* Gravity: Earth's gravitational force constantly pulls on the ball, even when it's in the air. This force acts downwards.

* Unbalanced Force: When the ball is thrown upwards, its initial upward motion is countered by the downward force of gravity. Because the ball is still being pulled towards Earth, the forces are unbalanced.

* Result: This unbalanced force causes the ball to slow down as it goes up and eventually fall back to the ground.

In summary: While the ball is in the air, Earth's gravitational pull creates an unbalanced force, causing the ball's trajectory.