The Earth exerts a gravitational force on a ball thrown in the air. This force is what pulls the ball back down to the ground after it is thrown upwards.

Here's a breakdown:

* Gravity: A fundamental force of attraction between any two objects with mass. The more massive the objects, the stronger the gravitational force.

* Earth's Gravity: The Earth is a massive object, so it exerts a strong gravitational force on everything near it, including a ball thrown in the air.

* Effect on the Ball: This force constantly pulls the ball towards the Earth's center, slowing its upward motion and eventually causing it to fall back down.