Is horizontal acceleration always 0?
Accordingly, is horizontal acceleration constant in projectile motion?
The horizontal velocity of a projectile is constant (a never changing in value), There is a vertical acceleration 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.
Besides, why is the horizontal component of a projectile velocity constant?
The horizontal velocity is constant. According to Newton's 1st Law and object in motion stays in motion until a force acts on it. There are no forces acting on a projectile in the horizontal direction (neglecting air resistance), therefore the velocity is constant.
There is no horizontal force acting on the projectile. So, there is no horizontal acceleration. That proposition of no horizontal acceleration is approximately correct in our world. The only horizontal force acting on the projectile is air friction, slowing it down.