Which force primarily acts on a projectile after it is launched?

Once a projectile is launched, it primarily experiences the force of gravity as it moves through the air. Gravity acts uniformly on all objects and causes them to accelerate downward at approximately 9.81 m/s², regardless of their mass. This downward force will influence the projectile's trajectory, causing it to follow a parabolic path.

While air resistance does act on the projectile as it moves through the atmosphere, its effect is generally secondary to the force of gravity. Air resistance opposes the motion and can slow the projectile down, but it does not significantly alter the overall force acting on it in the way that gravity does.

Inertia, while a key concept in understanding motion, describes an object's tendency to maintain its state of rest or uniform motion unless acted upon by a resultant external force; it is the property of the projectile itself rather than a separate force acting on it.

Friction primarily acts at surfaces in contact, and while it can be relevant in certain contexts (like when a projectile contacts the ground), it does not play a role during the flight of the projectile in the air.

Thus, the predominant force acting on a projectile after launch is indeed gravity, making it the most crucial force in determining the motion of a projectile in flight.