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I have a glass here.enter image description here

This is the bottom face of the glass. enter image description here

Now this is on what I had kept my glass. There is a little much of water spilled on it. Let us call this thing as C

enter image description here

Now , when I lifted the cup. The C on which I had kept my glass also lifted up. The bottom portion of glass and the top portion of the thing were in contact. They did not leave each other.

Now , the bottom portion of the cup is flat. So, I kept my glass on that C. The cup and the C both lifted up. Why is it like that ? They should not do this.

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1 Answer 1

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The surface of C where the glass sat is smooth, then the glass may have a slightly concave bottom and the water, with capillary action, managed to form a seal between the two surfaces. Even if the glass bottom and surface C are both flat, capillary action can still form a seal around the edge leaving a void in the centre, causing the same effect of lower v. higher pressure.

If the air trapped in there then cools down a few degrees - the outside air pressure holds the two items together. Until of course mass or vibration takes over and the item C falls off.

Happens often with coasters and glasses.

A hot cup of tea can show bubbles when there is some spilt tea in the saucer - the trapped air is being heated, until the tea in the cup cools and we have the same situation as above.

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  • $\begingroup$ Is it like pressure inside the seal is lesser than pressure outside (i.e air). $\endgroup$
    – S.M.T
    Commented Jan 29, 2021 at 7:21
  • $\begingroup$ @user15072279 that's why I mentioned the temperature change in my answer.... $\endgroup$
    – Solar Mike
    Commented Jan 29, 2021 at 7:23
  • $\begingroup$ @user15072279, and the force produced will be $ \Delta P \times A $ (the difference in pressure between the cavity and atmospheric times the area). $\endgroup$
    – Transistor
    Commented Jan 29, 2021 at 7:28
  • $\begingroup$ This may be overcomplicating things - two smooth surfaces with water to fill any gaps can stick together just from electrostatic attraction $\endgroup$ Commented Jan 29, 2021 at 13:19

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