1. Gravity: This force acts vertically downwards, constantly pulling the projectile towards the center of the Earth.

2. Air Resistance (Drag): This force opposes the projectile's motion and acts in the opposite direction to its velocity. It depends on factors like the projectile's shape, speed, and the density of the air.

It's important to note that for simplified projectile motion problems, air resistance is often ignored. This allows for easier calculations and provides a good approximation, especially for projectiles moving at relatively low speeds. However, in real-world scenarios, air resistance plays a significant role and can significantly affect the projectile's trajectory.