A projectile fired horizontally will accelerate toward Earth at a rate of 9.8 m/s², which is the acceleration due to gravity.

Here's why:

* Gravity acts vertically: The force of gravity always pulls objects downwards, regardless of their horizontal motion.

* Constant acceleration: This means the projectile's downward velocity will increase by 9.8 meters per second every second it's in the air.

* Horizontal motion is independent: The horizontal motion of the projectile is not affected by gravity. It will travel at a constant horizontal speed unless acted upon by an external force like air resistance.

Important Note: This assumes we are ignoring air resistance. In reality, air resistance will affect the projectile's trajectory and slow it down slightly.