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centripetal and centrifugal force


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What's the difference between centripetal and centrifugal force?


Centripetal is real and centrifugal is an apparent force. Using a satellite as an example, I see centripetal force as gravity and centrifugal force as inertia keeping the satellite going straight and resisting orbit decay.

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centripetal = center seeking

centrifugal = center fleeing


If you assume you are in an inertial frame and ignore the centripetal force, then you feel a fictitious centrifugal force. Just as the cartoon says.

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In a sense, neither is "real".


Suppose you tie a weight to one end of a rope, grab hold of the other end of the rope, and swing the rope so the weighted end is rotating around you. There are several real forces involved in this example. You are exerting a force on the rope, pulling it toward you. The rock's inertia pulls on the rope in the other direction, putting a tension in the rope. Some old-timers (older than me!) call that outward pull of the rope on you (a real force) a centrifugal reaction force.


This nomenclature is very outdated and is not what is usually meant by the centrifugal force. This is a fictitious force. It appears as a result of doing physics in a rotating frame and pretending that Newton's laws are still valid. In the rotating frame in which the rock is stationary, the tension in the rope exerts a real inward force on the rock. Pretending that Newton's laws apply in this frame, there must be some outward force acting on the rock to keep the rock stationary. This apparent outward force is called centrifugal force.


Note well: the centrifugal force only appears as a result of trying to explain motion in a rotating frame in terms of forces. All centrifugal forces vanish when one explains motion from the perspective of an inertial reference frame.


Centripetal force is simply centripetal acceleration (a kinematics concept) dressed up as a force. Kinematics doesn't care what causes motion to occur. All kinematics says is that the rock is perpetually accelerating toward you: the rock is undergoing centripetal acceleration. While that centripetal acceleration results in this case because you have tied the to the end of a flexible rope, that is irrelevant. The same motion can occur if you glue the rock to the end of a rigid rod, or if you give yourself and the rock opposite electrical charges, or if you make the rock orbit a very heavy mass, or if you use a Klingon tractor beam on the rock. Kinematics doesn't care about forces. Centripetal force is simply centripetal acceleration times mass. In other words, it is kinematics dressed up as dynamics.

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can't gravitational and electromagnetic forces be considered centripetal force in rotating systems?


Yes. Any force that results in circular motion is a centripetal force. Conversely, circular motion requires that there be a force directed toward the center of the circle (uniform circular motion means that it's the net force)

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