Here's why:
* Gravity: The only force acting on the ball after it's thrown is gravity.
* Constant Force: Gravity is a constant force, meaning it pulls on the ball with the same strength throughout its flight.
* Downward Acceleration: This constant force causes the ball to accelerate downwards, regardless of its upward motion.
Important Note: The ball's velocity changes throughout its flight. It starts with an upward velocity, slows down as it rises, reaches zero velocity at its highest point, and then accelerates downwards. However, the acceleration due to gravity remains constant throughout the entire flight.
