# B Rings in a glass of water

1. Jul 10, 2016

### qorizon

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?

2. Jul 10, 2016

### Staff: Mentor

Yes waves.

3. Jul 10, 2016

### qorizon

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?

4. Jul 10, 2016

### Staff: Mentor

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.

5. Jul 10, 2016

### pixel

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.

6. Jul 10, 2016

### Thejas15101998

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!!!!).

7. Jul 10, 2016

### Thejas15101998

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).