Why exactly is it that an object like a ping pong ball floating in water (say in a washbasin) will be held in the same location when a stream of water (say from a tap) pours down over it?


This works equally well in an empty sink or in a sink filled with water.

If instead of a ping pong ball, we had a wedge, it would move away from the stream (video). But a ping pong ball can roll and, as soon as it tries to get away from the stream, the stream forces it to roll back as shown in these videos: sink filled with water and empty sink.

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Besides the weight of the falling water, we have adhesion forces in play, which "grab" the ball and keeps it from bouncing off.

Some people believe that the ball is pulled back by low pressure due to the Bernoulli's principle, but I don't think it plays a significant role here, particularly, when the stream is tiny.


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