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A Simple Example to Illustrate Why Orbits Exist

Now that you know something about gravity, we will discuss a simple example that illustrates conceptually why orbits exist. Suppose you are standing on top of a tall building and you start throwing baseballs. We all know from experience that when you throw a ball, it follows a curved path until it hits the ground. The harder you throw it, that is, the more velocity you give it in the forward direction, the farther it travels before it hits the ground. The force with which you throw causes the ball to go outward and the force of gravity pulls the ball to the ground. Now back to the tall building on the surface of the Earth. Remember, because the Earth is spherical in shape, the surface curves downward.

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Case A: You throw the ball forward and it lands on the Earth some distance away from the building.

Case B: You throw the ball even further. Gravity still causes the ball to curve downward and strike the Earth, but further away from the building than in Case A.

Case C: You throw the ball just fast enough so that gravity causes its path to curve downward at exactly the same rate that the Earth curves downward. The ball keeps falling around the Earth at a constant height without ever hitting the Earth. The ball stays in a circular orbit around the Earth!

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