It’s flipping great to think like a physicist

The IOP’s press office is often the first number that journalists dial when looking for an explanation of something physics-related, and the relatively straightforward queries are usually dealt with by one of the several physics graduates working here.

One recent such call was from a tabloid reporter asking about the water bottle challenge – a current craze based on flipping a partly filled water bottle and trying to get it to land upright.

Our colleague Dr Taj Bhutta handled that query last week, and we were reflecting afterward on the way he dealt with it. Taj was self-conscious about the Sun’s description of him as a “leading physicist”, but his approach to the problem so exemplifies a number of characteristics of physics, and ways of physics thinking, that the moniker is well deserved.

The basis of this was a desire to find a deep and satisfying explanation, consistent with all he knew, to this unfamiliar phenomenon. His approach meant simplifying the situation, then synthesising the laws and models of physics – from a number of domains – to the dynamic elements within the bottle.

Taj then ran some tests and discussed his ideas with a number of collaborators to develop an everyday explanation that, although referring to mathematical relationships, relied a lot on analogy.

Above all, it’s a striking phenomenon that seems to defy expectations. The bottle seems to stop rotating and perform a vertical descent as it lands – something that is amazing and quite beautiful. There are those who would simply say it was magic, or a trick. But not a physicist: it occurs in the physical world, so it must be subject to explanation using what we know about the way the world behaves – ie, the universal laws of physics.

And yet, although there has to be an answer, exactly what that answer might be is not immediately apparent. That doesn’t faze us, however. Physics is not about knowing the answers off pat: it’s about puzzling away at something to figure out the explanation. As physicists, we will want to do that puzzling. We aren’t satisfied with an answer based on labelling of knowledge, such as saying something like “oh, it’s the Arisbottle effect”.

Coming up with a satisfying explanation will require us to bring together a number of aspects of physics. We therefore simplify the problem – in this case, considering the water and bottle to be separate. We then think of which laws or models are going to help us. In this case, probably: parabolic motion, angular momentum or moments of inertia, and centres of mass.

Each of these is quite straightforward relating to, for example, the flight of a golf ball, the rate of rotation of a ballet dancer, or the toppling of a London bus. But when all those different laws and models are combined, the system as a whole has quite complex behaviour. So physicists try to devise an explanation that is consistent with what we observe and with everything that we know. (We don’t always have to consider these things numerically, either: although there are mathematical solutions to things such as parabolic flight, it’s possible to explain ideas using qualitative versions of a numerical solution.)

Then we test it – in this instance, with repeated trials and by taking a video. We modify our explanation in light of the new, detailed evidence from the video, and check it with rough sketches and semi-quantitative reasoning. Often this involves collaboration with other physicists and deployment of their particular expertise.

Having done this, we use analogies and models to describe aspects of the behaviour, and, finally, arrive at an explanation with which we are satisfied. And there is only one thing more satisfying than a physics explanation – devising a physics explanation.

Physicists know this, and that’s why we plug away at it until we are content. A Sun journalist wants us to tell him the answer. We want just as much to figure it out.

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