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Water filled till the brim When the water is filled up-to the brim the plastic cap can easily stay in the middle (roughly ) of the glass.

enter image description here

When the water is filled till the middle the cap will always come at rest at the corner.

It will be great if somebody can explain why does this happen and also provide me with mathematical proof behind his reasoning

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    $\begingroup$ Does this answer your question? Why does a cork float to the side of a glass? $\endgroup$ Jun 17, 2020 at 4:03
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    $\begingroup$ @user7777777 that is half the answer. It doesn't explain why the cap remains in the middle when the glass is full. $\endgroup$ Jun 17, 2020 at 4:04
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    $\begingroup$ @JohnRennie actually it does: the top-voted answer (not the accepted one) touches this case too. $\endgroup$
    – Ruslan
    Jun 22, 2020 at 16:32
  • $\begingroup$ @Bhavay What kind of mathematical proof could be done? $\endgroup$ Jun 23, 2020 at 10:09
  • $\begingroup$ @AlexTrounev This in regard to the first pic .What am i asking is to prove that at centre sum of net forces = 0 , Also why won't it oscillate(damped oscillations due to viscous force) if the net force is always directed towards centre and rather just stay in centre when displace from the edge. $\endgroup$
    – Bhavay
    Jun 23, 2020 at 14:14

1 Answer 1

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This is something known as the Cheerios Effect. Because of the buoyancy of the water, a light object, for example a cork or bottle cap is forced to occupy the highest position on the surface of the water. Now as shown in the image:

enter image description here
The highest point of the water is at it's edges because of the strong force of adhesion between the glass and the water. So naturally the bottle cap moves to the edge to occupy the highest position available. Interestingly when filled upto the brim, the same force of adhesion pulls the water at the edges down but the water at the centre has no pull on it (due to force of adhesion, cohesive force is omnipresent) and now the highest point is at the centre so that's where the cap resides. This also means that in theory (never tried it) if you filled half the cup with mercury the bottle cap should stay at the centre because of its convex meniscus.

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  • $\begingroup$ For all those who downvoted this, what is the correct answer then? $\endgroup$ Jun 23, 2020 at 10:15
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    $\begingroup$ I would guess people are downvoting because your answer is wrong. The correct answer can be read in the question I linked. $\endgroup$ Jun 23, 2020 at 14:40
  • $\begingroup$ I didn't downvote. But this is what being explained in the top voted answer (not the accepted one) to the linked question. It also relates to the Cheerios effect. I guess there are two possible explanations to this observation. The other reason is explained by John Rennie in this answer. $\endgroup$
    – Vishnu
    Jun 23, 2020 at 14:56
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    $\begingroup$ @John Rennie the accepted answer did not have any given reason. It was a guess as the author clearly mentioned and it's ambiguity is apparent because it just says the sides are just lowest energy configurations. If you look deeply the Cheerios effect and what I have written IS the lowest energy configuration possible because of buoyancy (which in a way is related to gravity) and surface tension. It isn't wrong, I know what I'm saying. $\endgroup$ Jun 23, 2020 at 16:37

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