Here's why:

* Projectile Motion: A projectile is an object that is launched into the air and follows a curved path due to the force of gravity.

* Horizontal Velocity: In the absence of air friction, the horizontal component of a projectile's velocity remains constant. This is because there is no force acting horizontally to change it.

* Vertical Velocity: The vertical component of the velocity changes due to the constant downward acceleration caused by gravity.

Therefore, in the absence of air friction, the horizontal component of a projectile's velocity remains constant, while the vertical component changes.