* Gravity: The primary force acting on the ball is gravity. Gravity constantly pulls the ball downwards, causing its vertical velocity to change.

* Air Resistance: Even if we ignore air resistance for a moment, the ball's velocity is still changing. While the ball travels upwards, its vertical velocity decreases until it momentarily stops at its highest point. Then, as it falls, its vertical velocity increases.

* Air Resistance (Real-World): Air resistance, or drag, acts opposite to the ball's motion. This force also changes the ball's velocity, making it slow down both during ascent and descent.

In summary:

* Vertical velocity: Always changing due to gravity and air resistance.

* Horizontal velocity: If we ignore air resistance, it remains constant. In reality, air resistance will slightly decrease horizontal velocity.

Therefore, the ball's velocity is constantly changing throughout its flight, both vertically and horizontally (due to air resistance).