The forces acting on the rock include Normal Force, Gravitational force & Friction force
It's possible for it to stay on the boulder because the normal force balances it's weight. also because static friction acts on the boulder up to it's limiting friction even if it were on an attempt to move as a result of air resistance. gravitational forces act upon it by mainly affecting it it's weight. as altitude increases, it's weight decreases since gravity varies from a height to another.
