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centrifugal force
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Definition/Summary
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On a body forced to move in a circle, there must be a centripetal force (a force directed toward the centre of the circle).
On a body forced to move in a circle, there is no centrifugal force, when viewed from an inertial frame.
An inertial observer, applying the inertial laws of motion, measures no centrifugal force on a body forced to move in a circle.
An observer forced to move in a circle (and therefore non-inertial) can apply the inertial laws of motion only by adding a "fictitious" centrifugal force to the physical forces. |
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Recent forum threads on centrifugal force
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Breakdown
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Physics
> Classical Mechanics
>> Newtonian Dynamics
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Extended explanation
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The Principle of Equivalence (the basis of Einstein's General Theory of Relativity) says that anyone can be a valid observer, but that the inertial equations of motion may have to be adjusted to introduce imaginary (non-physical) forces.
Centrifugal force on a body is such a non-physical force.
In particular, a rotating observer invents an imaginary (non-physical) centrifugal force to explain why objects appear to move round him. 
"Centrifugal" means "away from the centre" … it comes from the Latin word fugo (I flee) … as does "refugee". It is the opposite of "centripetal", which means "toward the centre" (and comes from the Latin word peto, I seek … as does "petition").
On an object moving in a circular path, there is no centrifugal force as viewed by an inertial observer.
Centrifugal force on such an object only exists for non-inertial observers.
However, both inertial and non-inertial observers recognise a centrifugal force from such an object, on whatever is keeping it in the circle.
Two different meanings:
Most standard physics textbooks use the "modern" meaning of centrifugal force as a pseudoforce, existing only as an artefact of viewing things in a non-inertial frame.
It is not a "real" (physical) force, since it has no agent.
The "old-fashioned" meaning of centrifugal force as the Newton's-3rd-law pair ("reaction force") of the centripetal force is completely "real", in any frame.
These two different types of centrifugal force act on different bodies.
Whirling on a string:
An observer holding onto a string which is whirling him in a circle feels a force along his arm toward the centre of the circle.
However, he knows that he is not moving toward the centre.
So he also feels a force in the opposite direction, balancing the force along his arm.
In that sense, he genuinely feels a centrifugal force.
In a turning car:
The driver of a car turning sharply left notices that unsecured objects slide to the right … away from the centre of the turn.
In the driver's rotating (non-inertial) frame of reference, that can only be explained by a force to the right.
It is a centrifugal force, acting on everything in the car, but nothing physical is causing it.
If the driver regards that force as real, then he may apply the inertial laws of motion. |
Commentary
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