I recently visited an exhibition (at Questacon), where I saw a display that apparently exhibited the Coriolis effect.

photo of equipment

There are four water jets coming out of pipes connected to a central spindle. Two pipes point out (NE and SW in the photo), and the other pair (NW and SE) are C-shaped, so the jets point in, towards the spindle. With the handle, I could rotate the spindle, which was solidly connected to the pipes.

Upon rotation, the outward-pointing jets acted as I would expect. The pipes were "in front" of the jets, with the jets themselves dragging behind (in terms of their angular position).

outward jets

However, the inward-pointing jets were surprising to me. Instead of dragging behind the pipes, they instead appeared to push in front of the pipes.

inward jets

I couldn't wrap my head around this. The explanation mentioned the Coriolis effect, but was unfortunately lacking in a more specific description. Why are the inward-pointing water jets "in front" of the pipes?


Why are the inward-pointing water jets "in front" of the pipes?

This happens due the initial tangential component of the jet velocity, which is also a cause of the Coriolis effect. It is depicted as vector $v_t$ on the diagram below and, as you can see, it deflects the jet from the radial direction.

enter image description here

A three step time sequence, depicted on the same diagram, shows that, at $t_3$, the jet, formed by three yellow dots, will be ahead of the pipe. The two leftmost yellow dots are shown as green dots at $t_2$. The leftmost yellow dot is shown as a blue dot at $t_1$.

  • $\begingroup$ Ahhh! Thank you. So if I understand correctly, the linear displacement of the pipe tip and the water jet is similar, but the shorter arc at the radius of the water jet (compared to the arc at the pipe tip) makes it look like the jet is "in front" of the pipe. $\endgroup$ – Sparhawk Oct 25 '18 at 3:06
  • 1
    $\begingroup$ @Sparhawk Exactly. $\endgroup$ – V.F. Oct 25 '18 at 11:30

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