Here's why:

* Uniform increase in speed: This means the ball's velocity is changing at a steady rate.

* Acceleration: Acceleration is the rate of change of velocity. A constant change in velocity implies a constant acceleration.

* Gravity: Near the Earth's surface, the force of gravity causes objects to accelerate downwards at a nearly constant rate, approximately 9.8 m/s².

Therefore, the observed uniform increase in speed suggests the ball is being accelerated by gravity.