The situation you are experiencing is an example of Newton's first law of motion, also known as the law of inertia. This law states that an object at rest will remain at rest, and an object in motion will remain in motion with a constant velocity, unless acted upon by an external force. In this case, the external force is the force applied to the balls when they are thrown. The law of inertia explains why the balls continue to move forward after they are thrown, even though no force is being applied to them once they are in the air.