Musical Glass

What to do:

Fill up a wine glass almost to the top with water. Dip your finger in the water and them slowly run your finger around the rim of the glass. Play around with moving faster or slower until you get a note (most people go too fast to start with though).

Notice the surface of the water is rippling. Drink some water and then run your finger around the rim again – you’ll notice that the note changes.


Why does that happen?

You will have noticed that the pitch of the note goes up as the amount of water in the glass decreases.You vibrate both the glass and the water at a low frequency by running your finger around the rim of the glass. As you do this, your finger sticks and slides around the edge and this action causes the vibrations. You could see that it was both the glass and the water vibrating as you could see the ripples on the surface of the water. The vibration you put in was just the right frequency to get the whole glass and water vibrating. Everything has a frequency that it naturally likes to vibrate at. This is called its resonant frequency. It occurs when we put in just the right amount of energy for an object (and it can be any object) to get vibrating. The pitch of the note went up when you drank some water because there was less stuff (mass) to vibrate. This means that the resonant frequency is a bit higher and you end up with a higher pitch note.




So what?

Sometimes getting things to resonate can be quite pleasant (like our wine-glass trick), but sometimes it can cause really big problems. If a bridge starts to resonate because of a strong wind or the traffic walking over it this can cause wobbling and even crumbling. To try to prevent this engineers have to work out ways to dampen resonance when building bridges and even buildings, especially in earthquake-prone areas. It’s also why soldiers are commanded to walk out of step with each other when they cross a bridge.




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