically upwards from \(O\) at \(15 \mathrm {~ms} ^ { - 1 }\).
- Show that the particles collide 1.5 seconds later and determine the height above O at which the collision takes place.