(See “About the benefits of washing plates”)

Ernesto explained the rotation of the pan by the swirling of the jet.

But there was one weak point in this explanation:

Why does the jet of water from the slightly squeezed rubber nipple rotate in the opposite direction to the rotation of this light squeeze? After all, it seems to be supposed to twist in the same direction in which this pinch turns!

The other day I realized that this explanation is wrong!

It’s NOT about the swirling of the jet!!! And maybe there is no strong swirling at all!

The reason is different:

Let’s say (which was the case in practice) that there is a slight outward bulge near the center of the bottom of the pan.

The jet of water hitting the bottom of the pan tilts it slightly and the bottom turns into a kind of inclined plane. Moreover, the force of the dynamic pressure of the jet has a component that is horizontally directed, pushing this inclined plane towards its “elevation”. In addition, the inclined plane turns the jet and thereby creates an additional reactive force, too, pushing the pan towards the “elevation”.

When we move the jet in a circle, the resulting infinite inclined plane runs after the jet (beyond the point of contact of the jet with the bottom of the pan) and is pushed away by the jet in the opposite direction of the jet’s rotation!

The same will be observed if a pot with a perfectly flat bottom floats on the surface of a layer of water. The impacting jet will tilt the pan slightly, a “running” inclined plane is created again, and the pan will rotate in the direction opposite to the direction of the conical rotation of the jet.

Here is the true explanation of the “Pan Effect”

May 10th, 2012