# Pop quiz (with poll): Vibrating glasses with water

#### silverpig

##### Lifer
You have a wine glass with a small amount of water in it and make it vibrate by rubbing your wet finger around the rim to hear a tone. Now you add some more water to the glass and rub your finger on the rim again. Does the pitch go up or down?

I know the answer, but I thought it'd make an interesting poll on ATOT.

edit: thanks all for participating

edit2: This is the one I meant to post...

#### BrokenVisage

##### Lifer
I'm pretty sure it goes down.

#### effowe

##### Diamond Member
Well I haven't tried that, so I'm only relating this to drinking beer from a bottle. The more beer gone from the bottle, the lower the pitch. This is my reasoning for voting Up.

#### KK

##### Lifer
Originally posted by: effowe
Well I haven't tried that, so I'm only relating this to drinking beer from a bottle. The more beer gone from the bottle, the lower the pitch. This is my reasoning for voting Up.

Heh.. this was my logic as well

#### silverpig

##### Lifer
Hint: Think about what's actually vibrating

#### sdifox

##### No Lifer
Water is dampener, so if there is more water, there is more resistance to vibration. Thus lower frequency.

#### lyssword

##### Diamond Member
Hah, feels so good for being right at least once

#### DrPizza

##### Administrator Elite Member Goat Whisperer
I just want you to imagine a crowded bar (that serves food) that has an upper balcony section that overlooks the downstairs section. And, to that upstairs section, add 40 physics teachers and students who have all tuned their wine glasses to the same frequency. The downstairs section became dead quiet and 100 people had a wtf expression on their faces wondering what was going on.

#### silverpig

##### Lifer
Originally posted by: DrPizza
I just want you to imagine a crowded bar (that serves food) that has an upper balcony section that overlooks the downstairs section. And, to that upstairs section, add 40 physics teachers and students who have all tuned their wine glasses to the same frequency. The downstairs section became dead quiet and 100 people had a wtf expression on their faces wondering what was going on.

Nice!

#### Mo0o

##### Lifer
Adding water will decrease the fundamental wavelength, therefore increase the fundamental frequency.

atleast i think that's the case...

i am racked by selfdoubt

#### RichieZ

##### Diamond Member
Originally posted by: Mo0o
Adding water will decrease the fundamental wavelength, therefore increase the fundamental frequency.

atleast i think that's the case...

i am racked by selfdoubt

what he said, AP physics was a long time ago

#### MrPickins

##### Diamond Member
Originally posted by: effowe
Well I haven't tried that, so I'm only relating this to drinking beer from a bottle. The more beer gone from the bottle, the lower the pitch. This is my reasoning for voting Up.

That's what I was thinking. Of course I was thinking about blowing across the top like playing a jug, just assuming those vibrations would be similar to the type in question.

#### silverpig

##### Lifer
Okay, so I'll answer the question.

The pitch goes down.

What? But when I blow across a beer bottle, a full bottle produces a higher pitch than an empty one...

That's where my hint of "think about what's vibrating" comes in. When you blow across the bottle, you're making the air inside the bottle vibrate. Empty bottle = larger air cavity = longer wavelength of air vibration = lower pitch. This is not what happens when you rub a wine glass. When you rub the rim, you're making the glass vibrate. Yes, the air vibrates so you hear the sound, but the source of the vibration is the glass itself, not the air. As the glass is in contact with the water, the water becomes part of the vibrating system as well.

I'm sure the exact mechanics of what happen are rather complicated, but the glass, being quite rigid will have a restoring force to deformations (read: the glass is like a really stiff spring). Any deformation of the glass is faced with an increasing restoring force. The water does two things. It dampens the sound (the full glass will be quieter), and it provides a larger mass which must be vibrated.

The dampening component will have an effect on the pitch, but I'll ignore it.

The mass component definitely does have an effect on the pitch. Seeing as how we're ignoring dampening, we have reduced the system to something physicists love to reduce everything down to: a simple harmonic oscillator (a spring and a mass... a pendulum...). The resonant frequency of a harmonic oscillator goes as:

f = a*Sqrt[k/m]

where f is the frequency, a is some proportionality constant which makes the units work nicely, k is the spring constant, and m is the mass. If you increase m, you decrease f. Thus, adding water will decrease the frequency or pitch.

I was doing dishes earlier this afternoon. This didn't make it any more fun however.

[bill nye]NOW YOU KNOW...[/bill nye]

#### spidey07

##### No Lifer
Silverpig,

Thanks for the explantion!

But the question remains....can you calculate a perfect C and actually make it happen?

#### silverpig

##### Lifer
Originally posted by: spidey07
Silverpig,

Thanks for the explantion!

But the question remains....can you calculate a perfect C and actually make it happen?

Ha no. It'd take some finite element analysis to do properly, and a lot of cpu time I think. It could be done, but it'd be much more efficient to tune it with an eyedropper and a tuning fork

#### spidey07

##### No Lifer
Originally posted by: silverpig
Originally posted by: spidey07
Silverpig,

Thanks for the explantion!

But the question remains....can you calculate a perfect C and actually make it happen?

Ha no. It'd take some finite element analysis to do properly, and a lot of cpu time I think. It could be done, but it'd be much more efficient to tune it with an eyedropper and a tuning fork

mmmmmmmmmmmmmmmmmmmm

hooooooooooooooooooooooooooooooooon

mmmmmmmmmmmmmmmmmmmm

#### Rubycon

We need a glass with a hollow bottom to have the capability to add and remove water on the fly. Put it on a turntable so all one has to do is apply finger pressure to the edge. Sounds like fun.

#### Rubycon

Sort of but with just one surface to touch.

The player would be in an airtight room and the pressure would be controlled by a computer using direct displacement. The pressure doesn't have to change much (a few inches of WC) to make the levels go down and this would happen pretty fast. Even the musically challenged could play anything on the books as all they would have to do is press with their finger.

#### homercles337

##### Diamond Member
I cant believe so many thought it went down.

#### silverpig

##### Lifer
Originally posted by: homercles337
I cant believe so many thought it went down.

It does go down...

#### sdifox

##### No Lifer
Originally posted by: silverpig
Originally posted by: homercles337
I cant believe so many thought it went down.

It does go down...

too bad we can't see the list of people that answered wrongly

#### yankeesfan

##### Diamond Member
I had a great reason why I picked down. Now I can't remember what it was.