# Water not in solid body rotation yet

Confusing students even more by discussing how momentum is being transferred from the tank to the water.

As you remember, we are preparing for the Ekman experiment and need to spin up the tank to solid body rotation.

We had started discussing how, when observed from the co-rotating camera, particles seem to be slowing down relative to the coordinate system underneath the tank as we are approaching solid body rotation.

And this is where I usually confuse the students even more, because I start talking about how momentum is being transferred from the tank to the water. For that, I point out how when observing the tank from the non-rotating framework, the particles further away from the center are moving faster than the ones closer towards the center…

(and on the screen: particles closer to the center are moving faster than the ones further away).

Why is that?

Well, for exactly the same reason we can use this setup to simulate Ekman spirals: Because when the tank is sped up or slowed down, this initially creates friction with the water inside. And as the layer that is in direct contact with the tank is brought to the same speed as the tank, it changes its velocity relative to the next layer, which creates friction and influences the movement of this second layer. And so on and so forth.

I think that it is really useful to point this out, and in some of the groups students jump at it and understand where I am going right away, but in other groups I just cannot phrase it in a way that they understand me. Or maybe they are just not as fascinated as I am by being able to see how friction inside water propagates momentum and hence don’t get excited? Who knows.

[Thanks, Pierre, for your help with the filming!]

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