Centrifugal Force
When something is going straight, it always keeps going straight unless something else stops it or turns it. If it can't go straight, then it goes as straight as it can. So when you hit a tetherball, it tries to go straight away from you. But the rope pulls on it and keeps the tetherball from going straight. So the tetherball goes as straight as it can - around the pole in a circle. That's centrifugal force - the energy of something trying to go straight even though it can't.
The Earth is also affected by centrifugal force. It is moving, so it tries to keep moving in a straight line. But the gravity of the Sun pulls the Earth toward it, just as the rope pulls the tetherball. Gravity can't pull the Earth into the Sun, because the Earth keeps trying to go straight. So the Earth takes a middle road, going in a circle around the Sun.
To find out more about centrifugal force, check out these books from Amazon.com or from your library:
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Where did Egyptians bury your liver?
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What does a half-timbered house look like?
How do you spin wool?
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