* Gravity: The primary force acting on the marble is gravity. Gravity pulls the marble downwards, causing it to gain speed.

* Acceleration: Acceleration is the rate of change of velocity. Since the marble is speeding up, it's experiencing acceleration.

* Direction: The marble's velocity increases in the direction of its motion (downwards).

In summary: The marble's velocity increases in magnitude (it gets faster) and its direction remains downwards as it accelerates down the hill.