The mutual gravitational force between the apple and the Earth is responsible for the acceleration produced in the apple falling from the tree. Although the Earth will experience the same force, it does not get attracted towards the apple because of its large mass. The insect feels that the Earth is falling towards the apple with an acceleration g because of the the relative motion.letvae​ = velocity of apple w.r.t earth Vea​=velocity of earth w . r . t apple vae​=va​−ve​=−(ve​−va​)=−vea​ As the insect is in the frame of apple so he sees the earth moving with a relative velocity vae​=va​−ve​=−(ve​−va​)=−vea​.Any other observer on earth will see the apple moving towards earth with velocity vae​=va​−ve​=−(ve​−va​)=−vea​.Both are opposite in direction.