An alternative conception we may share with molluscs
Keith S. Taber
A book I was reading claimed that if a winkle is placed on a rotating turntable, it would move towards the centre (much like a record stylus). Moreover, this was explained as the mollusc getting its forces confused (Brown, 1950) .
A winkle knows which way is up – unless taken for a spin, apparently.
The common periwinkle or winkle (Photographed by Guttorm Flatabø, image from Wikipedia, Creative Commons Attribution-Share Alike 3.0 Unported license).
Yet, to my reading it was the author who was getting their forces confused, as the explanation relied on a non-existent 'centrifugal' force.
Given I considered the explanation flawed, I was intrigued to find whether the phenomena was genuine. Do these animals actually head for the spindle if placed upon a rotating turntable? I expected that if this was well known I could soon confirm this with a websearch, and no doubt would find videos on Youtube or similar sites offering empirical evidence. But I could not easily find any (only that for a cost of about €7000 I could purchase a record deck where the turntable levitated when playing a disc).
Life, death and taxes?
The explanation for this claimed (and I would like to think, genuine) phenomenon related to taxes. That is not taxes of the sort which are supposedly, according to scientist and statement Benjamin Franklin, the only certainty in life apart from death. Rather the biological types of taxes, such as ther phototaxis which leads to plant shoots growing upwards although the roots head in a different direction. There are various mechanisms that allow organisms to move or grow towards, or away from, certain features of the environment that act as stimuli. Even single-celled organisms exhibit some forms of taxis.
Brown described taxes exhibited by the winkle
"The winkle … is found just above the high tide level, and it has a set of automatic movements which enable it to regain this position if, as sometimes happens it falls back into the sea. In the sea, it moves away from light (towards the rock base), and against gravity (up the rock face)."
So this organism is sensitive to and responds to gravity – something known as geotaxis, as well as exhibiting phototaxis. This behavior will have evolved over a very long period of time, as very many generations of winkles interacted with features of their shoreline environment. For nearly all of that time their environment did not include any record turntables, so winkles have not had the opportunity to adapt to the (for them) unusual context of being rotated on stereo equipment.
Who has got their forces confused?
Brown argues that in the unusual context, the winkle gets confused in the sense of misidentifying a centrifugal force for gravity:
"This complicated set of movement is entirely automatic, so that if, for example, a winkle is placed on the rotating table of a gramophone, it necessarily moves towards the centre, that is to say, against the direction of the force, and 'mistakes' the centrifugal force for a gravitational force."
But this does not make a lot of sense, because the winkle is experiencing a gravitational force as normal, and there is no centrifugal force.
A centrifugal force is one which acts on a object radially away form the centre of a circle (whereas a centripetal force acts towards the centre.) But a common alternative conception (misconception) is to identify imaginary forces as centrifugal.
Read about misconceptions of centrifugal force
For example, when an object is moving in a circle, a force is needed to maintain that motion. A winkle on a turntable is constantly changing its velocity as its direction is being shifted, and a changing velocity is an acceleration – which requires a force to be acting. This is a centripetal force which is directed to the centre of the rotation. The force deflects the winkle just enough that it does not continue to move in a straight line, but rather along the circumference of a circle. When a centripetal force is maintained, the winkle continues to move in a circle.
But a common intuition is that a object moving with circular motion is stable (after all, it repeatedly returns to the same point) and subject to no overall force. A common alternative conception, then, is that in circular motion a centrifugal (outward) force must be balancing the centripetal (inward) force. This misconception is reflected in the concept cartoon below:
The winkle is subject to gravitational force (downwards, countered by a reaction force from the turntable), and also centripetal force acting towards the centre of rotation. The (unbalanced) centripetal force provides the acceleration that causes the turntable and winkle to move in a circular motion. If there is insufficient friction between winkle and turntable to provide the centripetal force, then the winkle's inertia would lead to it sliding off the turntable – but in the direction it was moving 1, not moving off radially! There is no centrifugal force acting.
I would be interested in learning more about this phenomenon, which I had not seen referenced anywhere else. If it is true, then why does the winkle head for the centre of the turntable?
This episode also intrigued me in another way. The author was Reader in Physics at University College London, and this seems an odd error for a physicist to make (but then, we are all prone to having alternative conceptions, and even those highly qualified in a subject may be mislead by their intuitions at time).
Winkles may be like us?
But then perhaps winkles are no different to us. Someone sitting in the back seat of a car may perceive a force pushing them outwards as the car goes around a roundabout. An observer located in a helicopter above could see that this is really just their inertia – the tendency to continue on a straight line – which a centripetal force has to overcome for the car to turn. There is no outward force – even if it feels like it.
So, perhaps what Brown meant was that, like us, the winkle does get confused – it mistakes the effect of inertia for an outward force that it then seeks to nullify by heading inward. If so, then the winkle, like many humans in equivalent situations, 'confuses its forces' in the sense of mistaking its own inertia for a force?
Work cited:
- Brown, G. Burniston (1950) Science. Its method and its philosophy. London. George Allen & Unwin Ltd.
Notes
1 If there was zero friction the winkle would move off the turntable in a straight line. That is not realistic, so more likely there would be some friction but insufficient to maintain circular motion, and the net force would have the winkle gradually move away from the centre of rotation till it reaches the edge of the turntable. BUT this does not mean it would leave radially (directed away from the centre) rather than tangentially (continuing in a straight line) – it would have quite the opposite effect in that the winkle would spiral out but continue to rotate at increasing distances from the centre till reaching the edge.
The book Student Thinking and Learning in Science: Perspectives on the Nature and Development of Learners' Ideas gives an account of the nature of learners' conceptions, and how they develop, and how teachers can plan teaching accordingly.
It includes many examples of student alternative conceptions in science topics.
