A projectile's horizontal velocity stays constant during flight because:

1. Only a force in a direction can change the velocity in that direction. 2. Because gravity is vertical, its horizontal component is zero. 3. With no horizontal acceleration, the horizontal velocity cannot change. 4. The vertical velocity meanwhile changes steadily under gravity. 5. That contrast between the two components is what curves the trajectory. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 3 'Two-Dimensional Kinematics', sections 3.1-3.4_