* Gravity acts only vertically: Gravity is the only force acting on the projectile (we're ignoring air resistance). Gravity acts straight down, influencing only the vertical component of the projectile's velocity.
* No horizontal force: Since there's no force acting horizontally, there's no acceleration in that direction.
* Constant velocity: Without acceleration, the horizontal component of velocity stays the same from the moment the projectile is launched until it hits the ground.
In simple terms: The projectile keeps moving forward at the same speed the whole time, even as it goes up and down.
