I have observed at my home that when there is a small puddle of water on a glass table and an inverted steel glass is kept on that puddle of water, the glass starts moving on its own after some time.

I was wondering how that was happening . I think it is due to the fluidity of water due to which the water flows and the glass also moves along with that flowing water. But then I am confused, how can such a small amount of water get adequate amount of energy to move a glass?

Is my reasoning correct? Or there is some more physics behind it?

(PS : I do not know the real reason behind this , so please excuse me if the tags I have given are incorrect.)


What happens is that the water forms a seal between the steel glass and the glass table.

Glass and seal

Due to the slightly higher pressure $p$ inside the glass it's kept afloat while the inside pressure is maintained because of the seal. The slight over-pressure arises when the glass 'sinks' into the water layer and is caused by the weight of the glass, which slightly compresses the air 'trapped' between the glass and the seal. The over-pressure ensures the glass never really touches the table, making movement almost frictionless.

Water also has quite a low viscosity compared to many other liquids, so with the seal in place, the steel glass is quite mobile with respect to the glass table.

But that is all these is to this phenomenon.

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  • $\begingroup$ can you please explain me more about the 'seal' you have mentioned? $\endgroup$ – Black Fire Jun 20 at 10:10
  • $\begingroup$ I've edited the answer accordingly. $\endgroup$ – Gert Jun 20 at 15:11
  • $\begingroup$ This is more likely to occur if the glass starts out cold (after holding a cold drink). As the glass warms, the pressure of the air trapped inside goes up and lifts the glass. The motion occurs if the surface is not perfectly level. $\endgroup$ – R.W. Bird Jun 20 at 17:42
  • $\begingroup$ Actually it works very well with a hot glass, because it heats up the cold air inside. I've seen this many times when washing up! $\endgroup$ – Gert Jun 20 at 17:48

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