Question

An apple falls from a tree. An insect in the apple finds that the earth is falling towards its with an acceleration g. Who exerts the force needed to accelerate the earth with this acceleration?

Answer 1

As the insect is inside the apple so it also falls with gravitational acceleration $g$ now as the insect is taken as frame so now no need to consider the apple just assume that the insect is falling towards the earth freely.

Now if mass of insect be $m$ and that of earth be $M$ and $r$ be the distance between the insect and center of earth at any instant of time.

Then gravitational attraction between $them$ i.e both attract $each-other$ with the force $F=GMm/r^2$

As insect is falling with $g$ so earth is appearing to coming towards the insect in the $insect-frame.$

The force force for this acceleration is exerted by the $insect.$