What stays constant in ideal projectile motion?

In ideal projectile motion, the horizontal velocity remains constant. This is due to the absence of horizontal forces acting on the projectile after it has been launched, assuming air resistance is negligible. Once the projectile is in the air, it moves with a constant horizontal speed as long as there are no external forces to affect that horizontal motion. This lack of force allows it to maintain its horizontal velocity throughout the entire flight.

In contrast, the vertical velocity changes due to the force of gravity acting on the projectile. The acceleration of the projectile also changes in the vertical direction, specifically it decreases as the object rises and increases as it falls back down. Therefore, it is the horizontal component of velocity that remains unchanged during the motion. The overall acceleration of the projectile is consistent and directed downwards at (9.81 , \text{m/s}^2) due to gravity, but because it is acting vertically, it does not affect the horizontal velocity.