Here's why:

* Gravity: Gravity is a force that pulls objects towards each other. The Earth has a large mass, so it exerts a strong gravitational pull on everything near it, including acorns.

* Air Resistance: As the acorn falls, it also encounters air resistance, which is a force that opposes its motion. Air resistance increases as the acorn's speed increases.

* Net Force: The net force on the acorn is the difference between the force of gravity and the force of air resistance. Initially, gravity is the dominant force, causing the acorn to accelerate downward. As the acorn falls faster, air resistance grows until it eventually balances the force of gravity, causing the acorn to reach a terminal velocity (a constant speed).

So, while gravity is the primary force causing the acorn to fall, air resistance also plays a role in influencing its motion.