Why do waves form in a glass of water when hitting the table?

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Discussion Overview

The discussion revolves around the phenomenon of wave formation in a glass of water when the table it rests on is struck. Participants explore the mechanics behind the generation of concentric waves on the water's surface, considering aspects of vibration transmission and symmetry.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant observes that striking the table causes concentric circles to form on the water surface and questions whether these are waves.
  • Another participant asserts that the observed phenomena are indeed waves.
  • A participant queries how waves can form in the glass when only the table is hit, and why the waves are concentric rather than random.
  • It is suggested that the glass vibrates due to the slap on the table, which excites various modes of oscillation in the glass, though the exact mode excited is uncertain.
  • One participant proposes that the circular symmetry of the glass and uniform force transmission from the table contribute to the formation of circular waves.
  • Another participant notes that the center of disturbance aligns with the center of the cup, suggesting that the average disturbance acts symmetrically, which influences the wave pattern.
  • It is reiterated that the circular nature of the waves is expected due to the symmetry and the nature of the disturbance being localized at a single point.

Areas of Agreement / Disagreement

Participants generally agree that the waves are a result of the disturbance caused by hitting the table, but there are multiple viewpoints regarding the mechanics of wave formation and the reasons for the concentric pattern. The discussion remains unresolved on some technical aspects.

Contextual Notes

The discussion includes assumptions about the uniformity of force transmission and the nature of oscillation modes in the glass, which are not fully explored or defined.

qorizon
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So I have a cup of cold water placed on a table, as I hit the table with my palm I'm seeing concentric circles forming and disappearing on the water surface. Why is this happening? Are they waves or something else?
 
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Yes waves.
 
How are they formed in the glass considering I've hit only the table and not the cup? And why the concentric circles instead of just random waves?
 
Ah, those are more interesting questions.

1) The glass rests on the table, Your slap vibrates both the table and the glass.
2) There are many modes of oscillation (wave patterns) possible in a glass. Why that particular mode was excited and not others, I can't say. Wait a bit and other PF members will probably give you the answer to that.
 
qorizon said:
And why the concentric circles instead of just random waves?

Given the circular symmetry of the glass, and the assumption that the force transmitted by the table is uniform over the bottom of the glass, it might be more surprising if the waves weren't circular.
 
Well, when you hit the table, it seems that the "centre of disturbance" turns out to be at the centre of the cup...which turns out that the average of the total disturbance acting on the basement of the cup acts at the centre ( nature love symmetry!).
 
pixel said:
Given the circular symmetry of the glass, and the assumption that the force transmitted by the table is uniform over the bottom of the glass, it might be more surprising if the waves weren't circular.
The waves wouldn't be circular if the disturbance would not show up at one point (consider the case of throwing a stick horizontally into water-rectilinear waves)...but in this case(the cup) the disturbance turns out to show up at a single point (obeying natures symmetry).
 

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