The horizontal velocity of a projectile remains constant as long as no external forces act upon it horizontally. In the absence of air resistance and other factors, horizontal motion is unaffected by vertical motion.
When a projectile is launched with an initial velocity that has both horizontal and vertical components, the two components are independent of each other. The vertical motion is influenced by the force of gravity, causing the vertical velocity to change over time. However, this does not affect the horizontal velocity.
As the vertical velocity decreases due to the downward acceleration caused by gravity, the projectile's vertical position decreases. However, the horizontal velocity remains constant throughout the motion. This means that the projectile covers equal horizontal distances in equal time intervals, assuming there are no external influences.
To summarize, the decrease in the vertical velocity of a projectile due to the force of gravity does not affect its horizontal velocity, which remains constant unless acted upon by external forces.