Question

why does the velocity in the horizontal direction remain constant while the velocity in the vertical direction changes?

Answer 1

The horizontal velocity of a projectile is constant (a never changing in value), There is a verticalacceleration caused by gravity; its value is 9.8 m/s/s, down, The vertical velocity of a projectile changes by 9.8 m/s each second, The horizontal motion of a projectile is independent of its vertical motion.