August 2016 - When something is going straight, it always keeps going straight unless something else stops it or turns it. (The Iranian scientist Ibn Sina figured this out about 1000 AD). 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 the ball 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 (and all the other planets) take a middle road, going in a circle around the Sun. Centrifugal force also makes the Moon go around the Earth in the same way, for the same reason.
Learn by doing: play some tetherball!
More about inertia
More about gravity
More about momentum
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