Actually you and the ball do not fly at 900 miles per hour relative to earth. Since you and the ball are on earth ( the system is earth and its atmosphere) you only feel movements with respect to earth. But if someone is observing ring you and the ball from outer space he will see that you and the ball are rotating around the earth's axis at 900 miles per hour.
It can be explained like this. If you are going in a car which travels at 100 km per hour you will not feel that you are moving at such a speed. Actually you feel like you are not moving. But for someone who is on the road will see you travelling at 100 km per hour. This is relative motion. You are not moving relative to car but moving at 100 km per hour relative to earth. Since you are in the system of car you do not feel that you are moving. In the same way since you are in the system of earth you do not feel that you are rotating at 900 miles per hour but actually you are rotating relative to outer space (actually to the solar system)
The correct answer would because of inertia. Inertia is the tendency of an object to stay at rest if it is at rest, or to stay in motion, if it is in motion, unless acted on by an outside force, such as gravity. When you are holding the ball in your hand, you are going at 900 miles per hour. The ball is also going at 900 miles per hour. So if you throw the ball into the air, it is still going at 900 miles per hour, while it is in the air. This is why you have to wear your seat belt when riding in a car, because if the car you are riding in at 60 miles per hour slams into the back of another car, your body will want to continue forward at that same 60 miles per hour. Inertia is dependent upon the amount of mass in an object, so objects with more mass are harder to move and harder to stop once they stop moving. Objects that are less in mass are easier to get to move and easier to stop moving, because they are less massive.