Final answer:
Gravity is the force that causes a baseball to accelerate downward when thrown, resulting in a constant negative vertical acceleration that contributes to the ball's parabolic trajectory.
Step-by-step explanation:
The force that causes a baseball to accelerate downward when thrown is gravity. Despite the ball traveling horizontally away from the pitcher, gravity acts downwards upon the ball from the moment it leaves the pitcher's hand until it contacts the ground. This creates a parabolic trajectory, characterized by a vertical acceleration that is constant over time, as indicated by the ball's motion being split into two perpendicular components: horizontal motion due to the initial throw, and vertical motion due to the force of gravity.
The graph of the ball's vertical acceleration versus time would show a negative value that remains constant, representing the unchanging acceleration due to gravity in the downward direction, which is typically approximately -9.8 m/s2 near Earth's surface. This downward acceleration is independent of the ball's horizontal velocity.