When I say centrifugal, I mean centrifugal!

Bad Astronomy
The entire universe in blog form
Aug. 30 2006 9:16 PM

When I say centrifugal, I mean centrifugal!

In various places on my site and in this blog, I have mentioned centrifugal force. Invariably when I do so, someone pedantically comes along and says "That's not a real force! It's fictional! It's really just centripetal force".

Let me clear and blunt here: that's wrong. Centrifugal force is every bit as real as centripetal force. It's just in a different frame. "Centripetal" means "center-seeking", and "centrifugal" means "outwards-seeking" or, more literally, "center-fleeing". You'd think these are opposites, but they are in fact the same thing! It just depends on your point of view.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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If you are standing outside a spinning object, and then draw a diagram of the forces, yes, you're better off using centripetal force. The math works out more easily. But if you're on that spinning object, then the forces are easier to draw assuming a centrifugal force. Really!

Think of it this way: watch a car make a right turn. The people inside have inertia, and they "want" to stay moving forward. The car pushes them to the right, toward the center of the circle it is making. Centripetal.

Now sit inside that car. When the car turns right, which way do you feel yourself leaning? Toward the left, away from the center. You feel a force towards the outside of the circle.

Tadaaa! Same thing, different viewpoints.

Centripetal = centrifugal. Got it? They're the same thing. If one is real, then so's the other. Once more, just to make sure:

Centripetal = centrifugal.

Cripes, I hope I don't ever have to explain that again. If I run across this non-issue again, I will link the perpetrator to this page and make them read it three times.

And if you don't get it, don't feel bad. James Bond didn't either.

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