More swimming pool physics

Have you ever thought why water is difficult to move through? What property does it have that air doesn’t, that makes it an effort to get anywhere in it?

The answer is utterly straightforward, but it is worth saying: it is simply more dense than air. If you want to move through it, you’ve got to push the water out of the way, and that means get it moving. And to get something with lots of mass to move requires energy. So it’s easier to move through air than water. We call this an inertial effect – i.e. an effect of inertia (mass). But that’s not the only effect that can happen with a fluid that will slow you down.

Imagine now trying to stir a tin of golden syrup with a teaspoon. Stirring syrup is a difficult thing to do – but not because of inertial effects – golden syrup has about the same density as water. In this case the reason is blindingly obvious again (as it is with much of physics) – golden syrup is sticky. The teaspoon literally sticks to the syrup – that means you need a big force to move it through. We call this effect viscosity.

Now, when something moves through a fluid, there are both inertial and viscous forces that act to slow the thing down. Which force is greater depends obviously on what fluid you are in, but not so obviously on your size. In fact, small things are much more sensitive to the effects of viscosity than large things. When we swim, the stickyness of water is pretty-well unimportant, but when a bacteria swims, the stickyness effect is everything. 

This is neatly described in a lovely lecture and article from the 1970’s by Edward Purcell, called ‘Life at Low Reynolds Number’. (Reynolds number is a measure of the relative size of inertial and viscous forces.) Things happen a bit differently from what we are used to. For example, a bacteria trying to move through water is rather like us trying to swim through golden syrup.  Quite tricky. And sticky. And there’s a deeper problem, which I’ll talk about next time, which explains  the curious propulsion mechanisms of bacteria, sperm etc


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