* Gravity only acts vertically: The only force acting on the ball once it leaves the table is gravity. Gravity pulls the ball downwards, affecting its vertical velocity.
* No horizontal forces: There are no forces acting on the ball in the horizontal direction. Since there's no force, there's no acceleration in that direction, meaning the horizontal velocity stays the same.
In simpler terms: Imagine the ball has a "forward momentum" as it rolls off the table. Gravity makes it fall down, but it doesn't stop the ball from moving forward. The ball continues moving horizontally at the same speed it had when it left the table.
Important note: In real life, air resistance will slightly slow down the ball's horizontal velocity. However, for many practical purposes, the effect of air resistance is small enough to be ignored.
