Centripetal Force and Newton's Third Law
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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Update: June 2012