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Name: Jhon
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My question has to do with centrifugal force. Everybody (mostly science teachers) says that centrifugal force does not exist, but doesn't every force have an equal and opposite? So... wouldn't centrifugal force be the equal and opposite of centripetal force?

Centrifugal force does not exist in an inertial frame of reference. Yes all forces have action-reaction pairs. But the NET force on an object is not the same as a force on the object. In circular motion, the NET force is the force pushing (or pulling) the object toward the center.

Let us say two people are pulling on a box in the opposite directions, one with 2 newtons of force and the other with 6 newtons of force. The net force is 4 newtons in the direction of the 6 newton force. There is no "equal and opposite" force to this 4 newton net force. The other two forces (the 2 newton and 6 newton forces) have equal and opposite forces with the earth.

When Space Shuttle orbits Earth, the only force acting on it is the gravitational field of Earth. There is no "outward" force acting. The action-reaction pair is Earth pulling on Shuttle and Shuttle pulling on Earth. The NET force on Shuttle is only the pull of Earth.

---Nathan A. Unterman

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