Going around in circles

If there’s one thing that is likely to promote disharmony amongst the physics community, its the use for the f-word. That’s ‘f’ as in ‘fugal’, as in centrifugal.  Now, part of me thinks I should reconsider before writing this entry, but I have recently read a particularly unhelpful  piece on the internet written by a well meaning but misguided physics teacher that rather concerns me. So I shall wade precariously into the mire of physics controversy and say why I think there shouldn’t be any controversy at all

Ask a six year old – he will know what centrifugal force is. He may not know it by name, but he knows what it is – it’s the force that pushes him to the outside of the car when his dad goes round a corner to fast. (OK, perhaps not so obvious now that children have to wear seat belts, but they’ll still feel it). The trouble is, ten years later, the now teenager does high-school physics and gets taught (quite correctly) that to get things to move in circles, you need to apply an inwards  force towards the centre of the circle (the centripetal force). Think of the moon going around the earth, the earth’s gravity attracts the moon, and this inwards force keeps it moving in (pretty much) a circle.

At this point, the student will ask ‘what about centrifugal force?’ Unfortunately, the response is sometimes ‘forget centrifugal force, there is no such thing as centrifugal force.’ 

Errr, really? So the fighter pilot who feels his head being ripped from his shoulders when carrying out high-speed manouevres is just imagining it then? It is a very real force to him. The point is, the correct concept to use, centrifugal or centripetal, depends entirely on your point of view, by which I mean whether it is the point of view of the person going round in the circle, in which case centrifugal is the way to go, or the point of view of someone standing by and watching, in which case centripetal is the right concept. This internet-favourite cartoon gets it spot-on.

For those still unhappy with centrifugal force, let me mention just one more thing. Constructing Newton’s laws in a rotating system, as talked about by Blofeld in the cartoon leads to another term – the coriolis force. Now, this force is what is responsible for low-pressure systems spiralling anti-clockwise in the northern hemisphere and clockwise in the southern, and, more interestingly, is exploited in the workings of the electronic stability control system (ESC) on a modern car. It has the same status mathematically as centrifugal force. So the logical consequence of denying centrifugal force is the denial of the coriolis force, and the implication of that is the non-existence of hurricanes and the failure of your ESC system.

Centrifugal force does exist, so long as it is you that is going around in circles.

2 thoughts on “Going around in circles”

  • Thank god someone feels the way I do about calling the centrifugal force fake. My teacher the other day went out of his way to particularly describe why the force is “fake” using some type of argument that the force is not felt in all reference frames therefore can’t be labeled as a force… well sure. thats fine and dandy but why not just call it something else rather than a fake force – maybe the pushme effect? in reference to your speal – What’s in a name? 🙂
    funny thing too was that the chapter we were in had all to do with reference frames ha…
    https://sci.waikato.ac.nz/physicsstop/2008/11/whats-in-a-name.shtml

  • Marcus Wilson says:

    No force is felt in ALL reference frames. (So, by your teacher’s argument, if I understand correctly, then every force is fake.) If there is a force F on you (of mass m), just choose a frame that is accelerating at a = F/m in the direction of that force and, hey presto, the force vanishes. That’s the ‘Equivalence Principle’, which underlies Einstein’s General Relativity. An often-quoted example is being in a free-falling elevator. From YOUR point of view, inside the elevator, there is no gravity. You will float about, being able to do all kinds of cool stuff like push globs of water about and watch pendulums that won’t oscillate at all, stand on bathroom scales and watch them read zero – at least until you smash into the bottom!
    From someone standing on top of the building (probably the guy who cut the lift cable) there very definitely is gravity, and he will watch you accelerate all the way to the bottom.

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