The horizontal acceleration of a projectile is zero when we consider ideal projectile motion in a vacuum. This is because, in the absence of air resistance, no horizontal forces act on the projectile once it is in motion.
To understand this better, let’s consider the motion of a projectile. When an object is launched, it typically has an initial horizontal velocity. While it’s in flight, under ideal conditions, the only force acting on it is gravity which acts downwards. Because there are no horizontal forces acting on the projectile, it maintains its horizontal velocity. Therefore, the horizontal acceleration, which is the change in horizontal velocity over time, remains constant at zero.
In summary, in ideal conditions, the horizontal acceleration of a projectile is zero, and the object continues to move horizontally at a constant velocity until it hits the ground.