The ATC notices that airplanes NJ267 and DL267 are flying at the same speed on intersecting courses. What is the problem and what would you tell the pilots in order to solve the problem

Astone weighing 1.5 kilograms is resting on a rock at a height of 20 meters above the ground. the stone rolls down 10 meters and comes to rest on a patch of moss. the gravitational potential energy of the stone on the moss is joules.