A surprising twist

When you work in a science centre, you tend to become quite familiar with all these interesting phenomena one gets to experience in a good hands-on exhibition. So when we make a new discovery, we tend to get all the more excited.

That happened to me when I visited the Exploratorium in San Francisco a few years back and my mind was boggled by a small, unassuming exhibit. It was just a circular sink with four metal pipes mounted on a vertical axis in the centre so they could spin freely. Two of these pipes extended almost to the edge of the sink and were bent such that the water streaming out of them was shooting back towards the axis.

There was a knob attached to the axis, and when I turned that, I was amazed – I’d expected the water jets to be deflected, and they were; but they bent in the opposite direction from what I’d expected!

See for yourself in this video by Hector Maciel:

This took me a long time to figure out, and I only really understood what was going on once I built my own simulation of the exhibit.

Just like with the Strobe Fountain I wrote about earlier, it helps when you can slow things down and play around with a few variables. In this case, you can not only change the rotation speed of the five nozzles, but also the water pressure (ie the speed of the water emerging from the nozzles). You can also trace an individual droplet to verify that it actually travels in a straight line. Finally, and this really helped me in understanding the Coriolis Effect, you can switch reference frames: you can look down at the whole arrangement either as a stationary observer or as part of the arrangement, rotating together with the nozzles.

Controls: (Click on applet to activate it.)
  a, z -  in-/decrease rotation speed
  s, x -  in-/decrease water pressure
  d, c -  in-/decrease frame rate of animation
     t -  toggle tracing
     f -  toggle reference frame 

The physical exhibit at the Exploratorium is still the best way to experience this perplexing phenomenon. I have since seen similar exhibits in other science centres, but this simple and very hands-on setup gives you very immediate control to play around and explore the effect. But if you then want to really understand what’s going on, I think an interactive simulation helps a lot…

[There are more online exhibits where this one came from.]

1 responses to A surprising twist

  1. Kiat Teng says:

    Intriguing! I found myself staring at the interactive simulation exhibit for quite sometime…


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