Skinning

Adding skins to functional models

Depending on the background and interests of your students, the following phrases may be useful.

Teacher Tip: It's not what it looks like, but what it does.

In physics, models don't have to resemble what they are models of. So the term model is used in a rather particular way. You'd be a bit surprised to buy a model car in a shop that did not have four wheels and windows. But models of cars used in physics may not have either.

The target in physics isn't representational models, but functional models. This is the origin of the impact of the phrase: Consider a spherical cow. It's not that physicists think that cows are spherical, rather that a sphere is simple enough that they can use it to capture some essential feature of how a cow works, which enables them to make predictions, and so understand what a cow does.

The idea of skinning – common in software and also in buying new covers for your mobile – may be useful here. Neither of these alter the functionality in any way: they only change the decorative veneer. Underneath is just the same reality: it's still just a browser, or just a Nokia.

Teacher Tip: A mass acted on by one or more forces can be skinned in many different ways. It could be skinned as a cat, a space rocket, or a thrown tennis ball.

Imagining the world in terms of forces and masses is seeing beyond the skin, down into the animating mechanisms. Since such a view makes many apparently different things appear identical, you might choose to call it a deeper reality. It's certainly a much more parsimonious and coherent view of the world – and those are key markers of the output of thinking like a physicist. You keep chipping away until the ultimate essence of a thing is revealed.

Einstein: A theory is the more impressive the greater the simplicity of its premises, the more different kinds of things it relates, and the more extended its area of applicability.