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.